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Nutrient Cycling in Pastures


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Nutrient Cycling in Pastures

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  • 1. NUTRIENT CYCLING IN PASTURES LIVESTOCK SYSTEMS GUIDE By Barbara Bellows NCAT Agriculture Specialist December 2001 Table of Contents Introduction and Summary ........................................................... 2 Publication Overview ................................................................... 5 Chapter 1. Nutrient Cycle Components, Interactions, and ............. Transformations ........................................................................ 6 Water Cycle ........................................................................... 6 Carbon Cycle ....................................................................... 10 Nitrogen Cycle ..................................................................... 13 Phosphorus Cycle ................................................................ 18 Secondary Nutrients ............................................................ 21Abstract: Good pasture management Chapter 2. Nutrient Availability in Pastures ................................ 23practices foster effective use and Soil Parent Material .............................................................. 23recycling of nutrients. Nutrient cycles Soil Chemistry ...................................................................... 23important in pasture systems are the Prior Management Practices ................................................ 24water, carbon, nitrogen, and phosphorus Soil Compaction ................................................................... 24cycles. This publication provides basic Organic Matter ..................................................................... 25descriptions of these cycles, and presents Soil pH ................................................................................. 27guidelines for managing pastures to Timing of Nutrient Additions ................................................. 27enhance nutrient cycling efficiency — Chapter 3. Nutrient Distribution and Movement in Pastures ...... 30with the goal of optimizing forage and Pasture Nutrient Inputs and Outputs .................................... 30livestock growth, soil health, and water Manure Nutrient Availability .................................................. 32quality. Includes 19 Tables and 14 Pasture Fertilization ............................................................. 33Figures. Grazing Intensity .................................................................. 34 Diversity and Density of Pasture Plants ............................... 36 Chapter 4. The Soil Food Web and Pasture Soil Quality ........... 40 Diversity of the Soil Food Web ............................................. 40 Organic Matter Decomposition............................................. 40 Primary Decomposers ......................................................... 41 Secondary Decomposers ..................................................... 43 Soil Organisms and Soil Health ........................................... 44 Chapter 5. Pasture Management and Water Quality ................ 47 Risk Factors for Nutrient Losses .......................................... 47 Pathogens ............................................................................ 48 Nitrate Contamination .......................................................... 49 Phosphorus Contamination .................................................. 49 Subsurface Drainage ........................................................... 51 Riparian Buffers ................................................................... 52 Riparian Grazing .................................................................. 53 References ................................................................................ 55 Resource List ............................................................................ 61 Agencies and Organizations ................................................ 61 Publications in Print ............................................................. 61 Web Resources ......................................................................... 63 ATTRA is the national sustainable agriculture information service operated by the National Center for Appropriate Technology under a grant from the Rural Business-Cooperative Service, U.S. Department of Agriculture. These organizations do not recommend or endorse products, companies, or individuals. ATTRA is headquartered in Fayetteville, Arkansas (P.O. Box 3657, Fayetteville, AR 72702), with offices in Butte, Montana and Davis, California.
  • 2. Introduction and Summary As a pasture manager, what factors do you look at as indicators of high production and maximumprofitability? You probably look at the population of animals stocked within the pasture. You probablylook at the vigor of plant regrowth. You probably also look at the diversity of plant species growing in thepasture and whether the plants are being grazed uniformly. But do you know how much water seepsinto your soil or how much runs off the land into gullies or streams? Do you monitor how efficiently yourplants are taking in carbon and forming new leaves, stems, and roots through photosynthesis? Do youknow how effectively nitrogen and phosphorus are being used, cycled, and conserved on your farm? Aremost of these nutrients being used for plant and animal growth? Or are they being leached into thegroundwater or transported through runoff or erosion into lakes, rivers, and streams? Do you know howto change your pasture management practices to decrease these losses and increase the availability ofnutrients to your forages and animals? .igure 1. Nutrient Cycles in Pastures. Animal Production T Manure Production T T T Water Infiltration TT T T Plant Vigor Minimal Soil T Water Availability T T T T TSoil Organisms Erosion T T T T Nutrient Availability T T T T T T T T Soil Organic Matter T T T Legumes T Nutrient Mineralization T T Plant Cover Soil Porosity/ T Minimal CompactionHealthy plant growth provides plant cover over the entire pasture. Cover from growing plants and plant residues protectsthe soil against erosion while returning organic matter to the soil. Organic matter provides food for soil organisms thatmineralize nutrients from these materials and produce gels and other substances that enhance water infiltration and thecapacity of soil to hold water and nutrients. Effective use and cycling of nutrients is practices that facilitate water movement into thecritical for pasture productivity. As indicated in soil and build the soil’s water holding capacityFigure 1 above, nutrient cycles are complex and will conserve water for plant growth and ground-interrelated. This document is designed to help water recharge, while minimizing waters poten-you understand the unique components of wa- tial to cause nutrient losses. Water-conservingter, carbon, nitrogen, and phosphorus cycles and pasture management practices include:how these cycles interact with one another. This • Minimizing soil compaction by not overgraz-information will help you to monitor your pas- ing pastures or using paddocks that have wettures for breakdowns in nutrient cycling pro- or saturated soilscesses, and identify and implement pasture man- • Maintaining a complete cover of forages andagement practices to optimize the efficiency of residues over all paddocks by not overgraz-nutrient cycling. ing pastures and by implementing practices WATER that encourage animal movement across each Water is necessary for plant growth, for dis- paddocksolving and transporting plant nutrients, and for • Ensuring that forage plants include a diver-the survival of soil organisms. Water can also be a sity of grass and legume species with a vari-destructive force, causing soil compaction, nutri- ety of root systems capable of obtaining wa-ent leaching, runoff, and erosion. Management ter and nutrients throughout the soil profilePAGE 2 //NUTRIENT CYCLING IN PASTURES
  • 3. CARBON called rhizobia, are able to transform atmospheric Carbon is transformed from carbon dioxide nitrogen into a form available for plant use. Ni-into plant cell material through photosynthesis. trogen in dead organic materials becomes avail-It is the basic structural material for all cell life, able to plants through mineralization. Nitrogenand following the death and decomposition of can be lost from the pasture system through thecells it provides humus and other organic com- physical processes of leaching, runoff, and ero-ponents that enhance soil quality. Plant nutri- sion; the chemical process of volatilization; theents such as nitrogen and phosphorus are chemi- biological process of denitrification; and throughcally bound to carbon in organic materials. For burning of plant residues. Since it is needed inthese nutrients to become available for plant use, high concentration for forage production and cansoil organisms need to break down the chemi- be lost through a number of pathways, nitrogencal bonds in a process called mineralization. If is often the limiting factor in forage and crop pro-the amount of carbon compared to other nutri- duction. Productive pasture management prac-ents is very high, more bonds will need to be tices enhance the fixation and conservation of ni-broken and nutrient release will be slow. If the trogen while minimizing the potential for nitro-amount of carbon compared to other nutrients gen losses. Practices that favor effective nitro-is low, fewer bonds will need to be broken and gen use and cycling in pastures include:nutrient release will proceed relatively rapidly. • Maintaining stable or increasing percentagesRapid nutrient release is preferred when plants of legumes by not overgrazing pastures andare growing and are able to use the nutrients by minimizing nitrogen applications, espe-released. Slower nutrient release is preferred cially in the springwhen plants are not actively growing (as in the • Protecting microbial communities involvedfall or winter) or if the amount of nutrients in in organic matter mineralization by minimiz-the soil is already in excess of what plants can ing practices that promote soil compactionuse. Pasture management practices that favor and soil disturbance, such as grazing of weteffective carbon use and cycling include: soils and tillage• Maintaining a diversity of forage plants with • Incorporating manure and nitrogen fertiliz- a variety of leaf shapes and orientations (to ers into the soil, and never applying these ma- enhance photosynthesis) and a variety of terials to saturated, snow-covered, or frozen root growth habits (to enhance nutrient up- soils take). A diversity of forages will provide a • Avoiding pasture burning. If burning is re- balanced diet for grazing animals and a va- quired, it should be done very infrequently riety of food sources for soil organisms and by using a slow fire under controlled con-• Promoting healthy regrowth of forages by ditions including a combination of grasses with both • Applying fertilizers and manure according low and elevated growing points, and by to a comprehensive nutrient management moving grazing animals frequently enough plan to minimize the removal of growing points PHOSPHORUS• Maintaining a complete cover of forages and Phosphorus is used for energy transforma- residues over all paddocks to hold soil nu- tions within cells and is essential for plant trients against runoff and leaching losses and growth. It is often the second-most-limiting min- ensure a continuous turnover of organic resi- eral nutrient to plant production, not only be- dues cause it is critical for plant growth, but also be- NITROGEN cause chemical bonds on soil particles hold the majority of phosphorus in forms not available Nitrogen is a central component of cell pro- for plant uptake. Phosphorus is also the majorteins and is used for seed production. It exists nutrient needed to stimulate the growth of algaein several chemical forms and various microor- in lakes and streams. Consequently, the inad-ganisms are involved in its transformations. Le- vertent fertilization of these waterways with run-gumes, in association with specialized bacteria off water from fields and streams can cause //NUTRIENT CYCLING IN PASTURES PAGE 3
  • 4. degradation of water quality for drinking, recre-ational, or wildlife habitat uses. Regulations on .igure 2. Components ofthe use of phosphorus-containing materials are Soil Organic Matter.becoming more widespread as society becomes Readily Bacteria & decomposable Fauna 10%increasingly aware of the impacts agricultural Actinomycetes 30% 7-21%practices can have on water quality. Pasture T T Yeast, algae, T Tmanagement practices must balance the need to T protozoa,ensure sufficient availability of phosphorus for nematodes 10%plant growth with the need to minimize move- Fungi 50%ment of phosphorus from fields to streams. Pas- T Tture management practices that protect this bal- Mineral Stable Humusance include: particles 70-90%• Minimizing the potential for compaction Soil Organic Soil Microbial while providing organic inputs to enhance Soil Matter Biomass activities of soil organisms and phosphorus mineralization Soil contains 1-6% organic matter. Organic matter• Incorporating manure and phosphorus fer- contains 3-9% active microorganisms. These organisms tilizers into the soil and never applying these include plant life, bacteria and actinomycetes, fungi, yeasts, materials to saturated, snow-covered, or fro- algae, protozoa, and nematodes. zen soils Humus, along with fungal threads, bacterial• Relying on soil tests, phosphorus index gels, and earthworm feces, forms glues that hold guidelines, and other nutrient management soil particles together in aggregates. These con- practices when applying fertilizers and ma- stitute soil structure, enhance soil porosity, and nure to pastures allow water, air, and nutrients to flow through SOIL LIFE the soil. These residues of soil organisms also en- hance the soils nutrient and water holding ca- Soil is a matrix of pore spaces filled with wa- pacity. Lichens, algae, fungi, and bacteria formter and air, minerals, and organic matter. Al- biological crusts over the soil surface. Thesethough comprising only 1 to 6% of the soil, liv- crusts are important, especially in arid range-ing and decomposed organisms are certainly of lands, for enhancing water infiltration and pro-the essence. They provide plant nutrients, cre- viding nitrogen fixation (2). Maintaining a sub-ate soil structure, hold water, and mediate nutri- stantial population of legumes in the pasture alsoent transformations. Soil organic matter is com- ensures biological nitrogen fixation by bacteriaposed of three components: stable humus, associated with legume roots.readily decomposable materials, and living or- Effective nutrient cycling in the soil is highlyganisms — also described as the very dead, the dependent on an active and diverse communitydead, and the living components of soil (1). of soil organisms. Management practices that Living organisms in soil include larger fauna maintain the pasture soil as a habitat favorablesuch as moles and prairie dogs, macroorganisms for soil organisms include:such as insects and earthworms, and microor- • Maintaining a diversity of forages, whichganisms including fungi, bacteria, yeasts, algae, promotes a diverse population of soil organ-protozoa, and nematodes. These living organ- isms by providing them with a varied dietisms break down the readily decomposable plant • Adding organic matter, such as forage resi-and animal material into nutrients, which are dues and manure, to the soil to provide foodthen available for plant uptake. Organic matter for soil organisms and facilitate the forma-residues from this decomposition process are tion of aggregatessubsequently broken down by other organisms • Preventing soil compaction and soil satura-until all that remains are complex compounds tion, and avoiding the addition of amend-resistant to decomposition. These complex end ments that might kill certain populations ofproducts of decomposition are known as humus. soil organismsPAGE 4 //NUTRIENT CYCLING IN PASTURES
  • 5. Publication OverviewThis publication is divided into five chapters:1. Nutrient cycle components, interactions, and transformations2. Nutrient availability in pastures3. Nutrient distribution and movement in pastures4. The soil food web and pasture soil quality5. Pasture management and water quality The first chapter provides an overview of nutrient cycles critical to plant production and water-quality protection: the water, carbon, nitrogen, phosphorus, and secondary-nutrient cycles. Thecomponents of each cycle are explained, with emphasis on how these components are affected bypasture management practices. The description of each cycle concludes with a summary of pasturemanagement practices to enhance efficient cycling of that nutrient. The second chapter focuses on the effects of soil chemistry, mineralogy, and land-managementpractices on nutrient cycle transformations and nutrient availability. Management impacts discussedinclude soil compaction, organic matter additions and losses, effects on soil pH, and consequences ofthe method and timing of nutrient additions. The chapter concludes with a summary of pasturemanagement practices for enhancing nutrient availability in pastures. The third chapter discusses nutrient balances in grazed pastures and the availability of manure,residue, and fertilizer nutrients to forage growth. Factors affecting nutrient availability include nutri-ent content and consistency of manure; manure distribution as affected by paddock location andlayout; and forage diversity. These factors, in turn, affect grazing intensity and pasture regrowth. Agraph at the end of the chapter illustrates the interactions among these factors. The fourth chapter describes the diversity of organisms involved in decomposing plant residuesand manure in pastures, and discusses the impact of soil biological activity on nutrient cycles andforage production. The impacts of pasture management on the activity of soil organisms are ex-plained. A soil health card developed for pastures provides a tool for qualitatively assessing thesoil’s ability to support healthy populations of soil organisms. The publication concludes with a discussion of pasture management practices and their effects onwater quality, soil erosion, water runoff, and water infiltration. Several topical water concerns arediscussed: phosphorus runoff and eutrophication, nutrient and pathogen transport through subsur-face drains, buffer management, and riparian grazing practices. A guide for assessing potential wa-ter-quality impacts from pasture-management practices concludes this final chapter. //NUTRIENT CYCLING IN PASTURES PAGE 5
  • 6. Chapter 1 Nutrient Cycle Components, Interactions, and Transformations The water, carbon, nitrogen, phosphorus, and sulfur cycles are the most important nutrient cyclesoperating in pasture systems. Each cycle has its complex set of interactions and transformations aswell as interactions with the other cycles. The water cycle is essential for photosynthesis and thetransport of nutrients to plant roots and through plant stems. It also facilitates nutrient loss throughleaching, runoff, and erosion. The carbon cycle forms the basis for cell formation and soil quality. Itbegins with photosynthesis and includes respiration, mineralization, immobilization, and humusformation. Atmospheric nitrogen is fixed into plant-available nitrate by one type of bacteria, con-verted from ammonia to nitrate by another set of bacteria, and released back to the atmosphere by yetanother group. A variety of soil organisms are involved in decomposition processes that release ormineralize nitrogen, phosphorus, sulfur, and other nutrients from plant residues and manure. Bal-ances in the amount of these nutrients within organic materials, along with temperature and mois-ture conditions, determine which organisms are involved in the decomposition process and the rateat which it proceeds. Pasture management practices influence the interactions and transformations occurring withinnutrient cycles. The efficiency of these cycles, in turn, influences the productivity of forage growthand the productivity of animals feeding on the forage. This chapter examines each of these cycles indetail and provides management guidelines for enhancing their efficiency. WATER CYCLE Water is critical for pasture productivity. It small, these soils can easily become compacted.dissolves soil nutrients and moves them to plant Sandy soils are porous and allow water to enterroots. Inside plants, water and the dissolved nu- easily, but do not hold water and nutrientstrients support cell growth and photosynthesis. against leaching. Organic matter in soil absorbsIn the soil, water supports the growth and re- water and nutrients, reduces soil compaction,production of insects and microorganisms that and increases soil porosity. A relatively smalldecompose organic matter. Water also can de- increase in the amount of organic matter in soilgrade pastures through runoff, erosion, and can cause a large increase in the ability of soils toleaching, which cause nutrient loss and water use water effectively to support plant produc-pollution. Pro- tion.ductive pastures A forage cover over INFILTRATION ANDare able to absorb the entire paddock pro- WATER HOLDING CAPACITYand use water ef- motes water infiltration Water soaks into soils that have a plant orfectively for plant and minimizes soil residue cover over the soil surface. This covergrowth. Good compaction. cushions the fall of raindrops and allows thempasture manage- to slowly soak into the soil. Roots create poresment practices promote water absorption by that increase the rate at which water can entermaintaining forage cover over the entire soil sur- the soil. Long-lived perennial bunch-grass formsface and by minimizing soil compaction by ani- deep roots that facilitate water infiltration by con-mals or equipment. ducting water into the soil (3). Other plant char- Geology, soil type, and landscape orientation acteristics that enhance water infiltration are sig-affect water absorption by soils and water move- nificant litter production and large basal cover-ment through soils. Sloping land encourages age (4). In northern climates where snow pro-water runoff and erosion; depressions and vides a substantial portion of the annual waterfootslopes are often wet since water from upslope budget, maintaining taller grasses and shrubscollects in these areas. Clay soils absorb water that can trap and hold snow will enhance waterand nutrients, but since clay particles are very infiltration.PAGE 6 //NUTRIENT CYCLING IN PASTURES
  • 7. .igure 3. Water Cycle. Rainfall T Water vapor T T Evaporation Transpiration The Water Cycle. Rain falling on soil can either be absorbed into the soil or be lost as it flows over the soil surface. Absorbed rain is used for the growth of plants and T Dissolved nutrients soil organisms, to transport nutrients to plant roots, or to T Photosynthesis T Crop harvest T TRunoff & recharge ground water. It can also leach nutrients through the soil profile, out of the reach of plant roots. Water flowing TPlant uptake T Infiltration T erosion off the soil surface can transport dissolved nutrients as T T Dissolved T to runoff, or nutrients and other contaminants associated with T Soil Leached T nutrients sediments as erosion. Uptake nutrients Soil by wells water T T Subsurface flow T T Groundwater flows Soils with a high water holding capacity ab- layer or high water table. Soils prone to satura-sorb large amounts of water, minimizing the po- tion are usually located at the base of slopes, neartential for runoff and erosion and storing water waterways, or next to seeps.for use during droughts. Soils are able to absorb Impact on crop production. Soil saturationand hold water when they have a thick soil pro- affects plant production by exacerbating soil com-file; contain a relatively high percentage of or- paction, limiting air movement to roots, andganic matter; and do not have a rocky or com- ponding water and soil-borne disease organismspacted soil layer, such as a hardpan or plowpan, around plant roots and stems. When soil poresclose to the soil surface. An active population of are filled with water, roots and beneficial soil or-soil organisms enhances the formation of aggre- ganisms lose access to air, which is necessary forgates and of burrowing channels that provide their healthy growth. Soil compaction decreasespathways for water to flow into and through the the ability of air, water, nutrients, and roots tosoil. Management practices that enhance water move through soils even after soils have dried.infiltration and water holding capacity include: Plants suffering from lack of air and nutrients• a complete coverage of forages and residues are susceptible to disease attack since they are over the soil surface under stress, and wet conditions help disease or-• an accumulation of organic matter in and on ganisms move from contaminated soil particles the soil and plant residues to formerly healthy plant roots• an active community of soil organisms in- and stems. volved in organic matter decomposition and Runoff and erosion potential. Soil satura- aggregate formation tion enhances the potential for runoff and ero-• water runoff and soil erosion prevention sion by preventing entry of additional water into• protection against soil compaction the soil profile. Instead, excess water will run SOIL SATURATION off the soil surface, often carrying soil and nutri-Soils become saturated when the amount of wa- ents with it. Water can also flow horizontallyter entering exceeds the rate of absorption or under the surface of the soil until it reaches thedrainage. A rocky or compacted lower soil layer banks of streams or lakes. This subsurface wa-will not allow water to drain or pass through, ter flow carries nutrients away from roots, wherewhile a high water table prevents water from they could be used for plant growth, and intodraining through the profile. Water soaking into streams or lakes where they promote the growththese soils is trapped or perched above the hard of algae and eutrophication. //NUTRIENT CYCLING IN PASTURES PAGE 7
  • 8. Artificial drainage practices are often used tures is remediated by root growth, aggregateon soils with a hardpan or a high water table to formation, and activities of burrowing soil organ-decrease the duration of soil saturation follow- isms. In colder climates, frost heaving is an im-ing rainfalls or snowmelts. This practice can in- portant recovery process for compacted soils (8).crease water infiltration and decrease the poten- RUNOFF AND EROSIONtial for water runoff (5). Unfortunately, most sub-surface drains were installed before water pollu- Runoff water dissolves nutrients and removestion from agriculture became a concern and thus them from the pasture as it flows over the soilempty directly into drainage ways. Nutrients, surface. Soil erosion transports nutrients and anypathogens, and other contaminants on the soil contaminants, such as pesticides and pathogens,surface can move through large cracks or chan- attached to soil particles. Because nutrient-richnels in the soil to drainage pipes where they are clay and organic matter particles are small andcarried to surface water bodies (6). lightweight, they are more readily picked up and moved by water than the nutrient-poor, but SOIL COMPACTION heavier, sand particles. Besides depleting pas- Soil compaction occurs when animals or tures of nutrients that could be used for forageequipment move across soils that are wet or satu- production, runoff water and erosion carry nu-rated, with moist soils being more easily com- trients and sediments that contaminate lakes,pacted than saturated soils (7). Compaction can streams, and rivers.also occur when animals or equipment continu- Landscape condi- Runoff water and ero-ally move across a laneway or stand around wa- tions and manage- sion carry nutrientstering tanks and headlands or under shade. Ani- ment practices that and sediments thatmals trampling over the ground press down on favor runoff and ero- contaminate lakes,soils, squeezing soil pore spaces together. Tram- sion include sloping streams, and rivers.pling also increases the potential for compaction areas, minimal soilby disturbing and killing vegetation. protection by forage or residues, intense rainfall, Soils not covered by forages or residues are and saturated soils. While pasture managerseasily compacted by the impact of raindrops. should strive to maintain a complete forage coverWhen raindrops fall on bare soil, their force causes over the soil surface, this is not feasible in prac-fine soil particles to splash or disperse. These tice because of plant growth habits and land-splash particles land on the soil surface, clog sur- scape characteristics. Plant residues from die- face soil pores, back and animal wastage during grazing provide Animals trampling over and form a crust a critical source of soil cover and organic matter. the ground press down over the soil. As mentioned above, forage type affects water on soils, squeezing soil Clayey soils are infiltration and runoff. Forages with deep roots pores together, which more easily com- enhance water infiltration while plants with a wide limits root growth and pacted than sandy vegetative coverage area or prostrate growth pro- the movement of air, soils because clay vide good protection against raindrop impact. water, and dissolved particles are very Sod grasses that are short-lived and shallow- nutrients. small and sticky. rooted inhibit water infiltration and encourage Compaction runoff. Grazing practices that produce clumps limits root growth of forages separated by bare ground enhance run-and the movement of air, water, and dissolved off potential by producing pathways for waternutrients through the soil. Compressing and flow.clogging soil surface pores also decreases water EVAPORATION AND TRANSPIRATIONinfiltration and increases the potential for runoff. Water in the soil profile can be lost throughThe formation of hardpans, plowpans, traffic evaporation, which is favored by high tempera-pans, or other compacted layers decreases down- tures and bare soils. Pasture soils with a thickward movement of water through the soil, caus- cover of grass or other vegetation lose little wa-ing rapid soil saturation and the inability of soils ter to evaporation since the soil is shaded andto absorb additional water. Compaction in pas- soil temperatures are decreased. While evapo-PAGE 8 //NUTRIENT CYCLING IN PASTURES
  • 9. ration affects only the top few inches of pasture This is because forage species differ in their abil-soils, transpiration can drain water from the en- ity to extract water from the soil and conserve ittire soil profile. Transpiration is the loss of wa- against transpiration (9). Some invasive plantter from plants through stomata in their leaves. species, however, can deplete water storesEspecially on sunny and breezy days, significant through their high water use (4). Water not usedamounts of water can be absorbed from the soil for immediate plant uptake is held within the soilby plant roots, taken up through the plant, and profile or is transported to groundwater reserves,lost to the atmosphere through transpiration. A which supply wells with water and decrease thediversity of forage plants will decrease transpi- impacts of drought.ration losses and increase water-use efficiency. Table 1. Water Cycle Monitoring. If you answer no to all the questions, you have soils with high water-use efficiency. If you answer yes to some of the questions, water cycle efficiency of your soil will likely respond to improved pasture management practices. See Table 2, next page. YES NO Water infiltration / Water runoff 1. Do patches of bare ground separate forage coverage?________________________________________________________________________________ 2. Are shallow-rooted sod grasses the predominant forage cover?________________________________________________________________________________ 3. Can you see small waterways during heavy rainfalls or sudden snowmelts?________________________________________________________________________________ 4. Are rivulets and gullies present on the land?________________________________________________________________________________ Soil saturation 5. Following a rainfall, is soil muddy or are you able to squeeze water out of a handful of soil?________________________________________________________________________________ 6. Following a rainfall or snowmelt, does it take several days before the soil is no longer wet and muddy?________________________________________________________________________________ 7. Do forages turn yellow or die during wet weather?________________________________________________________________________________ Soil compaction 8. Do you graze animals on wet pastures?________________________________________________________________________________ 9. Are some soils in the pasture bare, hard, and crusty?________________________________________________________________________________ 10. Do you have difficulty driving a post into (non-rocky) soils?________________________________________________________________________________ Water retention /Water evaporation and transpiration 11. Do you have a monoculture of forages or are invasive species prominent components of the pasture?________________________________________________________________________________ 12. Do soils dry out quickly following a rainstorm?________________________________________________________________________________ 13. During a drought, do plants dry up quickly? //NUTRIENT CYCLING IN PASTURES PAGE 9
  • 10. Table 2. Pasture Management Practices for Efficient Water Cycling in Pastures. Ensure forage and residue coverage across the entire pasture • Use practices that encourage animal movement throughout the pasture and discourage con- gregation in feeding and lounging areas • Use practices that encourage regrowth of forage plants and discourage overgrazing • Use a variety of forages with a diversity of root systems and growth characteristics Pasture management during wet weather • Use well-drained pastures or a “sacrificial pasture” that is far from waterways or water bodies • Avoid driving machinery on pastures that are wet or saturated • Avoid spreading manure or applying fertilizers Artificial drainage practices • Avoid grazing animals on artificially drained fields when drains are flowing • Avoid spreading manure or applying fertilizers when drains are flowing • Ensure that drains empty into a filter area or wetland rather than directly into a stream or drainage way CARBON CYCLE Effective carbon cycling in pastures depends on a diversity of plants and healthy populations ofsoil organisms. Plants form carbon and water into carbohydrates through photosynthesis. Plants aremost able to conduct photosynthesis when they can efficiently capture solar energy while also havingadequate access to water, nutrients, and air. Animals obtain carbohydrates formed by plants whenthey graze on pastures or eat hay or grains harvested from fields. Some of the carbon and energy inplant carbohydrates is incorporated into animal cells. Some of the carbon is lost to the atmosphere ascarbon dioxide, and some energy is lost as heat, during digestion and as the animal grows and breathes. Carbohydrates and other nutrients not used by animals are re- turned to the soil in the form of urine and manure. These organic Humus maintains soil materials provide soil organisms with nutrients and energy. As soil tilth and enhances water organisms use and decompose organic materials, they release nutri- and nutrient absorption. ents from these materials into the soil. Plants then use the released, inorganic forms of nutrients for their growth and reproduction. Soil organisms also use nutrients from organic materials to produce sub-stances that bind soil particles into aggregates. Residues of organic matter that resist further decom-position by soil organisms form soil humus. This stable organic material is critical for maintaining soiltilth and enhancing the ability of soils to absorb and hold water and nutrients. CARBOHYDRATE FORMATION ergy by a diversity of leaf shapes and leaf angles. For productive growth, plants need to effec- Taller plants with more erect leaves capture lighttively capture solar energy, absorb carbon diox- even at the extreme angles of sunrise and sunset.ide, and take up water from the soil to produce Horizontal leaves capture the sun at midday orcarbohydrates through photosynthesis. In pas- when it is more overhead.tures, a combination of broadleaf plants and Two methods for transforming carbon intograsses allows for efficient capture of solar en- carbohydrates are represented in diversified pas-PAGE 10 //NUTRIENT CYCLING IN PASTURES
  • 11. .igure 4. Carbon Cycle. T CO2 in atmosphere T T Consumption The Carbon Cycle begins with plants taking up carbon dioxide from the atmosphere in the process of photosynthesis. T Respiration T Photosynthesis Some plants are eaten by grazing animals, which return T T organic carbon to the soil as manure, and carbon dioxide to the atmosphere. Easily broken-down forms of carbon in Crop and Mineralization animal manure and plant cells are released as carbon dioxide when residues Loss via T decomposing soil organisms respire. Forms of carbon that Carbon in soil T erosion are difficult to break down become stabilized in the soil as humus. organic matter T T Humus and T aggregate T T Decomposition formulation T T in microbestures. Broadleaf plants and cool-season grasses they break down complex carbohydrates and pro-have a photosynthetic pathway that is efficient teins into simpler chemical forms. For example,in the production of carbohydrates but is sensi- soil organisms break down proteins into carbontive to dry conditions. Warm-season grasses dioxide, water, ammonium, phosphate, and sul-have a pathway that is more effective in produc- fate. Plants require nutrients to be in this sim-ing carbohydrates during hot summer condi- pler, decomposed form before they can use themtions. A combination of plants representing these for their growth.two pathways ensures effective forage growth To effectively decompose organic matter, soilthroughout the growing season. A diversity of organisms require access to air, water, and nu-root structures also promotes photosynthesis by trients. Soil compaction and saturation limit thegiving plants access to water and nutrients growth of beneficial organisms and promote thethroughout the soil profile. growth of anaerobic organisms, which are inef- ficient in the decomposition of organic matter. ORGANIC MATTER DECOMPOSITION These organisms also transform some nutrients Pasture soils gain organic matter from growth into forms that are less available or unavailableand die-back of pasture plants, from forage wast- to plants. Nutrient availability and nutrient bal-age during grazing, and from manure deposition. ances in the soil solution also affect the growthIn addition to the recycling of aboveground plant and diversity of soil organisms. To decomposeparts, every year 20 to 50% of plant root mass organic matter that contains a high amount ofdies and is returned to the soil system. Some carbon and insufficient amounts of other nutri-pasture management practices also involve the ents, soil organisms must mix soil- solution nu-regular addition of manure trients with this material tofrom grazing animals housed achieve a balanced diet. A healthy and diverse popu-during the winter or from poul- Balances between the lation of soil organisms istry, hog, or other associated amount of carbon and nitro- necessary for organic matterlivestock facilities. gen (C:N ratio) and the decomposition, nutrient min- A healthy and diverse amount of carbon and sulfur eralization, and the formationpopulation of soil organisms is (C:S ratio) determine of soil aggregates.necessary for organic matter whether soil organisms willdecomposition, nutrient miner- release or immobilize nutri-alization, and the formation of soil aggregates. ents when they decompose organic matter. Im-Species representing almost every type of soil mobilization refers to soil microorganisms takingorganism have roles in the breakdown of manure, nutrients from the soil solution to use in the de-plant residues, and dead organisms. As they use composition process of nutrient-poor materials.these substances for food and energy sources, Since these nutrients are within the bodies of soil //NUTRIENT CYCLING IN PASTURES PAGE 11
  • 12. organisms, they are temporarily unavailable to aggregated, soil pore size increases and soils be-plants. In soils with low nutrient content, this come resistant to compaction. The organic com-can significantly inhibit plant growth. However, pounds that hold aggregates together also in-immobilization can be beneficial in soils with ex- crease the ability of soils to absorb and hold wa-cess nutrients. This process conserves nutrients ter and bodies of soil organisms, where they are less As soil organisms decompose manure andlikely to be lost through leaching and runoff (10). plant residues, they release carbon dioxide and Populations of soil organisms are enhanced produce waste materials, which are further de-by soil that is not com- composed by other soil organisms. Be-pacted and has adequate In soils with low nutrient con- cause carbon is lost to respiration atair and moisture, and tent, nutrient immobilization each stage of this decomposition pro-by additions of fresh inhibits plant growth. In soils cess, the remaining material increasesresidues they can with excess nutrients, immo- in relative nitrogen content. The re-readily decompose. bilization conserves nutrients maining material also increases inSoil-applied pesticides in the bodies of soil organ- chemical complexity and requires in-can kill many beneficial isms, where they are less creasingly specialized species of de-soil organisms, as will likely to be lost through leach- composers. Efficient decompositionsome chemical fertiliz- ing and runoff. of organic matter thus requires a di-ers. Anhydrous ammo- versity of soil organisms. Humus isnia and fertilizers with the final, stable product of decompo-a high chloride content, such as potash, are par- sition, formed when organic matter can be bro-ticularly detrimental to soil organisms. Moder- ken down by soil organisms only slowly or withate organic or synthetic fertilizer additions, how- difficulty. Humus-coated soil particles form ag-ever, enhance populations of soil organisms in gregates that are soft, crumbly, and somewhatsoils with low fertility. greasy-feeling when rubbed together. SOIL HUMUS AND SOIL AGGREGATES PREVENTING ORGANIC MATTER LOSSES Besides decomposing organic materials, bac- Perennial plant cover in pastures not onlyteria and fungi in the soil form gels and threads provides organic matter inputs, it also protectsthat bind soil particles together. These bound against losses of organic matter through erosion.particles are called soil aggregates. Worms, Soil coverage by forages and residues protectsbeetles, ants, and other soil organisms move par- the soil from raindrop impact while dense roottially decomposed organic matter through the soil systems of forages hold the soil against erosionor mix it with soil in their gut, coating soil par- while enhancing water infiltration. Fine root hairsticles with organic gels. As soil particles become also promote soil aggregation. In addition, a Table 3. Typical C:N, C:S, and N:S Ratios. Typical C:N, C:S, and N:S ratios of plant residues, excreta of ruminant animals, and biomass of soil microorganisms decomposing in grassland soils (based on values for % in dry matter) %N C:N %S C:S N:S ______________________________________________________________ Dead grass 1.8 26.6:1 0.15 320:1 12:1 ______________________________________________________________ Dead clover 2.7 17.7:1 0.18 270:1 15:1 ______________________________________________________________ Grass roots 1.4 35:1 0.15 330:1 9:1 ______________________________________________________________ Clover roots 3.8 13.2:1 0.35 140:1 10:1 ______________________________________________________________ Cattle feces 2.4 20:1 0.30 160:1 8:1 ______________________________________________________________ Cattle urine 11.0 3.9:1 0.65 66:1 17:1 ______________________________________________________________ Bacteria 15.0 3.3:1 1.1 45:1 14:1 ______________________________________________________________ Fungi 3.4 12.9:1 0.4 110:1 8.5:1 From Whitehead, 2000 (reference #11)PAGE 12 //NUTRIENT CYCLING IN PASTURES
  • 13. dense forage cover shades and cools the soil. High temperatures promote mineralization and loss oforganic matter, while cooler temperatures promote the continued storage of this material within the plantresidues and the bodies of soil organisms. Table 4. Pasture Management Practices for Efficient Carbon Cycling. Ensure forage and residue coverage and manure deposition across the entire pasture • Use practices that encourage animal movement throughout the pasture and discourage con- gregation in feeding and lounging areas Promote healthy forage growth and recovery following grazing • Use a variety of forages with a diversity of leaf types and orientations • Use a combination of cool- and warm-season forages with a diversity of shoot and root growth characteristics • Conserve sufficient forage leaf area for efficient plant regrowth by monitoring pastures and moving grazing animals to another pasture in a timely manner • Maintain soil tilth for healthy root growth and nutrient uptake Encourage organic matter decomposition by soil organisms • Use management practices that minimize soil compaction and soil erosion • Minimize tillage and other cultivation practices • Maintain a diversity of forage species to provide a variety of food sources and habitats for a diversity of soil organisms • Avoid the use of soil-applied pesticides and concentrated fertilizers that may kill or inhibit the growth of soil organisms Encourage soil humus and aggregate formation • Include forages with fine, branching root systems to promote aggregate formation • Maintain organic matter inputs into the soil to encourage the growth of soil organisms • Maintain coverage of forages and plant residues over the entire paddock to provide organic matter and discourage its rapid degradation NITROGEN CYCLE Nitrogen is a primary plant nutrient and a return of manure to the land, and through themajor component of the atmosphere. In a pas- mineralization of organic matter in the soil. Ni-ture ecosystem, almost all nitrogen is organically trogen fixation occurs mainly in the roots of le-bound. Of this, only about 3% exists as part of a gumes that form a symbiotic association with aliving plant, animal, or microbe, while the re- type of bacteria called rhizobia. Some free-livingmainder is a component of decomposed organic bacteria, particularly cyanobacteria (“blue-greenmatter or humus. A very small percentage of algae”), are also able to transform atmosphericthe total nitrogen (less than 0.01%) exists as plant- nitrogen into a form available for plant growth.available nitrogen in the form of ammonium or Fertilizer factories use a combination of highnitrate (12). pressure and high heat to combine atmospheric Nitrogen becomes available for the growth nitrogen and hydrogen into nitrogen fertilizers.of crop plants and soil organisms through nitro- Animals deposit organically-bound nitrogen ingen fixation, nitrogen fertilizer applications, the feces and urine. Well-managed pastures accu- //NUTRIENT CYCLING IN PASTURES PAGE 13
  • 14. .igure 5. Nitrogen Cycle. Nitrogen gas (N2) (78% of atmosphere) T atmospheric fixation or fertilizer production The Nitrogen Cycle. Nitrogen enters the cycle when T T atmospheric nitrogen is fixed by bacteria. Nitrogen in the T NH4+ nitrogen crop harvest ammonical form is transformed into nitrite and nitrate by fixation T + NO3- T volatilization bacteria. Plants can use either ammonia or nitrate for legumes T growth. Nitrogen in plant cells can be consumed by animals NH4++ OH- T and returned to the soil as feces or urine. When plants die, T NH3 + H2O T T crop and soil organisms decompose nitrogen in plant cells and release T free-living bacteria animal residues T it as ammonia. Nitrate nitrogen can be lost through the T T in soil plant uptake physical process of leaching or through the microbially- T T T nitrogen organic matter erosion mediated process of denitrification. Nitrogen in the T ammonical form can be lost to the atmosphere in the T ammonium T denitrification chemical process of volatilization. TN NH4+ immobilization NO3- + N2O T 2 T nitrate NO3- leaching to groundwater Tmulate stores of organic matter in the soil and in NITROGEN FIXATIONplant residues. Decomposition and mineraliza- Plants in the legume family, including alfalfa,tion of nutrients in these materials can provide clover, lupines, lespedeza, and soybeans, form asignificant amounts of nitrogen to plants and relationship with a specialized group of bacteriaother organisms in the pasture system. called rhizobia. These bacteria have the ability to Plants use nitrogen for the formation of pro- fix or transform atmospheric nitrogen into a formteins and genetic material. Grazing animals thatconsume these plants use some of the nitrogen of nitrogen plants can use for their growth.for their own growth and reproduction; the re- Rhizobia form little balls or nodules on the rootsmainder is returned to the earth as urine or ma- of legumes. If these balls are white or pinkishnure. Soil organisms decompose manure, plant on the inside, they are actively fixing nitrogen.residues, dead animals, and microorganisms, Nodules that are grey or black inside are dead ortransforming nitrogen-containing compounds in no longer active. Legume seeds should be dustedtheir bodies into forms that are available for use with inoculum (a liquid or powder containingby plants. the appropriate type of rhizobia) prior to plant- Nitrogen is often lacking in pasture systems ing to ensure that the plant develops many nod-since forage requirements for this nutrient are ules and has maximal ability to fix nitrogen.high and because it is easily lost to the environ- Other microorganisms that live in the soil are alsoment. Nitrogen is lost from pasture systems able to fix and provide nitrogen to plants.through microbiological, chemical, and physicalprocesses. Dry followed by wet weather pro-vides optimal conditions for bacteria to transform Table 5. Nitrogen .ixation by Legumes.nitrogen from plant-available forms into atmo- #N/acre/ yearspheric nitrogen through denitrification. Chemi- Alfalfa -------------------------------------- 150-350cal processes also transform plant-available ni- White clover ------------------------------ 112-190trogen into atmospheric nitrogen through vola- Hairy vetch ------------------------------- 110-168tilization. In pastures, this often occurs after ma- Red clover --------------------------------- 60-200nure or nitrogen fertilizers are applied to the soilsurface, especially during warm weather. Physi- Soybeans ---------------------------------- 35-150cal processes are involved in the downward Annual lespedeza ----------------------- 50-193movement of nitrogen through the soil profile Birdsfoot trefoil --------------------------- 30-130during leaching. From Joost, 1996 (Reference # 13)PAGE 14 //NUTRIENT CYCLING IN PASTURES
  • 15. Legumes require higher amounts of phospho- sults in the formation of ammonia. Protozoa,rus, sulfur, boron, and molybdenum than non- amoebas, and nematodes are prolific nitrogenlegumes to form nodules and fix nitrogen. If these mineralizers, cycling 14 times their biomass eachnutrients are not available in sufficient amounts, year. While bacteria only cycle 0.6 times theirnitrogen fixation will be suppressed. When ni- biomass, because of their large numbers in soiltrogen levels in the soil are high due to applica- they produce a greater overall contribution to thetions of manure or nitrogen fertilizers, nitrogen pool of mineralized nitrogen (17). Plants can usefixation by legumes decreases because nitrogen ammonical nitrogen for their growth, but underfixation requires more energy than does root up- aerobic conditions two types of bacteria usuallytake of soluble soil nitrogen. Nitrogen fixed by work together to rapidly transform ammonia firstlegumes and rhizobia is available primarily to the into nitrite and then into nitrate before it is usedlegumes while they are growing. When pasture by plants.legume nodules, root hairs, and aboveground Mineralization is a very important source ofplant material dies and decomposes, nitrogen in nitrogen in most grasslands. As discussed above,this material can become available to pasture for efficient decomposition (and release of nitro-grasses (14). gen), residues must contain a carbon-to-nitrogen However, while legumes are still growing, ratio that is in balance with the nutrient needs ofmycorrhizal fungi can form a bridge between the the decomposer organisms. If the nitrogen con-root hairs of legumes and nearby grasses. This tent of residues is insufficient, soil organisms willbridge facilitates the transport of fixed nitrogen extract nitrogen from the soil solution to satisfyfrom legumes to linked grasses. Depending on their nutrient needs. the nitrogen con- NITROGEN LOSSES TO THE ATMOSPHERE tent of the soil and Under wet or anaerobic conditions, bacteria Legumes can transfer the mix of legumes transform nitrate nitrogen into atmospheric ni- up to 40% of their fixed and grasses in a trogen. This process, called denitrification, re- nitrogen to grasses pasture, legumes duces the availability of nitrogen for plant use. during the growing can transfer be- Denitrification occurs when dry soil containing season. tween 20 and 40% nitrate becomes wet or flooded and at the edges of their fixed nitro- of streams or wetlands where dry soils are adja-gen to grasses during the growing season (15). cent to wet soils.A pasture composed of at least 20 to 45% legumes Volatilization is the transformation of ammo-(dry matter basis) can meet and sustain the ni- nia into atmospheric nitrogen. This chemicaltrogen needs of the other forage plants in the pas- process occurs when temperatures are high andture (16). ammonia is exposed to the air. Incorporation of Grazing management affects nitrogen fixation manure or ammonical fertilizer into the soil de-through the removal of herbage, deposition of creases the potential for volatilization. In gen-urine and manure, and induced changes in mois- eral, 5 to 25% of the nitrogen in urine is volatil-ture and temperature conditions in the soil. Re- ized from pastures (11). A thick forage cover andmoval of legume leaf area decreases nitrogen fixa- rapid manure decomposition can reduce volatil-tion by decreasing photosynthesis and plant com- ization from manure.petitiveness with grasses. Urine deposition de- NITROGEN LEACHINGcreases nitrogen fixation by adjacent plants sinceit creates an area of high soluble-nitrogen avail- Soil particles and humus are unable to holdability. Increased moisture in compacted soils nitrate nitrogen very tightly. Water from rainfallor increased temperature in bare soils will also or snowmelt readily leaches soil nitrate down-decrease nitrogen fixation since rhizobia are sen- ward through the profile, putting it out of reachsitive to wet and hot conditions. of plant roots or moving it into the groundwater. Leaching losses are greatest when the water table NITROGEN MINERALIZATION is high, the soil sandy or porous, or when rainfall Decomposition of manure, plant residues, or or snowmelt is severe. In pastures, probably thesoil organic matter by organisms in the soil re- most important source of nitrate leaching is from //NUTRIENT CYCLING IN PASTURES PAGE 15
  • 16. urine patches (18). Cattle urine typically leaches to Nitrate levels in excess of 10 ppm in drinkinga depth of 16 inches, while sheep urine leaches only water can cause health problems for human infants,six inches into the ground (19). Leaching may also infant chickens and pigs, and both infant and adultbe associated with the death of legume nodules sheep, cattle, and horses (21). Pasture forages canduring dry conditions (20). also accumulate nitrate levels high enough to cause Methods for reducing nitrate leaching include health problems. Conditions conducive for nitratemaintaining an actively growing plant cover over accumulation by plants include acid soils; low mo-the soil surface, coordinating nitrogen applications lybdenum, sulfur, and phosphorus content; soilwith the period of early plant growth, not applying temperatures lower than 550 F; and good soil aera-excess nitrogen to soils, and encouraging animal tion (22).movement and distribution of manure across pad- Nitrate poisoning is called methemoglobinemia,docks. Actively growing plant roots take up ni- commonly known as “blue baby syndrome” whentrate from the soil and prevent it from leaching. If seen in human infants. In this syndrome, nitratethe amount of nitrogen applied to the soil is in ex- binds to hemoglobin in the blood, reducing thecess of what plants need or is applied when plants blood’s ability to carry oxygen through the body.are not actively growing, nitrate not held by plants Symptoms in human infants and young animalscan leach through the soil. Spring additions of ni- include difficulty breathing. Pregnant animals thattrogen to well-managed pastures can cause exces- recover may abort within a few days. Personnelsive plant growth and increase the potential for from the Department of Health can test wells toleaching. This is because significant amounts of determine whether nitrate levels are dangerouslynitrogen are also being mineralized from soil or- high.ganic matter as warmer temperatures increase theactivity of soil organisms. Table 6. Estimated Nitrogen Balance (pounds/acre) for Two Grassland Management Systems. Moderately managed Extensively grazed grass-clover grass Inputs Nitrogen fixation 134 9 Atmospheric deposition 34 19 Fertilizer 0 0 Supplemental feed 0 0 Recycled nutrients Uptake by herbage 270 67 Herbage consumption by animals 180 34 Dead herbage to soil 90 34 Dead roots to soil 56 34 Manure to soil 134 28 Outputs Animal weight gain 28 4 Leaching/runoff/erosion 56 6 Volatilization 17 3 Denitrification 22 2 Gain to soil 56 13 From Whitehead, 2000 (Reference # 11)PAGE 16 //NUTRIENT CYCLING IN PASTURES
  • 17. Table 7. Pasture Management Practices for Efficient Nitrogen Cycling.Ensure effective nitrogen fixation by legumes• Ensure that phosphorus, sulfur, boron, and molybdenum in the soil are sufficient for effective nitrogen fixation• Apply inoculum to legume seeds when sowing new pastures to ensure nodulation of legume roots• Ensure that legumes represent at least 30% of the forage cover• Maintain stable or increasing ratios of legumes to grasses and other non-legumes in pastures over time• Establish forages so that legumes and grasses grow close to one another to allow for the transfer of nitrogen from legumes to grassesEncourage nitrogen mineralization by soil organisms• Use management practices that minimize soil compaction and soil erosion• Minimize tillage and other cultivation practices• Maintain a diversity of forage species to provide a variety of food sources and habitats for a diversity of soil organisms• Use grazing management practices that encourage productive forage growth and that return and maintain residues within paddocks• Avoid application of sawdust, straw, or other high-carbon materials unless these mate- rials are mixed with manure or composted prior to application• Avoid the use of soil-applied pesticides and concentrated fertilizers that may kill or inhibit the growth of soil organismsAvoid nitrogen losses• Minimize nitrogen volatilization by avoiding surface application of manure, especially when the temperature is high or there is minimal forage cover over the soil• Minimize nitrogen leaching by not applying nitrogen fertilizer or manure when soil is wet or just prior to rainstorms and by encouraging animal movement and distribution of urine spots across paddocks• Minimize nitrogen leaching by not applying nitrogen fertilizer or manure to sandy soils except during the growing season• Rely on mineralization of organic residues to supply most or all of your forage nitrogen needs in the spring. Minimize the potential for nitrogen leaching by limiting spring applications of nitrogen• Minimize nitrogen losses caused by erosion by using management practices that main- tain a complete cover of forages and residues over the pasture surfaceEnsure effective use of nitrogen inputs• Use management practices that encourage the even distribution of manure and urine across paddocks• Rely on soil tests and other nutrient management practices when applying fertilizers and manure to pastures //NUTRIENT CYCLING IN PASTURES PAGE 17
  • 18. NITROGEN LOSS THROUGH of forages and plant residues should be main- tained over the soil surface to minimize raindrop RUNOFF AND EROSION impact on the soil, enhance water infiltration, Runoff and erosion caused by rainwater or help trap sediments and manure particles, andsnowmelt can transport nitrogen on the soil sur- reduce the potential for runoff and erosion. Aface. Erosion removes soil particles and organic healthy and diverse population of soil organisms,matter containing nitrogen; runoff transports dis- including earthworms and dung beetles that rap-solved ammonia and nitrate. Incorporation of idly incorporate manure nitrogen into the soilmanure and fertilizers into the soil reduces the and into their cells, can further reduce the risk ofexposure of these nitrogen sources to rainfall or nitrogen runoff from manure. Since increasedsnowmelt, thus reducing the potential for ero- water infiltration decreases the potential for run-sion. In pasture systems, however, incorpora- off but increases the potential for leaching, riskstion is usually impractical and can increase the of nitrate losses from runoff need to be balancedpotential for erosion. Instead, a complete cover against leaching risks. PHOSPHORUS CYCLE Like nitrogen, phosphorus is a primary plant nutrient. Unlike nitrogen, phosphorus is not part ofthe atmosphere. Instead, it is found in rocks, minerals, and organic matter in the soil. The mineralforms of phosphorus include apatitite (which may be in a carbonate, hydroxide, fluoride, or chlorideform) and iron or aluminum phosphates. These minerals are usually associated with basalt and shalerocks. Chemical reactions and microbial activity affect the availability of phosphorus for plant up-take. Under acid conditions, phosphorus is held tightly by aluminum and iron in soil minerals.Under alkaline conditions, phosphorus is held tightly by soil calcium. Plants use phosphorus for energy transfer and reproduction. Legumes require phosphorus foreffective nitrogen fixation. Animals consume phosphorus when they eat forages. Phosphorus notused for animal growth is returned to the soil in manure. Following decomposition by soil organ-isms, phosphorus again becomes available for plant uptake. .igure 6. Phosphorus Cycle. Fertilizer T Consumption Crop harvest T The Phosphorus Cycle is affected by microbial and chemical transformations. Soil organisms mineralize or T T Loss via T release phosphorus from organic matter. Phosphorus is T T Immobilization Crop and erosion chemically bound to iron and aluminum in acid soils, and T animal T T residues Loss via to calcium in alkaline soils. Soil-bound phosphorus can be Plant runoff uptake lost through erosion, while runoff waters can transport T Phosphorus T T T T T soluble phosphorus found at the soil surface. in microbes Phosphorus in soil humus Mineralization T Mineral T T phosphorus Phosphorus Plant T held by clay available minerals T phosphorus TPAGE 18 //NUTRIENT CYCLING IN PASTURES
  • 19. MYCORRHIZAE izer or manure may be readily lost from fields and transported to rivers and streams through Mycorrhizal fungi attach to plant roots and runoff and erosion. The potential for phospho-form thin threads that grow through the soil and rus loss through runoff or erosion is greatestwrap around soil particles. These thin threads when rainfall or snowmelt occurs within a fewincrease the ability of plants to obtain phospho- days following surface applications of manurerus and water from soils. Mycorrhizae are espe- or phosphorus fertilizers.cially important in acid and sandy soils where Continual manure additions increase the po-phosphorus is either chemically bound or has tential for phosphorus loss from the soil and thelimited availability. Besides transferring phos- contamination of lakes and streams. This is es-phorus and water from the soil solution to plant pecially true if off-farm manure sources are usedroots, mycorrhizae also facilitate the transfer of to meet crop or forage nutrient needs for nitro-nitrogen from legumes to grasses. Well aerated gen. The ratio of nitrogen to phosphate in swineand porous soils, and soil organic matter, favor or poultry manure is approximately 1 to 1, whilemycorrhizal growth. the ratio of nitrogen to phosphate taken up by SOIL CHEMISTRY AND forage grasses is between 2.5 to 1 and 3.8 to 1. PHOSPHORUS AVAILABILITY Thus, manure applied for nitrogen requirements will provide 2.5 to 3.8 times the amount of phos- Phosphorus is tightly bound chemically in phorus needed by plants (23). While much ofhighly weathered acid soils that contain high con- this phosphorus will be bound by chemicalcentrations of iron and aluminum. Active cal- bonds in the soil and in the microbial biomass,cium in neutral to alkaline soils also forms tight continual additions will exceed the ability of thebonds with phosphorus. Liming acid soils and soil to store excess phosphorus, and the amountapplying organic matter to either acid or alka- of soluble phosphorus (the form available for lossline soils can increase phosphorus availability. by runoff) will increase. To decrease the poten-In most grasslands, the highest concentration of tial for phosphorus runoff from barnyard manurephosphorus is in the surface soils associated with or poultry litter, alum or aluminum oxide can bedecomposing manure and plant residues. added to bind phosphorus in the manure (24). PHOSPHORUS LOSS THROUGH Supplemental feeds are another source of phosphorus inputs to grazing systems, especially RUNOFF AND EROSION for dairy herds. Feeds high in phosphorus in- Unlike nitrogen, phosphorus is held by soil crease the amount of phosphorus deposited onparticles. It is not subject to leaching unless soil pastures as manure. To prevent build up of ex-levels are excessive. However, phosphorus can cess phosphorus in the soil, minimize feeding ofmove through cracks and channels in the soil to unneeded supplements, conduct regular soil testsartificial drainage systems, which can transport on each paddock, and increase nutrient remov-it to outlets near lakes and streams. Depending als from excessively fertile paddocks throughon the soil type and the amount of phosphorus haying.already in the soil, phosphorus added as fertil- Phosphorus runoff from farming operations can promote unwanted growth of algae in lakes and slow-moving streams. Regulations and nutrient-management guidelines are being de- veloped to decrease the potential for phospho- rus movement from farms and thus reduce risks of lake eutrophication. Land and animal man- agement guidelines, called “phosphorus indi- ces,” are being developed across the U.S. to pro- vide farmers with guidelines for reducing “non- point” phosphorus pollution from farms (25). These guidelines identify risk factors for phos- phorus transport from fields to water bodies based on the concentration of phosphorus in the //NUTRIENT CYCLING IN PASTURES PAGE 19
  • 20. Phosphorus index guidelines consider: streams include location near streams of barn- • the amount of phosphorus in the soil yards or other holding areas without runoff con- • manure and fertilizer application rates, tainment or filtering systems, extensive grazing methods, and timing of animals near streams without riparian buff- • runoff and erosion potential ers, and unlimited animal access to streams. • distance from a water bodysoil, timing and method of fertilizer and manureapplications, potential for runoff and erosion, anddistance of the field from a water body (26). Al-though the total amount of phosphorus lost fromfields is greatest during heavy rainstorms, snow-melts, and other high-runoff events, relativelysmall amounts of phosphorus running off fromfields into streams at low water level in summerpose a higher risk for eutrophication. This is be-cause phosphorus is more concentrated in thesesmaller flows of water (27). Conditions for con-centrated flows of phosphorus into low-flow Table 8. Pasture Management Practices for Efficient Phosphorus Cycling. Encourage phosphorus mineralization by soil organisms • Use management practices that minimize soil compaction and soil erosion • Minimize use of tillage and other cultivation practices • Maintain a diversity of forage species to provide a variety of food sources and habitats for a diversity of soil organisms • Avoid application of sawdust, straw, or other high-carbon materials unless these materi- als are mixed with manure or composted prior to application • Avoid the use of soil-applied pesticides and concentrated fertilizers that may kill or inhibit the growth of soil organisms Avoid phosphorus losses • Minimize phosphorus losses caused by erosion by using management practices that maintain a complete cover of forages and residues over the pasture surface • Minimize phosphorus losses caused by runoff by not surface-applying fertilizer or ma- nure to soil that is saturated, snow-covered, or frozen • Avoid extensive grazing of animals in or near streams especially when land is wet or saturated or when streams are at low flow Ensure effective use of phosphorus inputs • Use management practices that encourage the even distribution of manure and urine across paddocks • Rely on soil tests, phosphorus index guidelines, and other nutrient management prac- tices when applying fertilizers and manure to pasturesPAGE 20 //NUTRIENT CYCLING IN PASTURES
  • 21. SECONDARY NUTRIENTS Potassium and the secondary nutrients, cal-cium, magnesium, and sulfur, play a critical role Phosphorus fertilization of tall fescuein plant growth and animal production. Potas- decreased the incidence of grasssium, calcium, and magnesium are components tetany in cattle since it stimulated grassof clay minerals. The soil parent-material pri- growth and increased the availabilitymarily influences the availability of these plant of magnesium during cool wet springnutrients. For example, soils derived from gran- conditions.ite contain, on the average, nine times more po-tassium than soils derived from basalt, while soils in the soil compared to magnesium, they willderived from limestone have half the amount. limit the ability of plants to take up magnesium.Conversely, soils derived from limestone have, Under these conditions, the magnesium concen-on the average, four times more calcium than tration needs to be increased relative to calcium.soils derived from basalt and thirty times more Dolomite lime, which contains magnesium car-than soils derived from granite (11). bonate, can be used to both lime soils and in- crease the availability of magnesium. Phospho- POTASSIUM rus fertilization of tall fescue in Missouri was also Potassium, like all plant nutrients, is recycled shown to increase the availability of magnesiumthrough plant uptake, animal consumption, and sufficiently to decrease the incidence of grassmanure deposition. The majority of potassium tetany in cattle (30). This probably resulted fromis found in urine. Potassium levels can become the stimulation of grass growth during cool wetexcessive in fields that have received repeated spring conditions that are conducive to the oc-high applications of manure. Application of fer- currence of grass tetany.tilizer nitrogen increases the potassium uptakeby grasses if the soil has an adequate supply of SULFURpotassium. Consumption of forages that contain Sulfur increases the protein content of pas-more than 2% potassium can cause problems in ture grasses and increases forage digestibility andbreeding dairy cattle and in their recovery fol- effectiveness of nitrogen use (31). In nature, sul-lowing freshening (28). High potassium levels, fur is contained in igneous rocks, such as graniteespecially in lush spring forage, can cause nutri- and basalt, and is a component of organic mat-ent imbalance resulting in grass tetany. ter. In areas downwind from large industrial and urban centers, sulfur contributions from the at- CALCIUM AND MAGNESIUM mosphere in the form of acid rain can be consid- Calcium and magnesium are components of erable. Fertilizer applications of nitrogen as am-liming materials used to increase soil pH and re- monium sulfate or as sulfur-coated urea also con-duce soil acidity. However, the use of lime can tribute to sulfur concentration in soils. However,also be important for increasing the amount of pasture needs for sulfur fertilization will increasecalcium in the soil or managing the balance be- as environmental controls for acid rain improve,tween calcium and magnesium. Increasing the as other sources of nitrogen fertilizer are used,calcium concentration may enhance biological ac- and as forage production increases.tivity in the soil (29). Managing this balance is Microbial processes affect sulfur availability.especially important for decreasing the occur- As with nitrogen, the sulfur content of organicrence of grass tetany, a nutritional disorder of matter determines whether nutrients will be min-ruminants caused by low levels of magnesium eralized or immobilized. Also as with nitrogen,in the diet. Magnesium may be present in the the sulfur content of grasses decreases as theysoil in sufficient amounts for plant growth, but become older and less succulent. Thus, soil or-its concentration may be out of balance with the ganisms will decompose younger plants morenutrient needs of plants and animals. When cal- rapidly and thereby release nutrients while theycium and potassium have a high concentration will decompose older plant material more slowly //NUTRIENT CYCLING IN PASTURES PAGE 21
  • 22. and may immobilize soil nutrients in the processof decomposition. Chemical and biological processes are involvedin sulfur transformations. In dry soils that becomewet or waterlogged, chemical processes transformsulfur from the sulfate to sulfide form. If these wetsoils dry out or are drained, bacteria transform sul-fide to sulfate. Like nitrate, sulfate is not readilyabsorbed by soil minerals, especially in soils witha slightly acid to neutral pH. As a result, sulfatecan readily leach through soils that are sandy orhighly permeable. Table 9. Pasture Management Practices for Efficient Cycling of Potassium, Calcium, Magnesium, and Sulfur. Encourage nutrient mineralization by soil organisms • Use management practices that minimize soil compaction and soil erosion • Minimize use of tillage and other cultivation practices • Maintain a diversity of forage species to provide a variety of food sources and habitats for a diversity of soil organisms • Avoid the use of soil-applied pesticides and concentrated fertilizers that may kill or in- hibit the growth of soil organisms • Encourage animal movement across paddocks for even distribution of manure nutrients Avoid nutrient losses • Minimize sulfur losses by using management practices that decrease the potential for leaching • Minimize nutrient losses caused by erosion by using management practices that main- tain a complete cover of forages and residues over the pasture surface Maintain nutrient balances in the pasture • Ensure magnesium availability to minimize the potential for grass tetany. This can be done by balancing the availability of magnesium with the availability of other soil cat- ions, particularly potassium and calcium. Phosphorus fertilization of pastures in spring can also enhance magnesium availability • Guard against a buildup of potassium in pastures by not overapplying manure. High potassium levels can cause reproductive problems, especially in dairy cowsPAGE 22 //NUTRIENT CYCLING IN PASTURES
  • 23. Chapter 2 Nutrient Availability in Pastures Nutrient balances and nutrient availability source of calcium and magnesium. Some claydetermine the fate of nutrients in pastures. In soils and soils with high percentages of organicthe simplest of grazing sys- matter contain a native storetems, forage crops take up of nutrients in addition tonutrients from the soil; Nutrient-depleted soils produce having the capacity to holdhaying and grazing remove low-yielding forages and unthrifty nutrients added by manure,forage crops and their asso- animals. crop residues, or fertilizers.ciated nutrients; and animal Excess soil nutrients can be dan- Soils formed under temper-manure deposition returns gerous to animal health and in- ate prairies or in flood plainsnutrients to the soil. Con- crease the potential for contami- have built up fertilitytinual nutrient removals de- nation of wells, springs, rivers, through a long history of or-plete soil fertility unless fer- and streams. ganic matter deposition andtilizers, whether organic or nutrient accumulation.synthetic, are added to re- Sandy soils and weathered,plenish nutrients. Nutrients may be added to reddish clay soils contain few plant nutrients andpastures by providing animals with feed supple- have a limited ability to hold added nutrients.ments produced off-farm. Soils formed under desert conditions are often Chemical and biological interactions deter- saline, since water evaporating off the soil sur-mine the availability of nutrients for plant use. face draws water in the soil profile upward. ThisBoth native soil characteristics and land manage- water carries nutrients and salts, which are de-ment practices affect these interactions. Phos- posited on the soil surface when water evapo-phorus can be held chemically by iron or alumi- rates. Tropical soils generally have low fertilitynum bonds while potassium can be held within since they were formed under conditions of highsoil minerals. Practices that erode topsoil and temperatures, high biological activity, and highdeplete soil organic matter decrease the ability rainfall that caused rapid organic matter decom-of soils to hold or retain nutrients. All crop nu- position and nutrient leaching.trients can be components of plant residues orsoil organic matter. The type of organic matteravailable and the activity of soil organisms de- SOIL CHEMISTRYtermine the rate and amount of nutrients miner-alized from these materials. Nutrient availabil-ity and balance in forage plants affect the health Many clay minerals are able to hold ontoof grazing animals. Depleted soils produce un- water and nutrients and make them available forhealthy, low-yielding forages and unthrifty ani- plant growth. The pH, or level of acidity or al-mals; excess soil nutrients can be dangerous to kalinity of the soil solution, strongly influencesanimal health and increase the potential for con- the strength and type of bonds formed betweentamination of wells, springs, rivers, and streams. soil minerals and plant nutrients. Soil pH also affects activities of soil organisms involved in the decomposition of organic matter and the disso- SOIL PARENT MATERIAL lution of plant nutrients from soil minerals. Many clay soil particles are able to bind large amounts of nutrients because of their chemical Chemical, physical, geological, and biologi- composition and because they are very small andcal processes affect nutrient content and avail- have a large surface area for forming bonds.ability in soils. As discussed in the previous Unfortunately, this small size also makes claychapter, soils derived from basalt and shale pro- particles prone to compaction, which can reducevide phosphorus to soils, granite contains high nutrient and water availability. Sandy soils areconcentrations of potassium, and limestone is a porous and allow water to enter the soil rapidly. //NUTRIENT CYCLING IN PASTURES PAGE 23
  • 24. But these soils are unable to hold water or nutri-ents against leaching. Organic matter has a high SOIL COMPACTIONcapacity to hold both nutrients and water. Soilaggregates, formed by plant roots and soil organ-isms, consist of mineral and organic soil compo- Animal movement compacts soil pores, es-nents bound together in soft clumps. Aggregates pecially when soils are wet or saturated. Con-enhance soil porosity, facilitate root growth, al- tinual trampling and foraging, especially in con-low for better infiltration and movement of wa- gregation areas and laneways, also depletes plantter and nutrients through soil, and help soils re- growth and produces bare spots.sist compaction. Soil compaction increases as soil mois- ture, animal weight, animal numbers, PRIOR MANAGEMENT PRACTICES and the length of stay in the paddock in- crease. In pastures, continual removal of nutrients Resistance to compaction increases asthrough harvests or heavy grazing without re- forage establishment and the percentageturn or addition of nutrients depletes the soil. of plants with fibrous roots increase.Land management practices that encourage soilerosion — such as heavy grazing pressure, Soil compaction reduces nutrient availabil-plowing up and down a slope, or leaving a field ity for plant uptake by blocking nutrient trans-bare of vegetation during times of heavy rains port to roots and restricting root growth throughor strong winds — also result in depletion of the soil profile. Treading and compaction cansoil fertility. Some pasture management prac- substantially reduce forage yields. One studytices involve the use of fire to stimulate growth showed that the equivalent of 12 sheep treadingof native forages (32). Burning readily miner- on mixed ryegrass, white clover, and red cloveralizes phosphorus, potassium, and other nu- pasture reduced yields by 25% on dry soil, 30%trients in surface crop residues. It also volatil- on moist soil, and 40% on wet soil compared toizes carbon and nitrogen from residues and re- no treading. On wet soils, root growth was re-leases these nutrients into the atmosphere, thus duced 23% (35).minimizing the ability of organic matter to ac- Compaction also decreases the rate of organiccumulate in the soil. Loss of residues also ex-poses soil to raindrop impact and erosion. Hot matter decomposition by limiting the access soiluncontrolled fires increase the potential for ero- organisms have to air, water, or nutrients. In ad-sion by degrading natural bio- dition, compacted soils limit water infiltrationlogical crusts formed by li- and increase the potential forchens, algae, and other soil or- water runoff and soil erosion. Inganisms, and by promoting the Arkansas, observers of over-formation of physical crusts grazed pastures found that ma-formed from melted soil min- nure piles on or near bare, com-erals (33, 34). The continual pacted laneways were morehigh application of manure, readily washed away by runoffwhey, sludge, or other organic than were manure piles in morewaste products to soils can vegetated areas of the pasturecause nutrients to build up to (24).excessive levels. Pasture man- The potential for animals toagement practices that influ- cause soil compaction increasesence soil compaction, soil satu- with soil moisture, the weight ofration, the activity of soil or- Compacted soils do not allow for the animal, the number of ani-ganisms, and soil pH affect normal root growth. This root mals in the paddock, and theboth soil nutrient content and grew horizontally when it amount of time animals stay inavailability. encountered a compacted layer. the paddock. The potential for aPAGE 24 //NUTRIENT CYCLING IN PASTURES
  • 25. paddock to resist compaction depends on the du- slow. Periods of wet weather alternating withration of forage establishment and the type of periods of dry weather can reduce compactionforage root system. Established forages with in some clay soils. Freezing and thawing de-strong and prolific root growth in the top six to creases compaction in soils subjected to cold10 inches of the soil profile are able to withstand weather. Taproots are effective in breakingtreading by grazing animals. Grasses with ex- down compacted layers deep in the soil profiletensive fibrous root systems, such as bermuda while shallow, fibrous roots break up compactedgrass, are able to withstand trampling better than layers near the soil surface (37). Active popula-grasses like orchardgrass that have non-branch- tions of soil organisms also reduce soil compac-ing roots or legumes like white clover that have tion by forming soil aggregates and burrowingtaproots (36). Bunch grasses expose more soil to into the soil.raindrop impact than closely seeded non-bunchgrasses or spreading herbaceous plants. How-ever, these grasses enhance water infiltration by ORGANIC MATTERcreating deep soil pores with their roots (3). Com-bining bunch grasses with other plant varieties NUTRIENT RELEASE FROMcan increase water infiltration while decreasingthe potential for soil compaction and water run- ORGANIC MATTER DECOMPOSITIONoff. Manure and plant residues must be decom- The risk of soil compaction can also posed by soil organ-be reduced by not grazing animals on isms before nutrients in Time required for organicpaddocks that are wet or have poorly- these materials are matter decomposition isdrained soils. Instead, during wet con- available for plant up- affected by:ditions, graze animals on paddocks that take. Soil organisms in- • the carbon to nitrogenhave drier soils and are not adjacent to volved in nutrient de- ratio of organic matterstreams, rivers, seeps, or drainage ways. composition require a • temperatureSoils that are poorly drained should be balance of nutrients to • moistureused only in the summer when the cli- break down organic • pHmate and the soil are relatively dry. matter efficiently. Ma- • diversity of soil organ- Compacted soils can recover from the nure and wasted for- ismsimpacts of compaction, but recovery is ages are succulent ma- Table 10. Effects of 11 Years of Manure Additions on Soil Properties. Manure application rate (tons/acre/year) None 10 tons 20 tons 30 tons Organic Matter (%) 4.3 4.8 5.2 5.5 CEC (me/100g) 15.8 17.0 17.8 18.9 PH 6.0 6.2 6.3 6.4 Phosphorus (ppm) 6.0 7.0 14.0 17.0 Potassium (ppm) 121.0 159.0 191.0 232.0 Total pore space (%) 44.0 45.0 47.0 50.0 From Magdoff and van Es, 2000 (Reference #1) //NUTRIENT CYCLING IN PASTURES PAGE 25
  • 26. terials that have high nitrogen content and a good Soil aggregates arebalance of nutrients for rapid decomposition. small, soft, water- Dried grasses, such as forages that died back stable clumps of soilover winter or during a drought, or manure held together by finemixed with wood bedding, have lower nitrogen plant-root hairs, fun-contents and require more time for decomposi- gal threads, humus,tion. In addition, soil organisms may need to and microbial gels.extract nitrogen and other nutrients from the soil Aggregates are alsoto balance their diet and obtain nutrients not formed through theavailable in the organic matter they are decom- activities of earth-posing. Composting these materials increases the worms. Research has shown that several spe-availability of nutrients and decreases the poten- cies of North American earthworms annuallytial for nutrient immobilization when materials consume 4 to 10% of the soil and 10% of the totalare added to the soil. Tem- organic matter in the top 7 inches of soilperature, moisture, pH, (39). This simultaneous consumption ofand diversity of soil organ- organic and mineral matter by earth-isms affect how rapidly or- worms results in casts composed of as-ganic matter is decom- sociations of these two materials. Earth-posed in the soil. worms, as well as dung beetles, incor- Nutrient release from porate organic matter into the soil as theyorganic matter is slow in burrow.the spring when soils are Besides enhancing the nutrient andcold and soil organisms water holding capacity, well-aggregatedare relatively inactive. soils facilitate water infiltration, guardMany farmers apply phos- against runoff and erosion, protectphorus as a starter fertil- against drought conditions, and are bet-izer in the spring to stimulate seedling growth. ter able to withstand compaction than less ag-Even though soil tests may indicate there is suf- gregated soils. Since aggregated soils are moreficient phosphorus in the soil, it may not be granular and less compacted, plant roots growreadily available from organic matter during cool more freely in them, and air, water, and dissolvedsprings. plant nutrients are better able to flow through them. These factors increase plant access to soil NUTRIENT HOLDING CAPACITY OF nutrients. To enhance aggregation within pasture soils, ORGANIC MATTER maintain an optimum amount of forages and Besides being a source of nutrients, soil or- residues across paddocks, avoid the formationganic matter is critical for holding nutrients of bare areas, and minimize soil disturbance.against leaching or nutrient runoff. Stabilized Grazing can degrade soil aggregates by encour-organic matter or humus chemically holds posi- aging mineralization of the organic glues thattively-charged plant nutrients (cations). The hold aggregates to-ability of soil particles to hold these plant nutri- gether. In areasents is called cation exchance capacity or CEC. Con- with a good covertinual application of organic materials to soils in- of plant residues,creases soil humus (38) and enhances nutrient animal movementavailability, nutrient holding capacity, and soil across pastures canpore space. enhance aggregate SOIL AGGREGATES formation by incor- Soil humus is most effective in holding wa- porating standingter and nutrients when it is associated with min- dead plant materi-eral soil particles in the form of soil aggregates. als into the soil (40).PAGE 26 //NUTRIENT CYCLING IN PASTURES
  • 27. nitrogen. Legumes that actively fix nitrogen use SOIL PH very little nitrate; consequently, they reduce soil pH by taking more cations than anions (9). A combination of legumes and non-legumes will Soil mineralogy, long-term climatic condi- tend to stabilize soil pH.tions, and land-management practices affect soil Pasture soils should be tested regularly to de-pH. The acidity or alkalinity of soils affects nu- termine soil nutrients, soil organic matter, andtrient availability, nitrogen fixation by legumes, pH. Based on test results and forage nutrient re-organic matter decomposition by soil organisms, quirements, management practices can adjustand plant root function. Most plant nutrients are soil pH. Lime and organic matter increase soilmost available for uptake at soil pH of 5.5 to 6.5. pH and decrease soil acidity. Soil organic mat-Legume persistence in pastures is enhanced by ter absorbs positive charges, including hydrogensoil pH of 6.5 to 7.0. In low-pH or acid soils, alu- ions that cause soil acidity (41). Lime increasesminum is toxic to root growth; aluminum and soil pH by displacing acid-forming hydrogen andiron bind phosphorus; and calcium is in a form aluminum bound to the edges of soil particleswith low solubility. In high-pH or alkaline soils, and replacing them with calcium or magnesium.calcium carbonate binds phosphorus while iron, Limestone that is finely ground is most effectivemanganese, and boron become insoluble. in altering soil pH since it has more surface area Application of some synthetic nitrogen fer- to bind to soil particles. All commercial lime-tilizers acidifies soils. Soil microorganisms in- stone has label requirements that specify its ca-volved in nitrification rapidly transform urea or pacity to neutralize soil pH and its reactivity,ammonia into nitrate. This nitrification process based on the coarseness or fineness of grind.releases hydrogen ions into the soil solution, “Lime” refers to two types of materials, cal-causing acidification, which decreases nutrient cium carbonate and dolomite. Dolomite is a com-availability, thus slowing the growth of plants bination of calcium and magnesium carbonate.and soil organisms. Nitrification Calcium carbonate is recommendedalso occurs in urine patches when for soils low in calcium; where grasssoil microorganisms transform Lime soils with cal- tetany or magnesium deficiency isurea into nitrate. cium carbonate if the an animal health problem, dolomite Another fertilizer that acidifies soil is low in calcium. limestone should be used. In sandythe soil is superphosphate. Super- Use dolomite lime- soils or soils with low to moderatephosphate forms a highly acid (pH stone if grass tetany levels of potassium, the calcium or1.5) solution when mixed with wa- or magnesium defi- magnesium in lime can displace po-ter. The impact of this acidification ciency is an animal tassium from the edges of soil par-is temporary and only near where health problem. ticles, reducing its availability.the fertilizer was applied, but, in Therefore, these soils should receivethis limited area, the highly acid so- both lime and potassium inputs tolution can kill rhizobia and other soil microor- prevent nutrient imbalances.ganisms (9). The timing of nutrient additions to fields or The type and diversity of forage species in pastures determines how effectively plants takepastures can alter soil pH. Rangeland plants such up and use nutrients while they are growing andas saltbush maintain a neutral soil pH. Grassesand non-legume broadleaf plants tend to increasepH, while legumes tend to decrease it. The im- TIMING O. NUTRIENT ADDITIONSpact of plant species on pH depends on the typeand amounts of nutrients they absorb. Range- setting seed. Different nutrients are importantland plants absorb equal amounts of cation (cal- during different stages of plant development. Ni-cium, potassium, magnesium) and anion (nitrate) trogen applied to grasses before they begin flow-nutrients from the soil. Grasses and non-legume ering stimulates tillering, while nitrogen appliedbroadleaf plants absorb more anions than cations during or after flowering stimulates stem and leafsince they use nitrate as their primary source of growth (9). However, fall nitrogen applications //NUTRIENT CYCLING IN PASTURES PAGE 27
  • 28. for cool-season grasses are more effective and tive nodule formation and nitrogen fixation. Ineconomical than spring applications. In most acid soils, liming may make phosphorus alreadyyears, nutrient releases through mineralization in the soil more available, thereby decreasing theare sufficient to stimulate forage growth in the need for fertilization.spring. Applications of nitrogen in the late sum- As discussed above, the type of organic ma-mer and fall allow cool-season grasses to grow terial added to the soil, as well as temperature,and accumulate nutrients until a killing frost. moisture, pH, and diversity of soil organisms,This provides stockpiled growth for winter graz- determines how rapidly soil organisms decom-ing (42). pose and release nutrients from organic matter. Both potassium and phosphorus are impor- Synthetic fertilizers are soluble and immediatelytant for increasing the nutrient quality of forages, available for plant uptake. Therefore, these fer-extending stand life, and enhancing the persis- tilizers should be applied during periods whentence of desirable species in the forage stand (42). plants can actually use nutrients for growth. APhosphorus is critical for early root growth, for lag time of two to 21 days may pass after fertiliz-seed production, and for effective nitrogen fixa- ers are applied before increased forage produc-tion by legume nodules. Potassium is important tion is observed.during the mid-to-late growing season. It in- Organic material releases nutrients over acreases the ability of plants to survive winter con- period of several years. On average, only 25 toditions, by stimulating root growth and reduc- 35% of the nitrogen ining water loss through stomata or leaf pores (43). manure is mineral-It also is important for legume vigor and for en- ized and available A lag time of two to 21hancing plant disease resistance (42). for plant use during days may pass after Nutrient uptake by plants corresponds to the year of applica- fertilizers are appliedtheir growth cycle. Warm-season forages exhibit tion. Another 12% before increased for-maximum growth during the summer, whereas is available in the age production is ob-cool-season forages exhibit maximum growth following year, 5% served.during the spring and early fall (32). Pastures in the second yearcontaining a diverse combination of forages will following application, and 2% in the third yearuse nutrients more evenly across the growing (44). Manure deposited in pastures causes an in-season while less-diverse pastures will show crease in forage growth approximately 2 to 3spikes in nutrient uptake requirements. months after deposition, with positive effects on Legumes provide nitrogen to the pasture sys- growth extending for up to two years (11). Al-tem through their relationship with the nitrogen- falfa can supply approximately 120 pounds offixing bacteria, rhizobia. If nitrogen levels in the nitrogen to crops and forages in the year after it soil are low, newly is grown, 80 pounds of nitrogen during the fol- planted legumes lowing year, and 10 to 20 pounds in the third Nitrogen fertilization require nitrogen year (44). Because of this gradual release of nu- depresses fixation by fertilization until trients from organic materials, continual addi- legumes since they re- rhizobia have de- tions of manure or legumes will compound the quire less energy to veloped nodules availability of nutrients over time. Accounting take up nitrogen from and are able to fix for nutrients available from previous years is the soil than they need nitrogen. Once critical for developing appropriate applications to fix nitrogen. they start fixing ni- rates for manure and fertilizers during each trogen, nitrogen growing season. Not accounting for these nutri- fertilization de- ents can result in unnecessary fertilizer expensespresses fixation by legumes since they require and risks of nutrient losses to the environment.less energy to take up nitrogen from the soil than Nutrients from both organic and syntheticthey need to fix nitrogen. Legumes can fix up to fertilizers can be lost through leaching, runoff,200 pounds of nitrogen per year, most of which or erosion. The potential for nutrient losses isbecomes available to forage grasses in the fol- greatest if these materials are applied in the falllowing years. Phosphorus is essential for effec- or winter, when plants are not actively growingPAGE 28 //NUTRIENT CYCLING IN PASTURES
  • 29. or during times when soils are frozen, snow-covered, or saturated. During times of high rainfall,nitrate may leach through the soil since it does not bind to soil particles. Rainfall also facilitates thetransport of phosphorus to water bodies in runoff water or through artificial drainage tiles. Rainfallor snowmelt water flowing over bare soil causes soil erosion and the transport of nutrients attachedto soil particles. Table 11. Pasture Management Practices for Enhancing Nutrient Availability. Ensure plant cover and diversity across pastures • Use management practices that maintain a complete cover of forages and residues across pastures • Combine bunch-grass species with a diversity of forage species, including plants with prostrate growth habit, to provide both good water infiltration and protection against erosion and soil compaction Grazing management practices during wet weather • Use well-drained pastures or a “sacrificial pasture” that is far from waterways or water bodies • Avoid driving machinery on pastures that are wet or saturated • Avoid spreading manure or applying fertilizers on soil that is saturated, snow-cov- ered, or frozen Ensure effective use of nutrient inputs • Use management practices that encourage the even distribution of manure and urine across paddocks • Rely on soil tests and other nutrient management practices when applying fertilizers and manure to pastures • Account for nutrients available form manure and legume applications during prior years when developing fertilizer or manure application rates for the current year • Sample the nutrient content of added manure to determine appropriate rates of ap- plication • Choose the appropriate type of limestone to apply for pH adjustment based on cal- cium and magnesium needs and balances in pastures • Either avoid the use of fertilizers that decrease soil pH or use lime to neutralize soils acidified by these fertilizers • Apply nitrogen fertilizer in the fall to enhance the amount of forages stockpiled for winter grazing • Apply sufficient phosphorus and potassium while limiting additions of nitrogen in or- der to favor growth of legumes in your pastures //NUTRIENT CYCLING IN PASTURES PAGE 29
  • 30. Chapter 3 Nutrient Distribution and Movement in Pastures Farmers and ranchers graze animals using avariety of management methods. In this docu- .igure 7. Pasture Nutrient Inputsment, extensive grazing refers to the practice of and Outputs.grazing animals continuously or for extendedperiods of time on a large land area. Rotational Inputs Farm Boundary Outputsgrazing is a management-intensive system thatconcentrates animals within a relatively small feedarea (a paddock) for a short period of time — animal fertilizer productsoften less than a day for dairy animals. The ani-mals are then moved to another paddock, while legume Nthe first paddock is allowed to recover and re- crops rainfallgrow. Animals are moved according to a flex-ible schedule based on the herd size, the amount Lossesof land available, quality of forages in the pad- ammonia volatilization, leaching,dock, and forage consumption. Grazing manag- denitrification, runoff, & erosioners determine when and how long to graze ani- From Klausner, 1995 (Reference #44)mals in specific paddocks based on climatic con-ditions, soil characteristics, land topography, and Grazing animals that receive no mineral orthe distance the paddock is from streams or riv- feed supplements will recycle between 75 anders. 85% of forage nutrients consumed. If no fertil- Pasture size, shape, and topography; stock- izer or outside manure inputs are applied, con-ing rate; grazing duration; and time of day all tinual grazing will cause a gradual depletion ofaffect how animals graze, lounge, drink water, plant nutrients. Animals provided feed or min-and use feed or mineral supplements. Also, dif- eral supplements also deposit 75 to 85% of theferent animal species vary in their use of nutri- nutrients from these inputs as urine and fecesents and their herding behavior. These factors, (42). These nutrients represent an input into thealong with soil characteristics, climate, and for- pasture system. Nutrient inputs from non-for-age and soil management practices, affect nutri- age feeds can be substantial for dairy and otherent cycling in pastures, animal growth and pro- animal operations that use a high concentrationductivity, and potential of manure nutrients to of grain and protein supplements, importing intocontaminate ground or surface water. the pasture approximately 148 lbs. N, 32 lbs. P, and 23 lbs. K per cow per year (42). Winter feeds also form a substantial input into the pasture nu- PASTURE NUTRIENT trient budget when animals are fed hay while be- ing kept on pasture. INPUTS AND OUTPUTS MANURE DEPOSITION AND DISTRIBUTION NUTRIENT BALANCES A cow typically has 10 defecations per day, Maintaining a balance between nutrients re- with each manure pile covering an area of ap-moved from pastures and nutrients returned to proximately one square foot (47). They will alsopastures is critical to ensure healthy and produc- urinate between eight and 12 times per day (48).tive forage growth, as well as to control nutrient Each urination spot produces a nitrogen appli-runoff and water-body contamination. Nutrient cation equivalent to 500 to 1,000 lbs./acre whilebalances in pastures are determined by subtract- each defecation represents a nitrogen applicationing nutrient removals in the form of hay harvested, rate of 200 to 700 lbs./acre (42). An even distri-feed consumed, and animals sold, from nutrient bution of nutrients throughout a paddock is re-inputs including feed, fertilizer, and manure. quired for productive plant and animal growth.PAGE 30 //NUTRIENT CYCLING IN PASTURES
  • 31. Table 12. Nutrient Consumption and Excretion by Grazing Animals. Dairy cows Beef /Sheep________________________________________________________________________________ Feed consumption/day 18–33 lbs.________________________________________________________________________________ Nutrients used for growth and reproduction 17% N 15 -25% N 26% P 20% P, 15% K________________________________________________________________________________ Nutrients removed from pasture in form N 84 lbs./cow N 10 lbs./cow-calf of milk and meat P 15 lbs./cow P 3 lbs./cow-calf K 23 lbs./cow K 1 lbs./cow-calf________________________________________________________________________________ Nutrients/ton manure 6–17 lbs.N 3–12 lbs.P2O5________________________________________________________________________________ 2-15 lbs. K2O________________________________________________________________________________ Nitrogen content of feces 2.0–3.6% 3.4–3.6%________________________________________________________________________________ Nitrogen excreted as feces 55 lbs./year________________________________________________________________________________ Nitrogen content of urine 0.42–2.16% 0.30–1.37%________________________________________________________________________________ Nitrogen excreted as urine 165 lbs./year________________________________________________________________________________ From Stout, et al. ( 45), Detling ( 46), Russelle ( 17), Wells and Dougherty ( 47) , Haynes and Williams ( 41), Klausner ( 44), Lory and Roberts ( 42)Unfortunately, grazing animals do not naturally ture to water, they deposit between 13 and 22%deposit urine and feces evenly across the paddocks of their manure on laneways (47, 52).where they graze. In one rotational grazing A study conducted in Iowa showed a buildupstudy, urine spots occupied 16.7% of the pasture, of nutrients extending 30 to 60 feet into the pas-while manure spots occupied 18.8%, following ture around water, shade, mineral supplements,504 grazing days per acre (49). Intensity of graz- and other areas where cattle congregated (53).ing rotations affects the distribution of manure Nutrients are concentrated in these congregationcoverage in paddocks. Under continuous, exten- areas because animals transport nutrients fromsive grazing practices, 27 years would be needed areas where they graze. Consequently, they alsoto obtain one manure pile on every square yard deplete nutrients from the grazing areas. Graz-within a paddock; if a two-day rotation were used ing practices that encourage foraging and manure distribution across paddocks and discourage con-instead only two years would be needed (42). gregation in limited areas will improve nutrient Nutrient concentration within pastures re- balances within pastures.sults from the tendency of grazing animals to con- The time of day when animals congregate ingregate. They tend to leave manure piles or urine different areas determines the amount and typespots around food and water sources, on side of nutrients that accumulate in each area. Ani-hills, in depressions, along fence lines, and un- mals tend to deposit feces in areas where theyder shade. Sheep have a greater tendency than rest at night or ruminate during the day, whilecattle to congregate and deposit manure in these they urinate more in the areas where they grazeareas (50). Prevailing wind direction and expo- during the day (47). Nitrogen is present in bothsure to sunlight can also affect animal movement, feces and urine while phosphorus is primarilycongregation, and manure deposition (51). deposited as feces, and potassium is foundLaneways that connect pastures or lead to wa- mostly in urine. While most urine is depositedtering areas are another area of animal congre- during the day, urine that is deposited at nightgation and manure deposition. When animals has a higher nutrient content than urine depos-have to walk more than 400 feet from the pas- ited while grazing (41). As a result of these fac- //NUTRIENT CYCLING IN PASTURES PAGE 31
  • 32. tors, phosphorus will accumulate in resting ar- .igure 8. eas (13) while nitrogen and potassium will accu- 3-Paddock System. mulate in both resting and grazing areas. O = water tank MINIMIZING ANIMAL CONGREGATION By working with the normal foraging and herding behavior of grazing animals, distribu- tion of animals across paddocks can be encour- aged. In larger paddocks, animals tend to graze and lounge as a herd, while they distribute them- selves more evenly across smaller paddocks (41). In larger paddocks, animals visit water, miner- als, shade, and fly-control devices as a herd, whereas animals concentrated within small pad- docks tend to visit these areas one-by-one. Lo- cating nutrients, shade, and pest-control devices farther apart in the paddock further discourages concentration of animals and manure. If a par- .igure 9. ticular area of a paddock is deficient in nutrients, 12-Paddock System. placement of supplemental feeds in that area can be used to encourage congregation and manure O = water tank deposition there. Subdividing depressions, side hills, and shady areas among several paddocks can en- hance nutrient distribution across the landscape. Research conducted in Missouri showed that manure nutrients were distributed more evenly across the landscape when a field was managed using 12 or 24 paddocks rather than only three paddocks (54). Animals in the smaller paddocks concentrated around favored areas for less time than did animals in larger paddocks. Since ani- mals tend to graze along the perimeter of fence lines, they distribute nutrients most evenly across .igure 10. paddocks that are small, square, and have water available (55). An efficiently designed paddock 24-Paddock System. allows animals to graze and drink with a mini- mum amount of time, effort, and trampling of O = water tank the pasture sod. MANURE NUTRIENT AVAILABILITY While feces contain nitrogen predominantly in the organic form, 60 to 70% of cow-urine ni- trogen and 70 to 80% of sheep-urine nitrogen is in the form of urea. Urea and potassium in urine are soluble and therefore immediately available for plant uptake. Phosphorus in feces is predomi-Manure deposition as affected by paddock size (from nantly in the organic form and must undergo de-Peterson and Gerrish, Reference 52). composition before it is available to plants. SoilPAGE 32 //NUTRIENT CYCLING IN PASTURES
  • 33. organisms will decompose moist, nitrogen-rich haying. Prior to the current concerns over watermanure piles derived from succulent grasses rela- quality, manure application recommendationstively quickly. They will have difficulty break- were made to meet forage needs for down manure derived from hay or older for- Continued nitrogen-based applications result inages that is stiff, dry, and crusty. When a hard a phosphorus build-up in the soil since manurecrust forms on manure piles during dry weather, usually contains about the same concentrationboth physical breakdown and biological decom- of phosphorus and nitrogen, while plants onlyposition are inhibited (41). By treading on ma- require one-half as much phosphorus as nitro-nure piles as they move around a pasture, ani- gen. In diverse pastures that contain a combina-mals physically break these piles into smaller tion of grasses and legumes, decreasing or elimi-pieces that are more easily consumed by soil or- nating manure applications can lower phospho-ganisms. rus imbalances while maintaining forage yields. Because nutrients are released slowly from Nitrogen fixation by legumes helps satisfy for-manure, forage plants in the vicinity of manure age nitrogen requirements while using excess soilpiles will grow slowly for about two months fol- phosphorus.lowing manure deposition (41, 42). However, asdecomposition of manure piles by soil organisms .igure 11. P Added in Manure/makes nutrients available for plant use, greater Removed by Crop.pasture regrowth and forage production occursin the vicinity of manure and urine compared to Applying manure to meet the nitrogen needsother pasture areas (49, 54, 56). Increases in ni- of corn (about 200 lbs. N/acre) adds muchtrogen availability in areas near manure piles can more phosphorus than corn needs.favor the growth of grasses compared to legumes(9), an impact that can last for up to two years P added (lb./acre)(41). Animals naturally avoid grazing near dung P removed (lb./acre)sites, but will feed closer to manure piles (41) anduse forages more efficiently as grazing pressure Dairy manure Poultry litterintensifies. In multispecies grazing systems,sheep do not avoid cattle manure as much ascattle do (57). While both sheep and cattle avoidsheep manure, the pellet form of sheep manure Cornhas a large surface area, and thus breaks downmore rapidly than cattle manure. Consequently, From Sharpley, et al. (Reference 58)forages are used more effectively when cattle arecombined with sheep. On some farms, manure is applied to soil as a waste product. Instead of being applied ac- PASTURE .ERTILIZATION cording to crop needs, manure is primarily ap- plied according to the need to dispose of manure, Manure and fertilizers are applied to pastures the location of fields in relation to the barn, andto provide nutrients necessary to obtain effective the accessibility of fields during bad weather. plant growth and These “waste application” practices present a animal production. high potential for nutrient buildup and move- Fertilizer and manure Applications should ment of excess nutrients to ground or surface applications should waters. be based on regular be based on regular To ensure that manure is used effectively as soil testing, the abil- soil testing, the abil- a source of plant nutrients and poses minimal ity of soil to provide ity of soil to provide risks to the environment, it should be applied and retain nutrients, and retain nutrients, according to a nutrient management plan. Natu- plant needs, and plant needs, grazing intensity, and nutri- ral Resources Conservation Service or Soil and grazing intensity. ent removals through Water Conservation District personnel, as well //NUTRIENT CYCLING IN PASTURES PAGE 33
  • 34. as many commercial crop consultants, are trained in the development of nutrient management plans.Software programs to develop your own nutrient management plan may be available from Coopera-tive Extension Service educators or Agronomy and Soil Science specialists at land grant universities. Table 13. Components of a Comprehensive Nutrient Management Plan. Components of a Comprehensive Nutrient Management Plan • Soil tests on all fields or paddocks • Manure tests • Load-capacity and rate-of-application of manure spreading equipment • Timing and method of manure and fertilizer applications • Prior land management practices including manure applications, legumes used as green manures, fallows, or hay removal • Assessments of runoff, erosion, and flooding potentials for each field or paddock • Crops or forages to be produced • Current pasture management practices, including stocking rates and hay removal Format of a nutrient management plan for each paddock or field • Soil and manure test results • Risk factors such as excess nutrient levels, or high runoff, erosion, or flooding potential • Recommended time, method, and rate for fertilizer and manure applications • Recommended time for grazing, especially on pastures with moderate to high potentials for runoff, erosion, or flooding • Management practices to minimize risk factors and maximize nutrient availability to forages cies, enhances the dispersal of forage seeds, and GRAZING INTENSITY helps conserve nutrient resources within the soil- plant system. GRAZING BEHAVIOR, PLANT GROWING DEFINITION POINTS, AND PLANT LEAF AREA Grazing intensity refers to the impact animals Grazing habits of different animal specieshave on forage growth and reproduction and on have different impacts on forage species compo-soil and water quality. It is influenced by ani- sition in pastures. For example, horses grazemal foraging habits, stock- more closely to the grounding rates, the length of time than cattle; sheep graze atanimals are allowed to Short-term high-intensity grazing soil level and can take awaygraze within a given pad- combined with a resting period (as the base of grass plants be-dock, and the relation these in rotational grazing practices) low the area of tiller emer-factors have to soil charac- causes an increase in the diversity gence (59); while cattle tendteristics and climatic condi- of forage species, enhances the dis- to graze taller grasses thattions. Continuous high-in- persal of forage seeds, and helps sheep may reject. Animaltensity grazing depletes soil conserve nutrient resources within grazing behavior, the loca-nutrients, decreases the di- the soil-plant system. tion of a plant’s growingversity of forage species, in- point, and the amount ofhibits the ability of some forage plants to regrow leaf area remaining when animals are rotated toand reproduce, and increases the potential for another pasture affect the ability of plants to re-nutrient runoff and erosion. Conversely, short- grow. If grazing animals remove the growingterm high-intensity grazing combined with a point and substantial leaf area of grasses, newresting period (as in rotational grazing practices) leaf growth must come from buds that have beencauses an increase in the diversity of forage spe- dormant and the energy for this growth mustPAGE 34 //NUTRIENT CYCLING IN PASTURES
  • 35. come from stored carbohydrates rather than from include treading impact on leaf and root growth,photosynthesis (60). forage composition impact on the ability of plants Early in the growing season, all grasses have to intercept sunlight for photosynthesis, and soiltheir growing points at or near ground level. conditions (35).Ryegrass, tall fescue, Kentucky bluegrass and NUTRIENT UPTAKEmany other species of cool-season grasses havegrowing points that remain at or below ground Forage plants that are cut or regrazed fre-level throughout most of the growing season. quently during the growing season take up moreOther, predominantly native, grass species — in- nutrients than forages that are not cut or grazed.cluding smooth broomgrass, timothy, reed Research conducted in Kansas indicated thatcanarygrass, switchgrass, and gamagrass — have cutting pasture forage six times during the grow-stems that elongate below the growing point ing season resulted in 4.3 times greater nitrogenabove the soil level (60). As long as the growing content and 5.2 times greater phosphorus con-point remains intact, the plant is capable of pro- tent in cut forages compared to uncut plots (63). Cutting pastures in the spring when seed headsducing new leaves. Grasses with low growing are forming can also increase the productivitypoints are able to recover from grazing relatively and nutrient uptake of pasture forages (64).quickly because the growing point is not dis- Other studies (56, 65) demonstrated that in-turbed. If the growing point is removed, growth creased grazing intensity resulted in younger,recommences from the emergence of new tillers. more succulent plants with a higher nitrogenUnder continuous, intensive grazing practices, content compared to plants growing in ungrazedwarm-season grasses recover more slowly than areas. The higher nitrogen content was attrib-cool-season grasses, especially during the spring uted to return of nitrogen to the system through(61). As a result, continuous grazing practices or urine and to the availability of nitrogen fixed bygrazing too early in the season tends to favor the legumes. In these studies legumes remainedgrowth of non-native grasses and decrease the prevalent in the more intensely grazed plotsdiversity of forages in pastures (62). while their populations decreased in the more lightly grazed paddocks (65). Continuous grazing tends to favor the YIELD growth of cool- season grasses since graz- ing animals remove the elevated growing During the first year of intensive grazing, points of native warm-season grasses increasing the intensity of cutting or grazing in- more readily than they remove the lower creases the amount of forage produced. Follow- growing points of non-native, cool-season ing grazing, photosynthesis is stimulated and grasses. plants take up more nutrients. This permits leaf regrowth in broadleaf plants and increased tillering in grasses. Increased leaf area then al- For areas with moderate rainfall, leaf area lows for greater photosynthesis. As photosyn-remaining after grazing is more critical for for- thesis and the formation of carbohydrates in-age recovery than the location of a forage plant’s crease, nutrient uptake by roots and subsequentgrowing point (J. Gerrish, personnal communi- movement of nutrients from roots to leaves alsocation). Most forbs and legumes, such as alfalfa increase. However, as more energy and nutri-and red clover, have aerial growing points rela- ents are allocated to leaf production and in-tively high up on the plant, which are easily re- creased photosynthesis, less energy and nutri-moved by grazing animals. This is not detrimen- ents are provided for root growth (63).tal to plant growth unless a majority of the leafarea or the basal portion of the plant is removed. Frequently grazed plots exhibit high bio-For optimal recovery, at least 3 to 4 inches of re- mass production and nutrient uptake dur-sidual leaf area should remain on cool-season ing the initial grazing season. But if graz-grasses while 4 to 8 inches of leaf area should ing intensity is too great, forage produc-remain for warm-season grasses following graz- tion will decrease in the following (61). Other factors that affect plant regrowth //NUTRIENT CYCLING IN PASTURES PAGE 35
  • 36. While frequently mowed or grazed plots ex- (65). Grazing or cutting pastures too short canhibit high biomass production and nutrient up- also expose bare soil to the impact of rainfall, in-take during the initial grazing season, if the in- creasing the potential for soil compaction and thetensity of grazing is too high, forage production loss of topsoil and nutrients through erosion.will decrease in following years (63, 66). This Nutrient cycling and effective nutrient use byproduction decline results from decreased plant plants depend on pasture management practicesability to take up nutrients because of decreased that minimize soil compaction, conserve organicroot growth and depletion of soil nutrients. Se- matter, and do not hinder plant regrowth follow-vere grazing will also impact plant diversity, ing grazing.since grazing during flowering removes seedheads and flowers, limiting the reseeding of for- Nutrient cycling and effective nutri-age plants (64). ent use by plants depend on pas- ture management practices that A sufficient resting period allows minimize soil compaction, conserve plants to regrow and produce ad- organic matter, and do not hinder equate leaf area for photosynthe- plant regrowth following grazing. sis. It also allows plants and soil organisms to reduce soil compac- tion and increase the availability of The nutrient content of forage plants affects nutrients through mineralization. animal feeding habits, the amount of nutrition animals obtain, and the type of manure they pro- duce. Succulent, nutritionally balanced pastures Root growth is critical for water and nutrient provide good animal productivity and cause ani-uptake. Plants can also store food reserves in mals to deposit moist manure piles (36). Ani-roots to allow for regrowth during periods of mals feeding on dry, older, or overgrazed for-stress. Plants grazed too frequently or cut too ages will obtain limited nutrient value. Manureshort have difficulty producing more leaves be- piles produced from these forages will be stiffcause of limited growth and food reserve stor- because of their high fiber content. Dry, stiffage by roots. In one study, plants that were not manure piles are difficult for soil organisms tocut until they reached eight inches tall produced decompose since there is little air within the pilemore growth than plants cut every time they (68). Conversely, animals often deposit very liq-reached two inches tall. Similarly, grasses sub- uid manureasthey begin feeding on pastures injected to continuous intensive grazing by sheep the spring after a winter of eating hay. The highproduced less vegetation than lightly grazed pas- moisture content of the pasture forages resultstures. In both cases, a longer resting period re- in a very wet manure pile that disperses acrosssulted in better plant growth, since the resting the soil. Soil organisms are able to decomposeperiod allowed plants to regrow and produce manure that has relatively high nitrogen andadequate leaf area for photosynthesis (63). Grass moisture content more readily than manure thattiller population and pasture production mark- is drier and more carbon-rich.edly increased in an extensively grazed pasturethat was fallowed for one year. This resting pe-riod allowed plants and soil organisms to reduce DIVERSITY AND DENSITY O.soil compaction and increase the availability ofnutrients through mineralization (67). PASTURE PLANTS Cutting grasses short not only depressesplant regrowth, it also increases soil temperature.As soil temperature increases so does nutrient Diverse forage mixtures of both broadleavedmineralization by soil organisms. While miner- plants and grasses use solar energy efficiently.alization is necessary to release nutrients from The shape and orientation of plant leaves affectplant and animal residues, if mineralization is how and when the plant can best conduct pho-too rapid, it can cause a loss of organic matter tosynthesis. Tall plants and upright grasses cap-PAGE 36 //NUTRIENT CYCLING IN PASTURES
  • 37. ture light at the extreme angles of sunrise and together in a Missouri Timing of grazingsunset while horizontal leaves of broadleaf plants pasture, pasture plant affects speciesuse sunlight better at midday. A combination of diversity remained high composition andtall sun-loving plants with shorter shade-toler- after three years of graz- diversity in pas-ant plants allow for the capture of both direct ing. Pastures with a di- tures.and filtered sunlight. A combination of warm- versity of forage speciesand cool-season grasses allows for effective pho- also maintained a highertosynthesis throughout the growing season. percentage of forage cover during this time thanWarm-season grasses like big bluestem are bet- pastures planted to monocultures or simple mix-ter able to grow and use solar energy at tempera- tures of forages (72). Productivity within pas-tures between 90 and 100°F, while cool-season tures is more stable when forages provide a di-grasses like tall fescue grow best between 75 and versity of function and structure, such as height,90°F (32). root growth habit, life cycle, and habitat require- ments (4). PERSISTENCE OF PASTURE LEGUMES Maintaining legumes as part of the forage Nitrogen transfer between grasses and le-mix is necessary if nitrogen fixation is to provide gumes is greatest when there is a closemost of the nitrogen input to the pasture system. population balance between these speciesLegumes with a deep taproot and a woody and they are growing close, such as alfalfa, red clover, and birdsfoottrefoil, are able to persist in a well-drained pas-ture because they are able to obtain water and Nitrogen fixation is directly related to the abil-nutrients from deep below the soil surface. They ity of legumes to accumulate energy through pho-also tolerate drought and cold, and are able to tosynthesis. Thus, leaf removal decreases nitro-regrow unless their growing points are elevated gen fixation, and leaf regrowth increases the po-and exposed to defoliation. Rotational grazing tential for nitrogen fixation. Legumes not onlyhas been shown to increase the proportion of red fix nitrogen for their own needs, but are also ableclover and alfalfa in mixed pastures (69). to supply nitrogen to non-nitrogen-fixing forage White clover has rhizomes rather than a tap- crops. They primarily supply nitrogen to forageroot. This growth habit allows it to colonize bare plants following decomposition. Pastures domi- soils (64) by form- nated by clover produce around 200 pounds ni- .igure 12. ing additional trogen per acre per year through nitrogen fixa- Root Growth. plants through the tion. growth of stolons. Legumes can also provide nitrogen to com- White clover is panion grass species during the growing season. competitive with In New Zealand, perennial ryegrass obtained 6 grass at low pro- to 12% of its nitrogen from associated white clo- duction densities ver. Alfalfa and birdsfoot trefoil provided up to while legumes 75% of the nitrogen used by reed canarygrass in with taproots are Minnesota. This nitrogen transfer occurs when more competitive roots die, nodules detach, or neighboring grasses at high production and legumes become interconnected by their roots densities (70). or through mycorrhizae. Nitrogen transfer be- The diversity tween grasses and legumes is greatest when of forage species there is a close population balance between these also affects the species and they are growing close together (15). Root growth of alfalfa un- persistence of le- In the first year of legume establishment, nitrogen der irrigated (left) and dry gumes within a transfer is relatively low and is derived predomi- (right) conditions (Weaver, pasture. When six nantly from nodule decomposition; it increases in Reference 71). to eight forage spe- the second year as direct-transfer mechanisms cies were planted through mycorrhizae become established (14). //NUTRIENT CYCLING IN PASTURES PAGE 37
  • 38. NUTRIENT USE EFFICIENCY (73). Due to their high nitrogen content, decom- position of residues from these plants stimulates A diverse plant community uses soil nutri- the mineralization or release of nutrients into theents more effectively than a monoculture or soil solution. Cool-season and warm-season for-simple plant mixtures. Native grasses have a ages grow and take up nutrients at different timeslower requirement for nitrogen and subsequently of the year. A combination of cool- and warm-a lower concentration of nitrogen in their leaf tis- season forages ensures a relatively even uptakesue compared to non-native cool-season grasses. of nutrients throughout the growing season.As a result, these grasses thrive under low nutri- Just as a diverse plant canopy covers the en-ent conditions but they provide lower-quality tire soil surface, a diversity of root systems occu-feed and recycle nutrients more slowly back to pies the entire soil profile, from the soil surfacethe soil system. The low nitrogen content of the down as far as 15 feet. Grasses generally haveplant litter results in slow decomposition, immo- fine bushy roots. Legumes such as alfalfa or redbilization of nitrogen by organisms involved in clover have taproots. Some plants have longerdecomposition, and a decrease in the nitrogen or deeper root systems while other plants have aavailable for plant uptake. root system that grows primarily in the surface soil. Pastures that contain plants with a diver- Just as a diverse plant canopy covers the sity of root systems will be better able to harvest entire soil surface, a diversity of root sys- and use nutrients from the soil than a less di- tems occupies the entire soil profile. verse community. Plants with more shallow roots are effective in recycling nutrients released through the decomposition of thatch and manure Broadleaf plants require higher nitrogen in- on the soil surface, while deep-rooted plants areputs for productive growth and have higher ni- able to scavenge nutrients that have been leachedtrogen content in their plant tissues than grasses down through the soil profile. Manure is unevenly distributed, concentrated near the fencelinePAGE 38 //NUTRIENT CYCLING IN PASTURES
  • 39. Table 14. Enhance Nutrient Balances within the .arm and across Paddocks.Balance nutrient inputs and outputs• Replenish nutrients removed by grazing animals• Recognize that feed supplements, particularly for dairy cows, represent significant nutri- ent inputs onto farms• Replace nutrients based on a comprehensive nutrient management plan that takes into account prior manure additions, nitrogen contributions from legumes, and soil tests• Apply manure based on the phosphorus needs of forages in order to avoid phosphorus build up on pastures; rely on legumes to supply much of the nitrogen needed for forage growthPromote even distribution of manure nutrients across paddocks• Subdivide pastures to distribute congregation areas among several paddocks• Keep paddock dimensions as close to square as possible• Provide animals with water in every paddock; avoid use of laneways to access water• Locate nutrient supplements, shade, water, and pest-control devices far apart from one anotherEnhance nutrient availability• Enhance growth of soil organisms involved in the decomposition of manure by maintain- ing good soil quality and minimizing use of soil-applied insecticides and high-salt fertiliz- ers• A combination of cattle and sheep enhances the amount of land available for grazing since sheep graze closer to cattle manure than cattle do and feed on coarser vegetation than cattle will useEncourage a diversity of forage species within paddocks• Maintain a diversity of forages representing a variety of leaf and root growth habits, life cycles, and habitat preferences• Rotate pastures while at least 4 inches of the leaf area remains. This allows plants to regrow rapidly and roots to recover• Maintain a high percentage of legumes in the forage mix by not overgrazing and by minimizing nitrogen fertilizer additions• Provide paddocks with sufficient rest time to allow forages to regrow //NUTRIENT CYCLING IN PASTURES PAGE 39
  • 40. Chapter 4: The Soil .ood Web and Pasture Soil Quality ganisms, ranging from visible insects and earth- DIVERSITY O. THE worms to microscopic bacteria and fungi. An acre of living soil may contain 900 pounds of SOIL .OOD WEB earthworms, 2400 pounds of fungi, 1500 pounds of bacteria, 133 pounds of protozoa, and 890 Soil organisms play a critical role in nutrient pounds of arthropods and algae, as well as smallcycling. Not only are they responsible for de- mammals. The term soil food web refers to thecomposing organic matter, forming soil aggre- network of dynamic interactions among these or- gates, solubilizing min- ganisms as they decompose organic materials eral nutrients, and ad- and transform nutrients. Soil food web justing soil pH; they are While some of the organisms in this diverse refers to the net- also responsible for ni- community are plant pests, many more serve as work of dynamic trogen fixation, nitrifi- antagonists of plant pests and diseases. Other interactions cation, phosphorus up- soil organisms, particularly bacteria, are able to among these or- take through my- use toxic chemicals, such as pesticides, as a source ganisms as they chorrizae, degradation of food. As they consume these toxic chemicals, decompose or- of soil minerals, and they break them down into substances, such as ganic materials formation of plant hor- carbon dioxide, water, and atmospheric nitrogen, and transform mones. A healthy soil that are either non-toxic or less-toxic to plants, nutrients. contains millions of or- animals, and humans. .igure 13. .oodweb of Grassland Soil. T Root-feeding Springtails T nematodes Roots TT Fungus-feeding T T T T mites Predatory mites T T Mycorrhizae T Fungus-feeding T TT Soil organic matter & T Fungi nematodes T Predatory nematodes T T T residues T T Bacteria Flagellates T Amoebas T Bacteria-feeding nematodes From Killham, 1994 (Reference #74) ents. Primary decomposers make greater use of ORGANIC MATTER DECOMPOSITION carbon than of nitrogen in their growth and res- piration processes. As a result, the feces and casts deposited by them have a lower carbon content SOIL ORGANISMS and a lower ratio of carbon to nitrogen than the Many soil organisms are involved in the de- original organic matter. By transforming organiccomposition of organic matter. Larger soil or- matter into a simpler chemical form as well asganisms, including small mammals, insects, and physically breaking it down into smaller pieces,earthworms, are primary decomposers, involved in primary decomposers make these materials morethe initial decomposition and cycling of nutri- available to microorganisms or secondary decom-PAGE 40 //NUTRIENT CYCLING IN PASTURES
  • 41. posers for further breakdown. Because the dead The type of organic matter will influence thebodies of earthworms and insects are high in ni- type of soil organisms involved in the decompo-trogen, they are easily decomposed by soil mi- sition process. As each decomposer feeds, it usescroorganisms (75). Fungi and bacteria are pre- some nutrients for its own growth and reproduc-dominant secondary decomposers, but algae, tion and releases other nutrients into the soil so-protozoa, amoebas, actinomycetes, and nema- lution where they are available for plant growthtodes also play important roles in transforming and production. Decomposer organisms maysoil nutrients. also excrete organic materials that can either be The chemistry of organic further broken down bymaterials and environmental other soil organisms orconditions determines how The chemistry of organic materials become part of the soilrapidly organic matter is bro- and environmental conditions deter- humus.ken down, which soil organ- mines: In general, bacteriaisms are involved in the de- • how rapidly organic matter is require more nitrogen incomposition process, and broken down order to break down or-whether organic matter de- • which soil organisms are in- ganic matter than docomposition will cause an ini- volved in the decomposition pro- most fungi. Fungi aretial decrease or increase in cess the dominant decom-available nutrients. The soil • whether nutrient availability will poser in forest environ-environment determines increase or decrease in the short ments since they requirewhich soil organisms are term less nitrogen in their dietdominant and which soil or- and are able to feed onganisms are less active. Some woody, older, or morebacterial species thrive under flooded, anaero- fibrous materials. They are also able to survivebic conditions but most soil organisms require and replicate in environments that are less moistaccess to oxygen. Earthworms and some soil in- than those required by bacteria. Bacteria are moresects require soil that is aggregated and relatively prevalent in garden and pasture environmentsuncompacted so they can burrow through it. because they require higher amounts of nitrogenEnvironments with limited nitrogen availability and moisture, and because they feed readily onare dominated by organisms that are able to fix fresh manure, young grasses, legumes, and othernitrogen from the atmosphere, such as algae, li- easy-to-decompose materials (10).chens, and rhizobia associated with legumes.Many soil organisms are killed by non-specificinsecticides as well as by highly concentrated fer- PRIMARY DECOMPOSERStilizers such as anhydrous ammonia. As discussed previously, organic materials Earthworms are primary decomposers of leafthat are old or woody, such as tree branches, old litter and manure piles. Research conducted inroots, or dried grass, contain a large amount of Denmark showed that earthworms were respon-carbon compared to nitrogen. To decompose sible for 50% of the breakdown and disappear-these materials, soil organisms may need to ex- ance of cow manure, while dung beetle larvaetract nitrogen from the soil solution in order to accounted for between 14 and 20% (76). Thesebalance their carbon-rich diet, thus temporarily organisms also consume fresh organic materials,reducing the amount of nitrogen available for then deposit their feces in the soil. When theyplant use. Young, succulent, “first-growth” plant burrow, they move manure and other organicmaterials, fresh manure, and materials that havegone through primary decomposition processes Earthworms and dung beetles are visibleby larger soil organisms contain a higher con- indicators of soil health: their presencecentration of nitrogen in relation to carbon. Soil shows that nutrient decomposition pro-organisms more readily decompose these mate- cesses are occurring and the soil foodrials and make the nutrients in them available web is effectively operating.for plant uptake. //NUTRIENT CYCLING IN PASTURES PAGE 41
  • 42. materials into the soil, where it is more acces- mix the top six inches of a humid grassland soilsible to other organisms involved in decomposi- in 10 to 20 years (80).tion. Burrowing organisms also aerate the soil. Factors that contribute to an abundant popu-Good aeration promotes the growth of the ma- lation of earthworms include inputs of fresh or-jority of organisms involved in organic matter de- ganic matter, a medium-textured soil, thick top-composition. For this reason, earthworms and soil, a near-neutral pH, moist but well-aerateddung beetles are visible indicators of soil health: soil, and moderate temperatures. Tillage, acid-their presence shows that nutrient decomposi- producing fertilizers, insecticides, and poorly-tion processes are occurring and the soil food web drained soils inhibit earthworm survival (79).is effectively operating. DUNG BEETLES EARTHWORMS Dung beetles According to research studies, the weight of improve nutrient An adequate popula-earthworms in the soil is directly related to pas- cycling, enhance tion and mix of dungture productivity (77). In healthy soils with abun- soil aeration, and beetle species candant earthworms, these or- improve forage remove a completeganisms consume between growth while feed- dung pile from the65 and 80 tons of manure ing on manure and soil surface within 24per acre per year (39). using it to provide hours.Earthworms also break housing and fooddown pasture thatch and for their young.incorporate organic matter Adult dung beetlesfrom the thatch into the are drawn to manure by odor. They use the liq-soil. Where few or no earthworms are present, uid contents for nourishment and the roughagepastures develop a thick thatch layer, slow rates to form a brood ball in which the female lays aof organic matter decomposition, and a poor single egg. This brood ball is buried in the soilcrumb structure (39). where the larva grows, eating about 40 to 50% of Decomposition of organic the interior contents of thematter by earthworms speeds Through their feeding and bur- ball while depositing its ownup the breakdown and release rowing activities, earthworms excrement. After the larvaof plant nutrients, particularly • break down large residues emerges, secondary decom-nitrogen and phosphorus. • produce nutrient-rich casts posers readily break downEarthworms consume low-ni- • move organic matter through the remaining dung ball (81).trogen plant materials as well the soil An adequate populationas high-nitrogen manure (39). • enhance soil aeration, water and mix of dung beetle spe-Under pasture conditions, infiltration, and soil structure cies can remove a completeearthworms have been shown • improve root growth dung pile from the soil sur-to mineralize 10 pounds per face within 24 hours (82).acre per year of phosphorus This process decreases the po-in their casts (5). Earthworms also facilitate the tential for ammonia volatilization and nutrienttransformation of straw and leaf litter into soil runoff while making manure nutrients availablehumus (78). The earthworm gut combines de- to secondary decomposers within the soil pro-composed organic matter with particles of min- file. While moving dung into the soil, dungeral soil and microorganisms, forming soil ag- beetles create tunnels that en-gregates and humus-coated soil minerals. hance soil aeration and water Through their feeding and burrowing activi- infiltration. Dung removalties, earthworms move organic matter through also increases forage availabil-the soil enhancing soil aeration, water infiltra- ity, since it minimizes the ar-tion, and soil structure. They also improve root eas that animals are avoidinggrowth by creating channels lined with nutrients because of the presence of ma-(79) and help till the soil. They can completely nure.PAGE 42 //NUTRIENT CYCLING IN PASTURES
  • 43. Environmental conditions that enhance activi- carbon-rich forms of organic matter. They alsoties of dung beetles include adequate soil mois- form soil aggregates by binding them with fun-ture levels and warm temperatures. Dung beetle gal threads or hyphae.larvae are susceptible to some insecticides used Mycorrhizal fungi en-for fly and internal-parasite control for cattle. hance the nutrient andBoth injectable and pour-on formulations of water uptake of plantsIvermectin (Ivomec and Doramectin), applied to by extending thecattle at the recommended dosages, reduce sur- length and surface-vival of the larvae for one to three weeks. How- area of root uptake.ever, when administered as a bolus, effects on In dry rangelands,dung beetle populations last up to 20 weeks (83). crusts composed of green algae, bacteria, cyanobacteria, lichens, SECONDARY DECOMPOSERS and fungi form over the soil surface. These crusts provide surface Soil organisms are not only responsible for cover, erosion control, and soil aggregation. Theythe mineralization and release of nutrients from are also involved in nitrogen fixation and nutri-organic material; they are also important for re- ent decomposition. Crust organisms are mosttaining nutrients in the soil, improving soil struc- active during the cooler, moister part of the yearture through the formation of aggregates and when plant cover is minimal (2).humus, degrading toxic substances, and sup- AMOEBAS, NEMATODES, AND PROTOZOApressing diseases. Nutrients held in the bodiesof soil organisms gradually become available for Amoebas, nematodes, and protozoa feed onplant uptake and meanwhile they are protected bacteria and fungi. Nematodes may consumeagainst being lost through leaching, runoff, or up to 25% of the bacteria in the soil (84). Accord-other processes. Soil organisms involved in nu- ing to one study, nematodes feeding on bacteriatrient cycling release nutrients as they defecate accelerated litter decomposition by 23% (85). Bothand die. While they are still alive, these organ- protozoa and nematodes release nutrients to theisms conserve nutrients within their bodies. soil system, making them available to plants and other soil organisms. MUTUALISTIC RELATIONSHIPS Soil microorganisms are responsible for • mineralizing nutrients In undisturbed ecosystems, plants and soil • retaining nutrients in the soil organisms have coevolved to form mutualistic • forming aggregates relationships. Plants provide carbohydrates and • degrading toxic substances other nutrient-rich substances through their root • suppressing diseases system, providing an excellent source of food for soil organisms. As a result, populations of soil organisms involved in nutrient decomposition are BACTERIA AND FUNGI greatest next to plant roots (85). These organ- Bacteria and fungi are the most prevalent soil isms provide plants with nutrients necessary fororganisms. Bacterial decomposers feed on root their growth, produce hormones and other chemi-exudates as well as on plant litter and manure. cals that improve plant vigor, and protect the plantMaintaining actively growing soil roots provides against diseases. When the plant’s need for nu-a nutrient-rich habitat for the growth of many trients is low, soil organisms will hold nutrientsbacterial species. Some species of bacteria are in their bodies rather than release them into theable to detoxify pollutants while other species, soil solution (10). This mutualistic relationshipparticularly rhizobia and cyanobacteria is disturbed by cultivation and harvesting. When(“bluegreen algae”), are able to fix nitrogen. Bac- plant roots are removed, populations of soil or-terial gels are an important component of soil ganisms decrease since they no longer have aaggregates. Fungi decompose complex or more source of nourishment and habitat. //NUTRIENT CYCLING IN PASTURES PAGE 43
  • 44. mation, degradation of toxins, creation of soil SOIL ORGANISMS pores, and absorption of water and nutrients are all functions of soil organisms. AND SOIL HEALTH The activities of soil organisms serve as ef- fective indicators of current land productivity Soil health refers to the ability of soils to func- and its ability to withstand degradation. Bytion as a productive environment for plant monitoring these indicators, farmers, soil conser-growth, an effective filter, and an efficient regu- vationists, and other land managers can imple-lator of water flow. Soil mineralogy and chem- ment appropriate practices to minimize soil oristry form the basis for soil composition and soil nutrient losses, enhance nutrient cycling, andhealth. However, much of soil health and func- increase plant productivity. The Soil Qualitytion depends on an active community of diverse Institute has taken the lead in developing andsoil organisms. Nutrient cycling, aggregate for- promoting the use of soil health indicators (86). Table 15. Pasture Soil Health Card. Indicator Good Medium Poor Complete cover of for- Limited bare patches. Extensive bare patches Pasture cover ages and litter over en- No extensive bare ar- especially near watering tire pasture. eas near drainage ar- or other congregation ar- eas. eas. Diversity of plant spe- Limited number of Less than three different Plant diversity cies, including forbs, le- species and limited species, or invasive spe- gumes, and grasses, diversity of growth cies are a major compo- and differences in leaf habit. Some invasive nent of the plant mix. and root growth habits. plants present. Plant roots Abundant vertical and More horizontal roots Few roots; most are hori- horizontal roots. than vertical. zontal. Soil life – Many dung beetles and Few dung beetles and No dung beetles or macroorganisms earthworms present. earthworms present. earthworms present. Wire flag enters soil Wire flag pushed into Wire flag cannot be Soil compaction easily, and does not en- soil with difficulty, or pushed into soil. counter hardened area encounters hardened at depth. area at depth. No gullies present; wa- Small rivulets pre- Gullies present; water Erosion ter running off pasture sent; water running running off pasture is is clear . off pasture is some- very muddy. what muddy. Soil in clumps; holds to- Soil breaks apart af- Soil breaks apart within Soil aggregation gether when swirled in ter gentle swirling in one minute in water. water. water. Water soaks in during Some runoff during Significant runoff during Water infiltration moderate rain; little run- moderate rainfall, moderate rainfall; much off or water ponding on some ponding on soil water ponding on soil soil surface. surface. surface. Adapted from the Georgia, Mon-Dak, and Pennsylvania Soil Health Cards (86) Sullivan (88) and USDA (89).PAGE 44 //NUTRIENT CYCLING IN PASTURES
  • 45. Qualitative “farm-based” and “farmer friendly” of concern, and enhance awareness of the rela-indicators are incorporated into soil health cards tionships between soil health and crop produc-specific to location and farming practice (87). tion. Below is a soil health card for pastures basedThese cards can be used to monitor the relative on a compilation of indicators from soil healthhealth and productivity of soils, identify areas cards developed in various locations. Table 16. Soil Organisms. Bacteria – the most numerous microorganism in the soil. Every gram of soil contains at least a million of these tiny one-celled organisms. Decompose simple or nitrogen-rich organic matter. Require moist environments. Also responsible for nitrogen fixation, soil aggregate formation, and detoxification of pollutants. Actinomycetes – thread-like bacteria, which look like fungi. They are decom- posers and are responsible for the sweet, earthy smell of biologically active soil. Fungi – multicelluar microorganisms that usually have a thread-like structure. Mycorrhizae form extensions on roots, increasing their ability to take up nutri- ents and water. They also transport nitrogen from legumes to grasses. Yeasts, slime molds, and mushrooms are other species of fungi. Algae – microorganisms that are able to make their own food through photosyn- thesis. They often appear as a greenish film on the soil surface following a rainfall. Protozoa – free-living animals that crawl or swim in the water between soil particles. Many soil protozoan species are predatory and eat other microorgan- isms. By feeding on bacteria they stimulate growth and multiplication of bacteria and the formation of gels that produce soil aggregates. Nematodes – small wormlike organisms that are abundant in most soils. Most nematodes help decompose organic matter. Some nematodes are predators on plant-disease-causing fungi. A few species of nematodes form parasitic galls on plant roots or stems, causing plant diseases. Earthworms – multicellular organisms that decompose and move organic mat- ter through the soil. Earthworms thrive where there is little or no tillage, espe- cially in the spring and fall, which are their most active periods. They prefer a near neutral pH, moist soil conditions, an abundance of plant residues, and low light conditions. Other species of soil organisms – Many other organisms, including dung beetles, sowbugs, millipedes, centipedes, mites, slugs, snails, springtails, ants, and birds facilitate nutrient cycling. They make residues more available to smaller organisms by breaking them down physically and chemically and by burying them in the soil. //NUTRIENT CYCLING IN PASTURES PAGE 45
  • 46. Table 17. Pasture Management Practices to Maintain a Healthy Soil .ood Web. Provide soil organisms with a balanced diet • Manure and perennial pastures provide food for soil organisms • Succulent materials that are more nitrogen-rich are more rapidly decomposed than materials that are older and woodier and contain less nitrogen Provide soil organisms with a favorable environment • Most beneficial soil organisms prefer a well-aerated environment • Decomposer bacteria generally prefer an environment that is moist, has a near neutral pH, and has easy-to-decompose materials • Decomposer fungi generally prefer an environment that is acid, moderately dry, and has more carbon-rich, complex organic materials • Continuous plant growth maintains environment of actively growing roots in the soil. The root or rhizosphere environment is a very nutrien-rich habitat for the growth of many soil organisms Use practices that favor the growth of soil organisms • Maintain a balance between intense grazing and adequate rest or fallow time • Encourage movement of grazing animals across pastures to feed and distribute manure evenly as well as to kick and trample manure piles • Maintain a diversity of forage species to provide a variety of food sources and habitats for a diversity of soil organisms Avoid practices that kill or destroy the habitat of soil organisms • Avoid the use of Ivomectin deworming medications, soil-applied insecticides, and concentrated fertilizers such as anhydrous ammonia and superphosphate • Minimize tillage and other cultivation practices • Minimize practices that compact the soil, such as extended grazing practices or grazing wet soilsPAGE 46 //NUTRIENT CYCLING IN PASTURES
  • 47. Chapter 5: Pasture Management and Water Quality Judicious applications of fertilizers and ma- off. High levels of phosphorus in surface waternure enhance plant growth. However, if nutri- cause eutrophication and algal blooms. Whenents are applied at the wrong time or in excess of sources of drinking water have significant algalwhat plants can use, they increase the potential growth, chlorine in the water-treatment processcontamination of nearby rivers and lakes. Simi- reacts with compounds in the algae to producelarly, grazing practices can degrade water qual- substances that can increase cancer risks.ity if grazing intensity is too great, if paddocks Unlike phosphorus, nitrogen does not readilyare used when the soil is too wet, or if the dura- bind to soil minerals or organic matter. As a re-tion of rest periods is too short. Long-term in- sult, it easily leaches through the soil, especiallytensive grazing practices can negatively affect if high rainfall follows manure or nitrogen fertil-water quality, especially when combined with izer applications and the soil is sandy or grav-heavy fertilization with either mineral or manure elly. High levels of nitrate in ground water usednutrients. Likely impacts include contamination for drinking can cause health problems for hu-of groundwater with nitrates and contamination man babies and immature animals. Managementof surface water with phosphate, sediments, and practices that minimize the potential for nitrogenpathogens (90, 91). leaching include not applying excessive nitrogen and avoiding manure or nitrogen fertilizer appli- RISK .ACTORS cations during times when plants are not actively growing. .OR NUTRIENT LOSSES Erosion occurs when water or wind moves soil particles, resulting in the loss of topsoil and of the nutrients, toxins, and pathogens attached NUTRIENT LOSS PATHWAYS to these particles. Erosion by water can also trans- If more manure or fertilizer nutrients are ap- port surface-applied manure into lakes, rivers,plied to pastures than are used in the growth of and streams. Water quality concerns associatedforage crops, excess nutrients will either accu- with erosion include siltation, fish kills, eutrophi-mulate in the soil or be lost through leaching, cation, and degraded quality for recreational andrunoff, or erosion. Nutrient accumulation occurs drinking-water uses.when minerals in the soil have the ability to bind NUTRIENT BALANCESor hold particular nutrients. Sandy or silty soilsor soils with a near-neutral pH do not bind phos- Water contamination problems associatedphorus well. When more phosphorus is applied with farming are becoming an increasing societalto these soils than is used for plant growth, the and political concern. The Federal Clean Waterexcess phosphorus can easily be dissolved and Act mandates states to minimize non-point-sourcecarried away by runoff water to lakes and pollution or pollution associated with runoff andstreams. Both acid clay soils and soils with a erosion, much of this originating from agricul-high calcium carbonate content have a strong abil- tural lands (92). Currently, water quality regu-ity to bind large amounts of phosphorus. If only lations are primarily focused on larger farms thatmoderate excesses of phosphorus are applied to have a high concentration of animals and use highthese soils or if excess phosphorus is applied to inputs of purchased feeds. Societal concernsthe soil only occasionally, these soils will be able about farming operations are increasing as moreto bind the excess phosphorus and hold it against non-farm families move into rural areas and ur-leaching. However, if phosphorus fertilizers or ban growth decreases the distance between farmmanure are continually applied at high rates, and non-farm community members.phosphorus levels will eventually build up in the On farms that have high numbers of animals,soil to the extent that soils will no longer be able a limited land area, and high use of feeds thatto hold onto the additional phosphorus and these are not grown on the farm, nutrient imbalancesexcesses will be susceptible to loss through run- exist. This is because the amount of nutrients //NUTRIENT CYCLING IN PASTURES PAGE 47
  • 48. that accumulate in the animal manure is cern associated with manure management. Ani-greater than the needs of all crops being grown mal grazing and manure applications can con-on the farm. Maintaining a balance between the taminate water bodies not only with excess nu-amount of nutrients added to the soil as manure trients but also with parasites in feces. Parasitesand fertilizer and the amount of nutrients removed of greatest concern are E. coli, Giardia, andas forages, hay, crops, or livestock is critical for Cryptosporidium. E. coli is of most concern to ru-productive crop growth and water quality pro- ral residents dependent on well water and of lim-tection. If more nutrients are removed than are ited concern to public water users since this para-returned to the system, crop production will de- site is killed by municipal water purification andcline. If more nutrients are added than can be treatment processes. Typically, this parasiteused for productive crop growth, nutrients will causes mild to moderate gastrointestinal prob-build up in the soil, creating a high risk for leach- lems. However, new strains of E. coli have killeding, runoff, and water contamination. people who are very young, very old, or have While environmental regulations primarily weakened immune systems. Giardia andtarget large farms, these are not the only live- Cryptospordium are pathogens with a dormantstock operations at risk for contaminating water stage that is very resistant to purification treat-quality. Often, smaller livestock farms pose more ment. Almost all municipal water treatment fa-risk than larger operations. For instance, on cilities are required to use secondary filtrationsmaller dairy farms the barn is commonly located processes that remove these resistant forms fromnear a stream because it was built prior to rural the water supply. Most private wells, however,electrification and the ability to pump water from do not have the capability of filtering out thesewells or streams to watering troughs. On many pathogens. Like the virulent strain of E. coli, Gia-small livestock operations, animals have access rdia and Cryptospordium cause gastrointestinalto paddocks located near a well head or over problems that can be fatal for people with weakhighly permeable soils because land area is lim- or undeveloped immune systems.ited. Riparian areas are less likely to be protected Minimizing the risk of pathogen movementby fencing or buffer areas. On farms without into water bodies involves ensuring that animals,adequate manure storage facilities, manure is of- especially young calves, are not exposed to, orten applied to poorly drained or frozen fields kept in conditions that make them susceptibleduring the winter, resulting in a high potential to, these diseases. Any manure that potentiallyfor surface water contamination. In addition, on contains pathogens should either be completelysmaller farms, necessary equipment or labor is composted before application, or applied to landoften not available to properly apply manure ac- far from streams and at low risk of erosion orcording to a nutrient management plan. Careful runoff (93).management of grazing and manure-handling PASTURE MANAGEMENT PRACTICES TOpractices is critical on all farms in order to pro-tect water resources. REDUCE RISKS OF PATHOGEN CONTAMINATION Manure or fertilizers should be applied Manure or fertilizers should be applied when when the nutrients in these materials can the nutrients in these materials can be most ef- be most effectively used for plant growth fectively used for plant growth and production, and production, and never to ground that and never to ground that is snow-covered, fro- is snow-covered, frozen, or saturated. zen, or saturated. Under such wet or frozen con- ditions, manure or fertilizer nutrients are not bound by soil particles. Instead, these nutrients PATHOGENS are lying unbound on the soil surface where they have a high potential to be carried away by run- off into lakes or streams. Pathogens in manure PATHOGENS IN MANURE applied to frozen or snow-covered soil will not Although not directly related to nutrient cy- be in contact with other soil organisms. In addi-cling, pathogens are a critical water quality con- tion, most predatory soil organisms will be in aPAGE 48 //NUTRIENT CYCLING IN PASTURES
  • 49. dormant state and unable to decrease pathogen local health departments can test wells to deter-numbers before snowmelts or heavy rainfalls mine nitrate concentrations.cause runoff. Areas that need to be protected from con-tamination by nutrients and parasites in animal PHOSPHORUS CONTAMINATIONfeces include well heads, depressions at the baseof hills, drainage ways, rivers, streams, and lakes. Phosphorus can be transported from fieldsWell heads and water bodies need protection or pastures into lakes or streams either as a com-because they serve as drinking and recreational ponent of erosion or within runoff water. Phos-water sources, while foot slopes and drainage phorus that is dissolved in runoff water has aways have a high potential for nutrient runoff greater effect on water quality than phosphorusand transport of contaminants to water bodies. that is attached to soil particles transported to water bodies by erosion (94). This is because the NITRATE CONTAMINATION dissolved phosphorus is more available for use by algae and other aquatic organisms that cause eutrophication, noxious greening of water bod- Nitrate is not held by soil particles and is eas- ies, and fish kills. Phosphorus associated withily leached, especially through porous soils, such soil particles tends to settle to the lake or riveras sandy soils or soils with cracks or fissures that bottom where it remains biologically stable orallow for rapid movement of excess nitrogen only slowly available for use by aquatic organ-through the soil profile. Where excess nitrogen not applied, nitrate leaching in pastures is mini-mal. High nitrate leaching losses were observed, DISSOLVED PHOSPHORUShowever, when orchard-grass pastures in Penn- In pastures, sources of dissolved phospho-sylvania were fertilized with 200 pounds per acre rus include manure or phosphorus fertilizers ly-of nitrogen as ammo- ing on the soil surface,nium nitrate (45). These and wet soils that have aresearchers also calcu- High nitrate leaching occurs when a severe high phosphorus concen-lated that a stocking rate drought follows good growing conditions, tration. Runoff waterfor Holstein dairy cows causing legume nodules to die and release can readily dissolveof 200 animal days nitrogen into the soil. soluble phosphorus inwould result in nitrate manure or phosphorusleaching from urine in fertilizers. When theexcess of drinking-water standards (10 mg/liter) amount of phosphorus in soil exceeds the ability(45). In pastures where nitrogen was provided of soil particles to bind onto it, the excess phos-by nitrogen-fixing legumes, nitrate leaching was phorus can readily be dissolved and transportedminimal when environmental conditions were by runoff water, especially when soils are satu-normal. But high nitrate leaching was observed rated. Dissolved phosphorus has the greatestwhen a severe drought followed good growing potential for being transported from pastures intoconditions, causing legume nodules to die and water bodies when rainfall is heavy, when highrelease nitrogen into the soil (20). levels of phosphorus are present either on the Nitrate concentrations greater than 10 ppm surface of the soil or within the soil, and whenin well water may cause nitrate toxicity or meth- pastures are located within 350 feet of water bod-emoglobinemia. This ailment, which affects in- ies (95).fant children as well as young chickens and pigs, Increasing forage diversity generally de-and both infant and adult sheep, cattle, and creases runoff potential. Care should be takenhorses, increases nitrate concentration in the to combine species, such as bunch-grasses, thatbloodstream and prevents the uptake and use of enhance water infiltration but expose the soil sur-oxygen, thus causing suffocation. Pregnant ani- face between clumps (96), with closer-growingmals that are affected may recover, then abort species such as tall fescue or prostrate specieswithin a few days (21). Personnel associated with such as white clover. A combination of native //NUTRIENT CYCLING IN PASTURES PAGE 49
  • 50. streams, which then causes algae and other nui- When the amount of phosphorus in soil ex- sance plants to grow (27). ceeds the ability of soil particles to bind onto it, the excess phosphorus can readily be dissolved and transported by runoff Table 18. Impacts of excess water, especially when soils are saturated. phosphorus on water quality.forages and low-growing, shade-tolerant plants • Excessive phosphorus stimulatescould enhance both water infiltration and cattle growth of algae and aquatic weedsproduction (D. Brauer, personal communication). in lakes and streamsSetting up paddocks on the contour can also al- • Rapid algal and aquatic-weed growthlow downslope paddocks that are regenerating depletes oxygen from water, leadingafter grazing to serve as buffer strips for upslope to death of fishpaddocks that are currently being grazed. Po- • Outbreaks of certain aquatic organ-tential runoff from manure can also be reduced isms dependent on high phosphorusby applying it to alternate paddocks set up as levels can cause health problems incontour strips (D.E. Carman, personal commu- humans, livestock, and other animalsnication). • When water that has high algal growth is chlorinated for use as drink- PHOSPHORUS ASSOCIATED WITH EROSION ing water, carcinogenic substances Soil-attached phosphorus can be transported are formedto water bodies by erosion. Low-level sheet ero-sion contributes more phosphorus than higher-impact rill or gully erosion. This is because sheet The type of phosphorus fertilizer used influ-erosion primarily transports nutrient-enriched ences the potential risk of water contamination.topsoil, manure, and plant residues while gully Highly soluble fertilizers such as superphosphateerosion transports more nutrient-poor subsoil present a greater short-term potential for phos-(27). As with runoff, the amount of phosphorus phorus loss since they are easily dissolved andtransported by erosion is greatest during intense transported. In the long term, however, less-rainfalls or snowmelts. Pasture soils that are soluble fertilizers, such as dicalcium phosphate,completely covered by vegetation are protected may pose a greater risk. This is because less-against the forces of erosion. Erosion occurs pri- soluble fertilizer remains on the soil surface andmarily when soils are bare and land is sloping. available for dissolution and runoff for a longer IMPACT OF PHOSPHORUS time (27). Impacts on water quality from sedi- CONTAMINATION ON WATER QUALITY ment-attached phosphorus fertilizer have been observed to persist for up to six months (97). The impact of phosphorus runoff on stream Runoff risks can be substantially decreased if fer-or lake water quality is greatest during summer tilizers are incorporated into soil and applied ac-and fall. While spring rains or snowmelts may cording to a nutrient management plan.transport a greater total amount of phosphorus,the large amount of water in the runoff dilutes PHOSPHORUS INDEXthe phosphorus so that it is in a relatively low The phosphorus index was developed to ad-concentration when it reaches water bodies. In dress federal and state water quality guidelinescontrast, intense rains falling on soils and pas- while recognizing that phosphorus movement istures during the summer are likely to run off influenced by local environmental conditionsrather than soaking into dry, hard soils. When and land management practices. Each state isintense rains fall on pastures with surface-ap- developing their own phosphorus index to en-plied manure or phosphorus fertilizers, runoff sure that it is appropriate to local conditions.water will carry a high concentration of dissolved Each phosphorus index contains a componentphosphorus into streams. If these streams have related to phosphorus sources, soil-test phospho-relatively low water flows, the runoff water will rus, manure phosphorus, and fertilizer phospho-create a high concentration of phosphorus in rus. (Soil-test phosphorus accounts for the plant-PAGE 50 //NUTRIENT CYCLING IN PASTURES
  • 51. available or soluble phosphorus in the soil, de- CONTAMINANT TRANSPORTrived either from the mineral base of the soil or THROUGH DRAINSfrom decomposing organic matter.) The poten-tial for manure or fertilizer phosphorus to be lost Unfortunately, the advantage that subsurfacethrough runoff depends on the amount applied, drainage provides in decreasing runoff potentialhow it was applied, and when it was applied. may be overshadowed by the ability of drainageManure and fertilizer incorporated into the soil systems to directly transport nutrients from fieldsat rates required for crop growth, and at or just to waterways. Most subsurface drainage sys-prior to the time of crop production, pose mini- tems were installedmal risk to water quality. Conversely, surface- primarily for produc- During wet periodsapplying excessive amounts of manure or fertil- tion reasons: to allow of the year, artifi-izer when crops are not actively growing or when farmers to work their cially drained fieldsthe soil is saturated, frozen, or snow-covered will fields earlier in the should be managedpose high risks for phosphorus runoff. However, year and to minimize as though they werea high concentration of phosphorus in the soil or plant stunting and not drained. Grazing,applied to the soil will not pose a risk to water disease problems as- manure spreading,quality unless there is a means of transporting sociated with satu- and fertilizer applica-this phosphorus to water bodies. Methods for rated soils. Since tions should betransporting phosphorus from farm fields to many of these sys- avoided while drainswater bodies include erosion, runoff, and flood- tems were installed are Locations that have a high source of phos- before agriculturalphorus and a high risk of transport are critical impacts on watersource areas or locations where land managers quality became a societal concern, agriculturalneed to carefully consider risks of phosphorus drains often empty directly into streams and riv-losses. ers. Because artificial drainage makes fields drier, .igure 14. Phosphorus Index farmers can drive tractors or other equipment Components. onto these fields earlier in the year. Farmers who have a full manure-storage system to empty, or Phosphorus Phosphorus who have time constraints for spreading manure Source Transport in advance of planting, may be tempted to apply • Soil test P manure and fertilizers to these fields during times • Manure and fertilizer P • Soil erosion when the soil would be wet if it were not drained. T • Application method and • Water runoff timing • Flooding frequency Farmers with a limited land base may want to • Grazing manage- graze these fields when the weather is wet. How- ment ever, to protect water quality, these fields should be managed as though artificial drainage had not Critical Source Area for P been installed. Nutrients in fertilizers or manure can leach through the soil to drainage pipes. If artificially drained fields are used for nutrient ap- plications or grazing while water is flowing out SUBSUR.ACE DRAINAGE of drainage outlets, drainage water can carry these leached nutrients directly to streams or riv- ers. Artificial subsurface drainage makes normally In soils with subsurface drainage, cracks andwet soils drier, and decreases the wet period for channels provide a direct pathway for nitrate,seasonally wet soils, by allowing more water to phosphorus, or soluble manure to move from theseep into the soil profile. Subsurface drainage soil surface to subsurface drains (6, 78). Becausehas been shown to decrease water runoff by 72% these channels are relatively large, contaminantsand total phosphorus losses due to runoff by 50% are not absorbed by soil particles or biologically(5). treated by soil organisms (27). Soil cracks or //NUTRIENT CYCLING IN PASTURES PAGE 51
  • 52. ers to capture sediments and absorb runoff wa- Tile drainage water should be directed to a ter. grassed filter or buffer area, or treated in a Riparian buffers are limited in their ability to wetlands area where biological and remove soluble phosphorus and nitrate from run- chemical processes lower contaminant off water, especially if flows are intense (98). levels. During heavy rainstorms or rapid snowmelts,channels develop through earthworm burrowing, buffers generally have limited effectiveness fordeath and decomposition of taproots, and soil controlling the movement of runoff-borne nutri-drying. The direct connection of cracks or chan- ents into water bodies. This is because waternels in soils to artificial drainage pipes can trans- from these heavy flows concentrates into rapidlyport phosphorus and pathogens from manure ap- moving channels that can flow over or throughplications directly to drainage outlets within an buffer areas.hour after the onset of a heavy rain (6). One re-search study showed that a single rotational graz- As phosphorus is continually transporteding event doubled the amount of sediment and into buffers, soils in the buffer area willincreased the amount of dissolved phosphorus eventually lose their ability to hold addi-in tile drainage water 15-fold compared to an tional phosphorus, thus limiting their ef-ungrazed site (5). fectiveness to control phosphorus move- ment into streams. MANAGEMENT PRACTICES To minimize the potential for water contami- The continual transport of phosphorus-richnation, land that is artificially drained should not sediments into buffers will cause a buildup ofbe grazed or have fertilizer or manure applied high concentrations of phosphorus in buffer ar-during times when drainage water is flowing from eas. Eventually, these areas will lose their abilitythe field or just prior to a rainstorm. Alterna- to hold additional phosphorus. Buffer areas cantively, contaminated water flowing out of tile actually become a source of phosphorus enteringdrains should not be allowed to empty directly water bodies, rather than an area that capturesinto rivers or streams. Instead, it should be di- phosphorus before it enters water bodies (99).rected to a grassed filter or buffer area, or treatedin a wetlands area where biological and chemicalprocesses lower contaminant levels through sedi- RIPARIAN GRAZINGmentation and absorption (27). When grazing animals have continuous, un- RIPARIAN BU..ERS limited access to riparian areas, their activities break down stream banks, alter stream flow, A well-designed buffer with a combination cause decreased vigor of stream-bank vegetation,of trees, shrubs, and herbaceous plants has the and diminish the species diversity and popula-ability to trap sediments and nutrients associated tions of fish and aquatic wildlife (100, 101). Cattlewith the sediments. Buffers also provide habitatfor river-bank and aquatic animals. An effectivebuffer for trapping sediments contains a combi-nation of grasses and herbaceous plants that areable to catch sediments in their foliage or resi-dues. The root channels around actively grow-ing plants will also absorb slow-moving runoffwater and plants in the buffer area will use trans-ported nutrients for their growth. Regular har-vest and removal of buffer vegetation can delayor prevent the buildup of nutrients in the bufferarea. However, harvests must be conducted in amanner that does not decrease the ability of buff-PAGE 52 //NUTRIENT CYCLING IN PASTURES
  • 53. grazing in riparian areas trample on streamside growth, bank protection, and sediment entrap-and aquatic organisms, disturb the habitat of these ment (104).organisms, decrease oxygen availability by sus- Pasture management practices should dis-pending bottom sediments, and contaminate courage animals from congregating in the streamstreams by directly depositing manure and urine or on the stream bank, where their manure can(102). Animal movement along streambanks or pollute water. Shade, salt licks, and other sourceswithin streams also contributes to bank erosion. of supplemental nutrients should be located atIn addition, grazing activities alter the amount least 15 feet from the stream bank to provide aand type of plant residues available for the growth buffer between areas of manure deposition andand reproduction of riparian organisms (90). the stream (100). The season in which riparian areas are grazed Limiting animal access to riparian areas is also an important consideration if water qual- allows a thick vegetative turf to develop ity is to be protected. Grazing in the spring or throughout the paddock, which stabilizes early summer followed by complete livestock stream banks and reduces stream-bank removal in the summer allows riparian plant re- erosion. growth to occur before the dormant period in the fall. Animals will damage stream banks if they are allowed to graze riparian areas in the Managed grazing of riparian areas can pro- winter when soils are freezing and thawing or intect water quality and improve riparian habitat. the spring when soils are wet. During droughtIn Wisconsin, researchers studying intensive ro- conditions, streambanks should not be grazedtational grazing practices restricted livestock ac- since vegetation will be slow to recover. Ani-cess to riparian areas to 5 to 20 days per season. mals should not be allowed to graze riparian ar-Limiting animal access to riparian areas allowed eas in the summer, when hot dry conditions woulda thick vegetative turf to develop throughout the encourage cattle to congregate in the water (103).paddock, which stabilized stream banks and re- Stream banks that have a high soil moisture con-duced streambank erosion (102). For dairy cattle, tent or a fine soil texture, or that are prone toeach grazing period should last only 12 to 24 erosion, are subject to early-season grazing dam-hours, while beef cattle and sheep can be grazed age and should not be grazed in the spring or notfor 3 to 4 days each time (103). Grazing should until they have dried (104). Use of floating fencesnot be allowed to reduce herbage stubble to less and graveled access areas can control animal ac-than 4 to 6 inches in height. This will protect cess to water, minimizing the impacts on stream-water quality by providing adequate plant bank stability and ecology. //NUTRIENT CYCLING IN PASTURES PAGE 53
  • 54. Table 19. Pasture Management Practices to Protect Water Quality. Minimize congregation of animals in pastures • Use practices that encourage the movement of animals across paddocks • Avoid overgrazing of pastures Minimize the potential for nitrate leaching • Encourage animal movement across paddocks • Maintain a healthy cover of actively growing forages across paddocks • Rotate pastures to maximize nutrient uptake by plants Minimize the potential for nutrient runoff • Do not apply fertilizers or manure to saturated, snow-covered, or frozen ground • If possible, compost manure before applying it to soil. This will minimize pathogen populations while transforming nutrients into more stable compounds • Do not use pastures that are wet, flooded, or saturated • Use practices that favor populations of soil organisms that rapidly incorporate ma- nure into the soil • During cold or wet weather, do not use pastures that are located next to a river, stream, or waterway • Recognize that buffers are not effective in controlling the movement of nutrients car- ried by runoff water, especially when flows are intense Minimize the potential for erosion • Maintain a complete cover of forages and residues across the surface of all pad- docks • Use practices that minimize the congregation of animals or the repeated trampling of animals on the same lounging area or pathway • Riparian areas should only be grazed using short-term intensive grazing practices, and then only during spring and early summer • Maintain riparian buffers (including a combination of herbaceous plants, trees, and shrubs) adjacent to rivers, streams, and lakes to act as a filter for eroded soil and other contaminants Minimize water contamination from artificial drainage systems • During wet weather, do not use pastures that are on artificially-drained land • Modify outlets from drainage ways to treat drainage water in wetlands or on filter areas before it flows into streams or other water bodiesPAGE 54 //NUTRIENT CYCLING IN PASTURES
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  • 61. Resource List General Grazing AGENCIES AND ORGANIZATIONS Clark, E.A. and R.P. Poincelot. 1996. The Contri- bution of Managed Grasslands to Sustainable Agriculture in the Great Lakes Basin. TheAmerican Forage and Grassland Council. Haworth Press Inc., New York.Gerorgetown, TX. An easy-to-read but technically-based discus- Mission statement: “To promote the use of sion of the relations between grazing manage- forages as economically and environmentally ment, forage production, and soil quality. sound agriculture through education, commu- nication, and professional development of producers, scientists, educators and commercial PUBLICATIONS IN PRINT representatives and through communication with policy makers and consumers in North America” <>. Emmick, D.L. and D.G. Fox. 1993. PrescribedGrazing Lands Conservation Initiative Grazing Management to Improve Pasture Productivity in New York. United States A national effort to provide high-quality techni- Department of Agriculture Soil Conservation cal assistance on privately owned grazing lands Service and Cornell University. <http:// and increase the awareness of the importance of grazing land resources. <>. index.html>. Grazing Lands Technology Institute (GLTI) Gerrish, J. and C. Roberts (eds.) 1996 Missouri Provides technical excellence to the Natural Grazing Manual. University of Missouri. Resources Conservation Service (NRCS) and Columbia, MO. other appropriate customers in the acquisition, development, coordination, and transfer of Short papers addressing forage production and technology that meets the needs of grazing land nutrient management in pastures. resources, landowners and managers, and the Pearson, C.J. and R.L. Ison. 1987. Agronomy of public. < Grassland Systems. Cambridge University homepage.html>, or contact your county or Press. Cambridge. regional NRCS office. Technical discussion of forage biology andCooperative Extension Service. production, nutrient availability, and animal nutrition on forages. Educational and technical assistance that links farmers and ranchers with university research Stockman Grass Farmer Magazine expertise. Many county or regional offices The nation’s leading publication on grass-based address grazing practices. To identify your livestock systems. Order information <http:// local office see <>. statepartners/usa.htm>. Terrill, T. and K. Cassida. 2001. AmericanNatural Resource, Agriculture, and Engineer- Forage and Grassland Council Service. Ithaca, NY. American Forage and Grassland Council. Coordinates and publishes proceedings from Gerorgetown, TX. conferences on agricultural and environmental issues. Also publishes technical and practical documents on manure management, composting, and animal housing. <http://>. //NUTRIENT CYCLING IN PASTURES PAGE 61
  • 62. Unsander, D., B. Albert, P. Porter, A. Crossley, Whitehead, D.C. 2000. Nutrient Elements inand N. Martin. No date. Pastures for Profit. A Grassland Soil-Plant-Animal Relationships.Guide to Rotational Grazing. University of CAB International Publishing. Wallingford,Wisconsin Extension. Madison, WI. Available Oxon, <>. An excellent technical resource on nutrient cycle Soil Quality and Soil Conservation components and interactions in grazing systems.Magdoff, F. and H. van Es. 2000. Building Undersander, D. and B. Pillsbury. 1999. Graz-Soils for Better Crops, Second Edition. Sus- ing Streamside Pastures. University of Wis-tainable Agriculture Network. Handbook consin Extension. Madison, WI.Series Book 4. Beltsville, MD. An easy-to-read overview of the potential risks and benefits associated with riparian grazing Easy-to-read descriptions of concepts regarding practices. soil quality and agricultural management practices to enhance soil quality. Although Water Quality Protection many of the practical management descriptions are oriented more towards field crops than to Sharpley, A.N., T. Daniel, T. Sims, J. grazing systems, the concepts of soil quality Lemunyon, R. Stevens, and R. Parry. 1999. protection and management are the same. Agricultural Phosphorus and Eutrophication. ARS-149. United State Department of Agricul-Edwards, C. 1999. Soil Biology Primer. ture / Agricultural Research Service. Washing-United States Department of Agriculture, ton, D.C. Order information: <http://Natural Resources Conservation Service.>.Washington, D.C. Order information <http:// An illustrated, easy-to-read discussion ing the impact of environmental conditions andsoil_biology.htm> land management practices on the risk for A well-illustrated overview of soil organisms phosphorus runoff from agricultural lands. and their impact on soil quality. Daniels, M., T. Daniel, D. Carman, R. Morgan,Cavigelli, M.A., S.R. Deming, L.K. Probyn, and J. Langston, and K. VanDevender. 1998. SoilR.R. Harwood (eds). 1998. Michigan Field Phosphorus Levels: Concerns and Recommen-Crop Ecology: Managing biological processes dations. University of Arkansas. Division offor productivity and environmental quality. Agriculture. Cooperative Extension Service.MSU Extension Bulletin E-2646. Michigan State Accessed at: < Extension Service, East Lansing, MI. sera17/publicat.htm>. A well-illustrated overview of nutrient cycles in NRAES. 2000. Managing Nutrients and agricultural systems, the organisms that affect Pathogens from Animal Agriculture. Natural these systems, and the impact of environmental Resource, Agriculture, and Engineering Ser- conditions and management practices on the vice. Ithaca, NY. activities of these organisms. Proceedings from a conference that includedJournal of Soil and Water Conservation. research reports, field experience, and policyTechnical articles on soil conservation research discussions. Available from <http://and practices. Many articles pertain to grazing>.systems. Order information: <>PAGE 62 //NUTRIENT CYCLING IN PASTURES
  • 63. Stockman Grass Farmer Magazine - The WEB RESOURCES nation’s leading publication on grass-based livestock systems <http://>. Rotational Grazing – University ProgramsCenter for Grassland Studies - University of American Farmland Trust information site onNebraska-Lincoln <http:// grass-based farming systems <>.>.Purdue Pasture Management Page- Purdue Sustainable Farming Connection’s GrazingUniversity Cooperative Extension <http:// Menu- good links to grazing sites <>. ing/home.htm>.Texas Agricultural Extension Service. Extension Why Grassfed Is Best - Jo Robinson explores theResource Center < many benefits of grassfed meat, eggs, and dairycatalog/query.cgi?id=433>. products <>.Controlled Grazing of Virginia’s Pastures – Archived listings from Graze-L, an internationalVirginia Cooperative Extension <http:// forum for the discussion of rotational and seasonal dairying <http://012.html>.>.Grazing Dairy Systems at the Center for Inte- Soil Qualitygrated Agricultural Systems – University of Soil Quality Institute. United States DepartmentWisconsin – Madison < of Agriculture. Natural Resources Conservationcias/research/livestoc.html#grazing>. Service. < Management & Grazing – University of survey/SQI/>.Wisconsin Extension <>. Soil quality information sheets, soil quality indicators, and soil quality assessment methodsGrasslands Watershed Management – ClemsonUniversity <>. Rangeland Soil Quality. Soil Quality Institute. United States Department of Agriculture Focuses on the role pasture and forage crop Natural Resources Conservation Service. production can play in helping insure a clean, < safe water supply. range.html>. Rotational Grazing – Information sheets on rangeland soil quality Organizations and Agencies Soil Biological Communities. United StatesGrazing Lands Conservation Initiative Department of Agriculture Bureau of Land A national effort to provide high-quality Management. Information sheets. <http:// technical assistance on privately owned grazing>. lands and increase the awareness of the impor- Ingham, E. 1996. The soil foodweb: Its impor- tance of grazing land resourses <http:// tance in ecosystem health. Accessed at <http:/>. />.Grazing Lands Technology Institute - Grazing The Soil Foodweb Incorporated. <http://information from the Natural Resources Con->.servation Service. <>. Soil microbiology, soil ecology information and laboratory analyses of soil biology. //NUTRIENT CYCLING IN PASTURES PAGE 63
  • 64. SoilFacts. Soil science related publications from EPA’s National Agriculture Compliance Assis-North Carolina State University. Includes: tance Center. <>.Poultry Manure as a Fertilizer Source, Good Information on environmental laws affectingSoil Management Helps Protect Groundwater, agricultural operations.Nitrogen Management and Water Quality, andSoils and Water Quality <http:// Riparian Riparian Grazing Project - University of Califor-Soilfacts/AG-439-05/>. nia Cooperative Extension <http://United States Department of Agriculture – Research Service. <http:// riparian_grazing_project.htm>>. Effects of Cattle Grazing in Riparian Areas of Research programs addressing soil and water the Southwestern United States <http:// quality, rangeland, pastures, and forests, and integrated agricultural systems. riparian.htm>. Managed Grazing and Stream Ecosystems. Water Quality Laura Paine and John Lyons. < http://Pellant, M., P. Shaver, D.A. Pyke, and J.E.>.Herrick. 2000. Interpreting Indicators of Driscoll, M. and B. Vondracek. 2001. AnRangeland Health. Version 3. Technical Refer- Annotated Bibliography of Riparian Grazingence 1734-6. National Sciene and Technology Publications. The Land Stewardship Project.Center Information and Communications < Denver, CO. <http:// resources-main.html>>.SERA-17. Minimizing Phosphorus Losses fromAgriculture. < By Barbara Bellowssera17/publicat.htm>. NCAT Agriculture Specialist National, multi-agency information on phospho- rus fate and transport, including the develop- Edited by Richard Earles ment of the phosphorus index. Formatted by Gail M. HardyUnited States Environmental Protection AgencyDepartment of Water. 1999. Laws and Regula- December 2001tions, Policy and Guidance Documents, andLegislation. Accessed at: <>.Concentrated Animal Feeding Operations(CAFOs) Effluent Guidelines for larger scalefarms. < IP136cafo/index.html>. Focuses on confinement systems but many of the nutrient management planning guidelines may The electronic version of Nutrient Cycling in also be appropriate for grazing systems. Pastures is located at: HTMLAnimal Feeding Operations. US EPA Office of < nutrientcycling.htmlhome.cfm?program_id=7>. PDF PDF/nutrientcycling.pdfPAGE 64 //NUTRIENT CYCLING IN PASTURES