Sustainable soil management


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Sustainable soil management

  1. 1. SUSTAINABLE SOIL USTAINABLE SOIL MANAGEMENT SOIL SYSTEMS GUIDE National Sustainable Agriculture Information Service Abstract: This publication covers basic soil properties and management steps toward building and maintaining healthy soils. Part I deals with basic soil principles and provides an understanding of living soils and how they work. In this section you will find answers to why soil organisms and organic matter are important. Part II covers manage- ment steps to build soil quality on your farm. The last section looks at farmers who have successfully built up their soil. The publication concludes with a large resource section of other available information. By Preston Sullivan NCAT Agriculture Specialist May 2004 ©2004 NCAT Table of Contents Part I. Characteristics of Sustainable Soils ....................................... 2 Introduction ................................................. 2 The Living Soil: Texture and Structure ............................................ 2 The Living Soil: The Importance of Soil Organisms .......................................... 3 Organic Matter, Humus, and the Soil Foodweb .................................................... 7 Soil Tilth and Organic Matter ................... 8 Tillage, Organic Matter, and Plant Productivity ........................................... 10 Fertilizer Amendments and Biologically Active Soils ....................... 13 Conventional Fertilizers .......................... 14 Top$oil – Your Farm’$ Capital .............. 15 Summary of Part I .................................... 18 Summary of Sustainable Soil Management Principles ............... 19 Part II. Management Steps to Improve Soil Quality ............................ 20 Part III. Examples of Successful Soil Builders (Farmer Profiles) ............ 25 Photo by Preston Sullivan References .................................................. 27 Additional Resources .............................. 28 Soybeans no-till planted into wheat stubble. ATTRA is the national sustainable agriculture information service operated by the National Center for Appropriate Technology, through a grant from the Rural Business-Cooperative Service, U.S. Department of Agriculture. These organizations do not recommend or endorse products, companies, or individuals. NCAT has offices in Fayetteville, Arkansas (P.O. Box 3657, Fayetteville, AR 72702), Butte, Montana, and Davis, California.
  2. 2. PART I. Characteristics of SUSTAINABLE SOILS Introduction Sustainable: capable of being maintained at length without interruption, weakening, or losing in power or quality. What are some features of good soil? Any farmer will tell you that a good soil: Textur exture The Living Soil: Texture • feels soft and crumbles easily • drains well and warms up quickly in and Structure the spring • does not crust after planting Soils are made up of four basic components: • soaks up heavy rains with little runoff minerals, air, water, and organic matter. In • stores moisture for drought periods most soils, minerals represent around 45% of • has few clods and no hardpan the total volume, water and air about 25% each, • resists erosion and nutrient loss and organic matter from 2% to 5%. The min- • supports high populations of soil eral portion consists of three distinct particle organisms sizes classified as sand, silt, or clay. Sand is the • has a rich, earthy smell largest particle that can be considered soil. • does not require increasing inputs for high yields Sand is largely the mineral quartz, though other • produces healthy, high-quality crops minerals are also present. Quartz contains no (1) plant nutrients, and sand cannot hold nutri- ents—they leach out easily with rainfall. Silt All these criteria indicate a soil that functions particles are much smaller than sand, but like effectively today and will continue to produce sand, silt is mostly quartz. The smallest of all crops long into the future. These characteris- the soil particles is clay. Clays are quite differ- tics can be created through management prac- ent from sand or silt, and most types of clay tices that optimize the processes found in na- contain appreciable amounts of plant nutrients. tive soils. Clay has a large surface area resulting from the plate-like shape of the individual particles. How does soil in its native condition function? Sandy soils are less productive than silts, while How do forests and native grasslands produce soils containing clay are the most productive and plants and animals in the complete absence of use fertilizers most effectively. fertilizer and tillage? Understanding the prin- ciples by which native soils function can help Soil texture refers to the relative proportions of farmers develop and maintain productive and sand, silt, and clay. A loam soil contains these profitable soil both now and for future genera- three types of soil particles in roughly equal pro- tions. The soil, the environment, and farm con- portions. A sandy loam is a mixture containing dition benefit when the soil’s natural produc- a larger amount of sand and a smaller amount tivity is managed in a sustainable way. Reli- of clay, while a clay loam contains a larger ance on purchased inputs declines year by year, amount of clay and a smaller amount of sand. while land value and income potential increase. These and other texture designations are listed Some of the things we spend money on can be in Table 1. done by the natural process itself for little or nothing. Good soil management produces crops Another soil characteristic—soil structure—is and animals that are healthier, less susceptible distinct from soil texture. Structure refers to the to disease, and more productive. To understand clumping together or “aggregation” of sand, silt, this better, let’s start with the basics. and clay particles into larger secondary clusters. PAGE 2 //SUSTAINABLE SOIL MANAGEMENT
  3. 3. be balanced in nutrients and high in humus, Table 1. Soil texture designations with a broad diversity of soil organisms. It will ranging from coarse to fine. produce healthy plants with minimal weed, dis- Texture Designation ease, and insect pressure. To accomplish this, we need to work with the natural processes and Coarse-textured Sand optimize their functions to sustain our farms. Loamy sand Sandy loam Considering the natural landscape, you might Fine sandy loam wonder how native prairies and forests func- Loam tion in the absence of tillage and fertilizers. Silty loam These soils are tilled by soil organisms, not by Silt machinery. They are fertilized too, but the fer- Silty clay loam tility is used again and again and never leaves Clay loam the site. Native soils are covered with a layer of Fine-textured Clay plant litter and/or growing plants throughout the year. Beneath the surface litter, a rich com- If you grab a handful of soil, good structure is plexity of soil organisms decompose plant resi- apparent when the soil crumbles easily in your due and dead roots, then release their stored hand. This is an indication that the sand, silt, nutrients slowly over time. In fact, topsoil is and clay particles are aggregated into granules the most biologically diverse part of the earth or crumbs. (3). Soil-dwelling organisms release bound-up minerals, converting them into plant-available Both texture and structure determine pore space forms that are then taken up by the plants grow- for air and water circulation, erosion resistance, ing on the site. The organisms recycle nutrients looseness, ease of tillage, and root penetration. again and again with the death and decay of While texture is related to the minerals in the each new generation of plants. soil and does not change with agricultural ac- tivities, structure can be improved or destroyed There are many different types of creatures that readily by choice and timing of farm practices. live on or in the topsoil. Each has a role to play. These organisms will work for the farmer’s ben- The Living Soil: The efit if we simply manage for their survival. Con- sequently, we may refer to them as soil livestock. Importance of Soil While a great variety of organisms contribute to soil fertility, earthworms, arthropods, and the Organisms various microorganisms merit particular atten- tion. An acre of living topsoil contains approximately 900 pounds of earthworms, 2,400 pounds of Earthworms fungi, 1,500 pounds of bacteria, 133 pounds of protozoa, 890 pounds of arthropods and algae, Earthworm burrows enhance water infiltration and even small mammals in some cases (2). and soil aeration. Fields that are “tilled” by Therefore, the soil can be viewed as a living com- earthworm tunneling can absorb water at a rate munity rather than an inert body. Soil organic 4 to 10 times that of fields lacking worm tun- matter also contains dead organisms, plant nels (4). This reduces water runoff, recharges matter, and other organic materials in various groundwater, and helps store more soil water phases of decomposition. Humus, the dark-col- for dry spells. Vertical earthworm burrows pipe ored organic material in the final stages of de- air deeper into the soil, stimulating microbial composition, is relatively stable. Both organic nutrient cycling at those deeper levels. When matter and humus serve as reservoirs of plant earthworms are present in high numbers, the nutrients; they also help to build soil structure tillage provided by their burrows can replace and provide other benefits. some expensive tillage work done by machin- ery. The type of healthy living soil required to sup- port humans now and far into the future will PAGE 3 //SUSTAINABLE SOIL MANAGEMENT
  4. 4. Table 2. Selected nutrient analyses of worm casts compared to those of the sur- rounding soil. Soil Nutrient Worm casts Lbs/ac Lbs/ac Carbon 171,000 78,500 Nitrogen 10,720 7,000 Phosphorus 280 40 Potassium 900 140 From Graff (6). Soil had 4% organic matter. Earthworms thrive where there is no tillage. Generally, the less tillage the better, and the shal- lower the tillage the better. Worm numbers can be reduced by as much as 90% by deep and fre- quent tillage (7). Tillage reduces earthworm populations by drying the soil, burying the plant residue they feed on, and making the soil more likely to freeze. Tillage also destroys vertical Figure 1. The soil is teeming with organisms that cycle worm burrows and can kill and cut up the nutrients from soil to plant and back again. worms themselves. Worms are dormant in the hot part of the summer and in the cold of win- ter. Young worms emerge in spring and fall— they are most active just when farmers are likely Worms eat dead plant material left on top of to be tilling the soil. Table 3 shows the effect of the soil and redistribute the organic matter and tillage and cropping practices on earthworm nutrients throughout the topsoil layer. Nutri- numbers. ent-rich organic compounds line their tunnels, which may remain in place for years if not dis- turbed. During droughts these tunnels allow Table 3. Effect of crop management on for deep plant root penetration into subsoil re- earthworm populations. gions of higher moisture content. In addition Crop Management Worms/foot2 to organic matter, worms also consume soil and soil microbes. The soil clusters they expel from Corn Plow 1 their digestive tracts are known as worm casts Corn No-till 2 or castings. These range from the size of a mus- Soybean Plow 6 tard seed to that of a sorghum seed, depending Soybean No-till 14 on the size of the worm. Bluegrass/ clover —- 39 The soluble nutrient content of worm casts is Dairy considerably higher than that of the original soil pasture —- 33 (see Table 2). A good population of earthworms From Kladivko (8). can process 20,000 pounds of topsoil per year— with turnover rates as high as 200 tons per acre having been reported in some exceptional cases As a rule, earthworm numbers can be increased (5). Earthworms also secrete a plant growth by reducing or eliminating tillage (especially fall stimulant. Reported increases in plant growth tillage), not using a moldboard plow, reducing following earthworm activity may be partially residue particle size (using a straw chopper on attributed to this substance, not just to improved the combine), adding animal manure, and grow- soil quality. ing green manure crops. It is beneficial to leave as much surface residue as possible year-round. PAGE 4 //SUSTAINABLE SOIL MANAGEMENT
  5. 5. Cropping systems that typically have the most millipedes, centipedes, slugs, snails, and spring- earthworms are (in descending order) perennial tails. These are the primary decomposers. Their cool-season grass grazed rotationally, warm- role is to eat and shred the large particles of plant season perennial grass grazed rotationally, and and animal residues. Some bury residue, bring- annual croplands using no-till. Ridge-till and ing it into contact with other soil organisms that strip tillage will generally have more earthworms further decompose it. Some members of this than clean tillage involving plowing and disking. group prey on smaller soil organisms. The Cool season grass rotationally grazed is highest springtails are small insects that eat mostly fungi. because it provides an undisturbed (no-tillage) Their waste is rich in plant nutrients released environment plus abundant organic matter from after other fungi and bacteria decompose it. Also the grass roots and fallen grass litter. Generally of interest are dung beetles, which play a valu- speaking, worms want their food on top, and able role in recycling manure and reducing live- they want to be left alone. stock intestinal parasites and flies. Bacteria Earthworms prefer a near-neutral soil pH, moist soil conditions, and plenty of plant residue on the soil surface. They are sensitive to certain Bacteria are the most numerous type of soil or- pesticides and some incorporated fertilizers. ganism: every gram of soil contains at least a Carbamate insecticides, including Furadan, million of these tiny one-celled organisms. There Sevin, and Temik, are harmful to earthworms, are many different species of bacteria, each with notes worm biologist Clive Edwards of Ohio its own role in the soil environment. One of the State University (4). Some insecticides in the major benefits bacteria provide for plants is in organophosphate family are mildly toxic to making nutrients available to them. Some spe- earthworms, while synthetic pyrethroids are cies release nitrogen, sulfur, phosphorus, and harmless to them (4). Most herbicides have little trace elements from organic matter. Others effect on worms except for the triazines, such break down soil minerals, releasing potassium, as Atrazine, which are moderately toxic. Also, phosphorus, magnesium, calcium, and iron. anhydrous ammonia kills earthworms in the Still other species make and release plant injection zone because it dries the soil and tem- growth hormones, which stimulate root porarily increases the pH there. High rates of growth. ammonium-based fertilizers are also harmful. Several species of bacteria transform nitrogen For more information on managing earthworms, from a gas in the air to forms available for plant order The Farmer’s Earthworm Handbook: Man- use, and from these forms back to a gas again. aging Your Underground Moneymakers, by David A few species of bacteria fix nitrogen in the roots Ernst. Ernst’s book contains details on what of legumes, while others fix nitrogen indepen- earthworms need to live, how to increase worm dently of plant association. Bacteria are respon- numbers, the effects of tillage, manure, and live- sible for converting nitrogen from ammonium stock management on earthworms, how 193 to nitrate and back again, depending on cer- chemicals affect earthworms, and more. See the tain soil conditions. Other benefits to plants Additional Resources section of this publica- provided by various species of bacteria include tion for ordering information. Also visit the increasing the solubility of nutrients, improving earthworm Web sites listed in that section. soil structure, fighting root diseases, and detoxi- fying soil. As a rule, earthworm numbers can be in- Fungi creased by reducing or eliminating tillage. Fungi come in many different species, sizes, and shapes in soil. Some species appear as thread- Arthropods like colonies, while others are one-celled yeasts. Slime molds and mushrooms are also fungi. In addition to earthworms, there are many Many fungi aid plants by breaking down or- other species of soil organisms that can be seen ganic matter or by releasing nutrients from soil by the naked eye. Among them are sowbugs, PAGE 5 //SUSTAINABLE SOIL MANAGEMENT
  6. 6. Protozoa minerals. Fungi are generally quick to colonize larger pieces of organic matter and begin the decomposition process. Some fungi produce Protozoa are free-living microorganisms that plant hormones, while others produce antibiot- crawl or swim in the water between soil par- ics including penicillin. There are even species ticles. Many soil protozoa are predatory, eat- of fungi that trap harmful plant-parasitic nema- ing other microbes. One of the most common is todes. an amoeba that eats bacteria. By eating and digesting bacteria, protozoa speed up the cy- The mycorrhizae (my-cor-ry´-zee) are fungi that cling of nitrogen from the bacteria, making it live either on or in plant roots and act to extend more available to plants. the reach of root hairs into the soil. Mycorrhizae increase the uptake of water and nutrients, es- Nematodes pecially phosphorus. They are particularly im- portant in degraded or less fertile soils. Roots Nematodes are abundant in most soils, and only colonized by mycorrhizae are less likely to be a few species are harmful to plants. The harm- penetrated by root-feeding nematodes, since the less species eat decaying plant litter, bacteria, pest cannot pierce the thick fungal network. fungi, algae, protozoa, and other nematodes. Mycorrhizae also produce hormones and anti- Like other soil predators, nematodes speed the biotics that enhance root growth and provide rate of nutrient cycling. disease suppression. The fungi benefit by tak- ing nutrients and carbohydrates from the plant Soil organisms and soil quality roots they live in. All these organisms—from the tiny bacteria up Actinomycetes to the large earthworms and insects—interact with one another in a multitude of ways in the Actinomycetes (ac-tin-o-my´-cetes) are thread- soil ecosystem. Organisms not directly involved like bacteria that look like fungi. While not as in decomposing plant wastes may feed on each numerous as bacteria, they too perform vital other or each other’s waste products or the other roles in the soil. Like the bacteria, they help substances they release. Among the substances decompose organic matter into humus, releas- released by the various microbes are vitamins, ing nutrients. They also produce antibiotics to amino acids, sugars, antibiotics, gums, and fight diseases of roots. Many of these same an- waxes. tibiotics are used to treat human dis- eases. Actinomycetes are respon- Roots can also release into the Research on life in the soil has sible for the sweet, earthy smell soil various substances that determined that there are noticed whenever a biologically stimulate soil microbes. These ideal ratios for certain key or- active soil is tilled. substances serve as food for se- ganisms in highly productive lect organisms. Some scientists soils. Algae and practitioners theorize that plants use this means to stimulate the specific Many different species of algae live in the up- population of microorganisms capable of releas- per half-inch of the soil. Unlike most other soil ing or otherwise producing the kind of nutri- organisms, algae produce their own food tion needed by the plants. through photosynthesis. They appear as a greenish film on the soil surface following a satu- Research on life in the soil has determined that rating rain. Algae improve soil structure by pro- there are ideal ratios for certain key organisms ducing slimy substances that glue soil together in highly productive soils (9). The Soil Foodweb into water-stable aggregates. Some species of Lab, located in Oregon, tests soils and makes algae (the blue-greens) can fix their own nitro- fertility recommendations that are based on this gen, some of which is later released to plant understanding. Their goal is to alter the makeup roots. PAGE 6 //SUSTAINABLE SOIL MANAGEMENT
  7. 7. of the soil microbial community so it resembles Organic matter and humus are terms that de- that of a highly fertile and productive soil. There scribe somewhat different but related things. are several different ways to accomplish this Organic matter refers to the fraction of the soil goal, depending on the situation. For more on that is composed of both living organisms and the Soil Foodweb Lab, see the Additional Re- once-living residues in various stages of decom- sources section of this publication. position. Humus is only a small portion of the organic matter. It is the end product of organic Because we cannot see most of the creatures liv- matter decomposition and is relatively stable. ing in the soil and may not take time to observe Further decomposition of humus occurs very the ones we can see, it is easy to forget about slowly in both agricultural and natural settings. them. See Table 4 for estimates of typical In natural systems, a balance is reached be- amounts of various organisms found in fertile tween the amount of humus formation and the soil. There are many Web sites that provide in- amount of humus decay (11). This balance also depth information on soil organisms. Look for occurs in most agricultural soils, but often at a a list of these Web sites in the Additional Re- much lower level of soil humus. Humus con- sources section. Many of these sites have color tributes to well-structured soil that, in turn, pro- photographs of soil organisms and describe their duces high-quality plants. It is clear that man- benefits to soil fertility and plant growth. agement of organic matter and humus is essen- tial to sustaining the whole soil ecosystem. The benefits of a topsoil rich in organic matter Table 4. Weights of soil organisms in the and humus are many. They include rapid de- top 7 inches of fertile soil. composition of crop residues, granulation of soil Organism Pounds of liveweight/acre into water-stable aggregates, decreased crust- ing and clodding, improved internal drainage, Bacteria 1000 better water infiltration, and increased water Actinomycetes 1000 and nutrient holding capacity. Improvements Molds 2000 in the soil’s physical structure facilitate easier Algae 100 tillage, increased water storage capacity, re- Protozoa 200 duced erosion, better formation and harvesting Nematodes 50 of root crops, and deeper, more prolific plant Insects 100 root systems. Worms 1000 Plant roots 2000 Soil organic matter can be compared to a bank From Bollen (10). account for plant nutrients. Soil containing 4% organic matter in the top seven inches has 80,000 pounds of organic matter per acre. That Organic Matter, Humus, Organic Matter, 80,000 pounds of organic matter will contain about 5.25% nitrogen, amounting to 4,200 and the Soil Foodweb pounds of nitrogen per acre. Assuming a 5% release rate during the growing season, the or- ganic matter could supply 210 pounds of nitro- Like cattle and other farm animals, soil live- gen to a crop. However, if the organic matter is stock require proper feed. allowed to degrade and lose nitrogen, pur- chased fertilizer will be necessary to prop up crop yields. Understanding the role that soil organisms play is critical to sustainable soil management. Based All the soil organisms mentioned previously, on that understanding, focus can be directed except algae, depend on organic matter as their toward strategies that build both the numbers food source. Therefore, to maintain their popu- and the diversity of soil organisms. Like cattle lations, organic matter must be renewed from and other farm animals, soil livestock require plants growing on the soil, or from animal ma- proper feed. That feed comes in the form of nure, compost, or other materials imported from organic matter. PAGE 7 //SUSTAINABLE SOIL MANAGEMENT
  8. 8. off site. When soil livestock are these aggregates become wet fed, fertility is built up in the soil, again, however, their stability Ultimately, building organic and the soil will feed the plants. is challenged, and they may matter and humus in the soil break apart. Aggregates can is a matter of managing the Ultimately, building organic mat- soil’s living organisms. also be held together by plant ter and humus levels in the soil is roots, earthworm activity, and a matter of managing the soil’s by glue-like products pro- living organisms—something akin to wildlife duced by soil microorganisms. Earthworm-cre- management or animal husbandry. This entails ated aggregates are stable once they come out working to maintain favorable conditions of of the worm. An aggregate formed by physical moisture, temperature, nutrients, pH, and aera- forces can be bound together by fine root hairs tion. It also involves providing a steady food or threads produced by fungi. source of raw organic material. Aggregates can also become stabilized (remain intact when wet) through the by-products of Soil Tilth and Organic organic matter decomposition by fungi and bac- teria—chiefly gums, waxes, and other glue-like Matter substances. These by-products cement the soil particles together, forming water-stable aggre- A soil that drains well, does not crust, takes in gates (Figure 2). The aggregate is then strong water rapidly, and does not make clods is said enough to hold together when wet—hence the to have good tilth. Tilth is the physical condi- term “water-stable.” tion of the soil as it relates to tillage ease, seed- bed quality, easy seedling emergence, and deep USDA soil microbiologist Sara Wright named root penetration. Good tilth is dependent on the glue that holds aggregates together aggregation—the process whereby individual “glomalin” after the Glomales group of common soil particles are joined into clusters or “aggre- root-dwelling fungi (12). These fungi secrete a gates.” gooey protein known as glomalin through their hair-like filaments, or hyphae. When Wright Aggregates form in soils when individual soil measured glomalin in soil aggregates she found particles are oriented and brought together levels as high as 2% of their total weight in east- through the physical forces of wetting and dry- ern U.S. soils. Soil aggregates from the West ing or freezing and thawing. Weak electrical and Midwest had lower levels of glomalin. She forces from calcium and magnesium hold soil found that tillage tends to lower glomalin lev- particles together when the soil dries. When els. Glomalin levels and aggregation were MICROBIAL AND FUNGAL BYPRODUCTS GLUE THE PARTICLES TOGETHER DISPERSED STATE AGGREGATED STATE Figure 2. Microbial byproducts glue soil particles into water-stable aggregates. PAGE 8 //SUSTAINABLE SOIL MANAGEMENT
  9. 9. higher in no-till corn plots than in tilled plots clog the pores immediately beneath the surface. (12). Wright has a brochure describing glomalin Following drying, a sealed soil surface results and how it benefits soil, entitled Glomalin, a Man- in which most of the pore space has been dras- ageable Soil Glue. To order this brochure see the tically reduced due to clogging from dispersed Additional Resources section of this publica- clay particles. Subsequent rainfall is much more tion. likely to run off than to flow into the soil (Fig- ure 3). A well-aggregated soil allows for increased water entry, increased air flow, and increased water-holding capacity (13). Plant roots occupy air water a larger volume of well-aggregated soil, high in organic matter, as compared to a finely pulver- ized and dispersed soil, low in organic matter. Roots, earthworms, and soil arthropods can pass more easily through a well-aggregated soil (14). Aggregated soils also prevent crusting of the soil surface. Finally, well-aggregated soils are more erosion resistant, because aggregates are much heavier than their particle compo- nents. For a good example of the effect of or- ganic matter additions on aggregation, as Crusted shown by subsequent increase in water entry into the soil, see Table 5. air water Table 5. Water entry into the soil after 1 hour Manure Rate (tons/acre) Inches of water 0 1.2 8 1.9 16 2.7 Boyle et al. (13). Well-Aggregated The opposite of aggregation is dispersion. In a dispersed soil, each individual soil particle is free Figure 3. Effects of aggregation on water and air to blow away with the wind or wash away entry into the soil. Derived from Land Stewardship Project with overland flow of water. Monitoring Toolbox (15). Clay soils with poor aggregation tend to be sticky when wet, and cloddy when dry. If the Since raindrops start crusting, any management clay particles in these soils can be aggregated practices that protect the soil from their impact together, better aeration and water infiltration will decrease crusting and increase water flow will result. Sandy soils can benefit from aggre- into the soil. Mulches and cover crops serve this gation by having a small amount of dispersed purpose well, as do no-till practices, which al- clay that tends to stick between the sand par- low the accumulation of surface residue. Also, ticles and slow the downward movement of a well-aggregated soil will resist crusting be- water. cause the water-stable aggregates are less likely to break apart when a raindrop hits them. Crusting is a common problem on soils that are poorly aggregated. Crusting results chiefly from Long-term grass production produces the best- the impact of falling raindrops. Rainfall causes aggregated soils (16). A grass sod extends a clay particles on the soil surface to disperse and mass of fine roots throughout the topsoil, con- PAGE 9 //SUSTAINABLE SOIL MANAGEMENT
  10. 10. tributing to the physical processes that help form • allowing the build-up of excess sodium from aggregates. Roots continually remove water irrigation or sodium-containing fertilizers from soil microsites, providing local wetting and drying effects that promote aggregation. Fine Tillage, Organic Matter, and Organic Matter, root hairs also bind soil aggregates together. Plant Productivity Roots also produce food for soil microorganisms and earth- worms, which in turn generate The best-aggregated soils are Several factors affect the level compounds that bind soil par- those that have been in long- of organic matter that can be maintained in a soil. Among ticles into water-stable aggre- term grass production. these are organic matter addi- gates. In addition, perennial tions, moisture, temperature, grass sods provide protection tillage, nitrogen levels, cropping, and fertiliza- from raindrops and erosion. Thus, a perennial tion. The level of organic matter present in the cover creates a combination of conditions opti- soil is a direct function of how much organic mal for the creation and maintenance of well- material is being produced or added to the soil aggregated soil. versus the rate of decomposition. Achieving this balance entails slowing the speed of organic mat- Conversely, cropping sequences that involve ter decomposition, while increasing the supply annual plants and extensive cultivation provide of organic materials produced on site and/or less vegetative cover and organic matter, and added from off site. usually result in a rapid decline in soil aggrega- tion. For more information on aggregation, see Moisture and temperature also profoundly af- the soil quality information sheet entitled Ag- fect soil organic matter levels. High rainfall and gregate Stability at the Soil Quality Institute’s temperature promote rapid plant growth, but home page, < these conditions are also favorable to rapid or- sq_eig_1.pdf>. From there, click on Soil Qual- ganic matter decomposition and loss. Low rain- ity Information Sheets, then click on Aggregate fall or low temperatures slow both plant growth Stability. and organic matter decomposition. The native Midwest prairie soils originally had a high Farming practices can be geared to conserve and amount of organic matter from the continuous promote soil aggregation. Because the binding growth and decomposition of perennial grasses, substances are themselves susceptible to micro- combined with a moderate temperature that did bial degradation, organic matter needs to be not allow for rapid decomposition of organic replenished to maintain microbial populations matter. Moist and hot tropical areas may ap- and overall aggregated soil status. Practices pear lush because of rapid plant growth, but should conserve aggregates once they are soils in these areas are low in nutrients. Rapid formed, by minimizing factors that degrade and decomposition of organic matter returns nutri- destroy aggregation. Some factors that destroy ents back to the soil, where they are almost im- or degrade soil aggregates are: mediately taken up by rapidly growing plants. • bare soil surface exposed to the impact of Tillage can be beneficial or harmful to a biologi- raindrops cally active soil, depending on what type of till- • removal of organic matter through crop pro- age is used and when it is done. Tillage affects duction and harvest without return of or- both erosion rates and soil organic matter de- ganic matter to the soil composition rates. Tillage can reduce the or- • excessive tillage ganic matter level in croplands below 1%, ren- • working the soil when it is too wet or too dering them biologically dead. Clean tillage in- dry volving moldboard plowing and disking breaks • use of anhydrous ammonia, which speeds down soil aggregates and leaves the soil prone up decomposition of organic matter to erosion from wind and water. The mold- • excess nitrogen fertilization board plow can bury crop residue and topsoil to a depth of 14 inches. At this depth, the oxy- PAGE 10 //SUSTAINABLE SOIL MANAGEMENT
  11. 11. gen level in the soil is so low that decomposi- In cold climates with a long dormant season, tion cannot proceed adequately. Surface-dwell- light tillage of a heavy residue may be benefi- ing decomposer organisms suddenly find them- cial; in warmer climates it is hard enough to selves suffocated and soon die. Crop residues maintain organic matter levels without any till- that were originally on the surface but now have age. been turned under will putrefy in the oxygen- deprived zone. This rotting activity may give a As indicated in Figure 4, moldboard plowing putrid smell to the soil. Furthermore, the top causes the fastest decline of organic matter, no- few inches of the field are now often covered till the least. The plow lays the soil up on its with subsoil having very little organic matter side, increasing the surface area exposed to oxy- content and, therefore, limited ability to support gen. The other three types of tillage are inter- productive crop growth. mediate in their ability to foster organic matter decomposition. Oxygen is the key factor here. The topsoil is where the biological activity hap- The moldboard plow increases the soil surface pens—it’s where the oxygen is. That’s why a area, allowing more air into the soil and speed- fence post rots off at the surface. In terms of ing the decomposition rate. The horizontal line organic matter, tillage is similar to opening the on Figure 4 represents the replenishment of or- air vents on a wood-burning stove; adding or- ganic matter provided by wheat stubble. With ganic matter is like adding wood to the stove. the moldboard plow, more than the entire or- Ideally, organic matter decomposition should ganic matter contribution from the wheat straw proceed as an efficient burn of the “wood” to is gone within only 19 days following tillage. release nutrients and carbohydrates to the soil Finally, the passage of heavy equipment in- organisms and create stable humus. Shallow creases compaction in the wheel tracks, and tillage incorporates residue and speeds the de- some tillage implements themselves compact the composition of organic matter by adding oxy- soil further, removing oxygen and increasing the gen that microbes need to become more active. chance that deeply buried residues will putrefy. Organic Matter loss 19 days after Tillage 4000 Pounds/ac OM loss 3500 Residue from wheat crop 3000 2500 2000 1500 1000 500 0 Mb.+ 2 Mb. plow No-till Disc Chisel Pl. disc Tillage type Reicosky & Lindstrom, 1995 Figure 4. Organic matter losses after various tillage practices (17). PAGE 11 //SUSTAINABLE SOIL MANAGEMENT
  12. 12. Tillage also reduces the rate of water entry into acres of vegetables, alfalfa, and grain crops on the soil by removal of ground cover and destruc- his Cedar Meadow Farm. Learn more about tion of aggregates, resulting in compaction and his operation in the Farmer Profiles section of crusting. Table 6 shows three different tillage this publication, by visiting his Web site, or by methods and how they affect water entry into ordering his video (see Additional Resources the soil. Notice the direct relationship between section). tillage type, ground cover, and water infiltra- tion. No-till has more than three times the wa- Other conservation tillage systems include ridge ter infiltration of the moldboard-plowed soil. tillage, minimum tillage, zone tillage, and re- Additionally, no-till fields will have higher ag- duced tillage, each possessing some of the ad- gregation from the organic matter decomposi- vantages of both conventional till and no-till. tion on site. The surface mulch typical of no-till These systems represent intermediate tillage sys- fields acts as a protective skin for the soil. This tems, allowing more flexibility than either a no- soil skin reduces the impact of raindrops and till or conventional till system might. They are buffers the soil from temperature extremes as more beneficial to soil organisms than a con- well as reducing water evaporation. ventional clean-tillage system of moldboard plowing and disking. Table 6. Tillage effects on water infiltration and Adding manure and compost is a recognized ground cover. means for improving soil organic matter and Water Infiltration Ground Cover humus levels. In their absence, perennial grass is the only crop that can regenerate and increase mm/minute Percent soil humus (18). Cool-season grasses build soil No-till 2.7 48 organic matter faster than warm-season grasses Chisel Plow 1.3 27 because they are growing much longer during Moldboard Plow 0.8 12 a given year (18). When the soil is warm From Boyle et al., 1989 (13). enough for soil organisms to decompose organic matter, cool-season grass is growing. While Both no-till and reduced-tillage systems provide growing, it is producing organic matter and benefits to the soil. The advantages of a no-till cycling minerals from the decomposing organic system include superior soil conservation, mois- matter in the soil. In other words, there is a net ture conservation, reduced water runoff, long- gain of organic matter because the cool-season term buildup of organic matter, and increased grass is producing organic matter faster than it water infiltration. A soil managed without till- is being used up. With warm-season grasses, age relies on soil organisms to take over the job organic matter production during the growing of plant residue incorporation formerly done by season can be slowed during the long dormant tillage. On the down side, no-till can foster a season from fall through early spring. During reliance on herbicides to control weeds and can the beginning and end of this dormant period, lead to soil compaction from the traffic of heavy the soil is still biologically active, yet no grass equipment. growth is proceeding (18). Some net accumu- lation of organic matter can occur under warm- Pioneering development work on chemical-free season grasses, however. In a Texas study, no-till farming is proceeding at several research switchgrass (a warm-season grass) grown for stations and farms in the eastern U.S. Pennsyl- four years increased soil carbon content from vania farmer Steve Groff has been farming no- 1.1% to 1.5% in the top 12 inches of soil (19). In till with minimal or no herbicides for several hot and moist regions, a cropping rotation that years. Groff grows cover crops extensively in includes several years of pasture will be most his fields, rolling them down in the spring us- beneficial. ing a 10-foot rolling stalk chopper. This rolling Effect of Nitrogen on Organic Matter chopper kills the rye or vetch cover crop and creates a nice no-till mulch into which he plants a variety of vegetable and grain crops. After Excessive nitrogen applications stimulate in- several years of no-till production, his soils are creased microbial activity, which in turn speeds mellow and easy to plant into. Groff farms 175 organic matter decomposition. The extra nitro- PAGE 12 //SUSTAINABLE SOIL MANAGEMENT
  13. 13. gen narrows the ratio of carbon to nitrogen in Fertilizer Amendments and the soil. Native or uncultivated soils have ap- Biologically Active Soils proximately 12 parts of carbon to each part of nitrogen, or a C:N ratio of 12:1. At this ratio, populations of decay bacteria are kept at a stable What are the soil mineral conditions that foster level (20), since additional growth in their popu- biologically active soils? Drawing from the lation is limited by a lack of nitrogen. When work of Dr. William Albrecht (1888 to 1974), large amounts of inorganic nitrogen are added, agronomist at the University of Missouri, we the C:N ratio is reduced, which allows the popu- learn that balance is the key. Albrecht advocated lations of decay organisms to explode as they bringing soil nutrients into a balance so that none decompose more organic matter with the now were in excess or deficient. Albrecht’s theory abundant nitrogen. While soil bacteria can ef- (also called base-saturation theory) is used to ficiently use moderate applications of inorganic guide lime and fertilizer application by measur- nitrogen accompanied by organic amendments ing and evaluating the ratios of positively (carbon), excess nitrogen results in decomposi- charged nutrients (bases) held in the soil. Posi- tion of existing organic matter at a rapid rate. tively charged bases include calcium, magne- Eventually, soil carbon content may be reduced sium, potassium, sodium, ammonium nitrogen, to a level where the bacterial populations are and several trace minerals. When optimum ra- on a starvation diet. With little carbon avail- tios of bases exist, the soil is believed to support able, bacterial populations shrink, and less of high biological activity, have optimal physical the free soil nitrogen is absorbed. Thereafter, properties (water intake and aggregation), and applied nitrogen, rather than being cycled become resistant to leaching. Plants growing through microbial organisms and re-released to on such a soil are also balanced in mineral lev- plants slowly over time, becomes subject to els and are considered to be nutritious to hu- leaching. This can greatly reduce the efficiency mans and animals alike. Base saturation per- of fertilization and lead to environmental prob- centages that Albrecht’s research showed to be lems. optimal for the growth of most crops are: Calcium 60—70% Excessive nitrogen stimulates Magnesium 10—20% increased microbial activity, Potassium 2—5% which in turn speeds organic Sodium 0.5—3% matter decomposition. Other bases 5% According to Albrecht, fertilizer and lime ap- To minimize the fast decomposition of soil or- plications should be made at rates that will bring ganic matter, carbon should be added with ni- soil mineral percentages into this ideal range. trogen. Typical carbon sources—such as green This approach will shift the soil pH automati- manures, animal manure, and compost—serve cally into a desirable range without creating this purpose well. nutrient imbalances. The base saturation theory also takes into account the effect one nutrient Amendments containing too high a carbon to may have on another and avoids undesirable nitrogen ratio (25:1 or more) can tip the balance interactions. For example, phosphorus is known the other way, resulting in nitrogen being tied to tie up zinc. up in an unavailable form. Soil organisms con- sume all the nitrogen in an effort to decompose The Albrecht system of soil evaluation contrasts the abundant carbon; tied up in the soil organ- with the approach used by many state labora- isms, nitrogen remains unavailable for plant tories, often called the “sufficiency method.” uptake. As soon as a soil microorganism dies Sufficiency theory places little to no value on and decomposes, its nitrogen is consumed by nutrient ratios, and lime recommendations are another soil organism, until the balance be- typically based on pH measurements alone. tween carbon and nitrogen is achieved again. While in many circumstances base saturation and sufficiency methods will produce identical PAGE 13 //SUSTAINABLE SOIL MANAGEMENT
  14. 14. soil recommendations and similar results, sig- Laboratories and Sources of Organic Fertilizers and nificant differences can occur on a number of Amendments. Both of these are also available soils. For example, suppose we tested a corn- on the ATTRA Web site located at <http:// field and found a soil pH of 5.5 and base satu->. ration for magnesium at 20% and calcium at 40%. Base saturation theory would call for lim- Conventional Fertilizers ing with a high-calcium lime to raise the per- cent base saturation of calcium; the pH would rise accordingly. Sufficiency theory would not Commercial fertilizer can be a valuable resource specify high-calcium lime and the grower might to farmers in transition to a more sustainable choose instead a high-magnesium dolomite lime system and can help meet nutrient needs dur- that would raise the pH but worsen the balance ing times of high crop nutrient demand or when of nutrients in the soil. Another way to look at weather conditions result in slow nutrient re- these two theories is that the base saturation lease from organic resources. Commercial fer- theory does not concern itself with pH to any tilizers have the advantage of supplying plants great extent, but rather with the proportional with immediately available forms of nutrients. amounts of bases. The pH will be correct when They are often less expensive and less bulky to the levels of bases are correct. apply than many natural fertilizers. Albrecht’s ideas have found their way onto Not all conventional fertilizers are alike. Many large numbers of American farms and into the appear harmless to soil livestock, but some are programs of several agricultural consulting com- not. Anhydrous ammonia contains approxi- panies. Neal Kinsey, a soil fertility consultant mately 82% nitrogen and is applied subsurface in Charleston, Missouri, is a major proponent as a gas. Anhydrous speeds the decomposition of the Albrecht approach. Kinsey was a stu- of organic matter in the soil, leaving the soil dent under Albrecht and is one of the leading more compact as a result. The addition of an- authorities on the base-saturation method. He hydrous causes increased acidity in the soil, re- teaches a short course on the Albrecht system quiring 148 pounds of lime to neutralize 100 and provides a soil analysis service (21). His pounds of anhydrous ammonia, or 1.8 pounds book, Hands On Agronomy, is widely recognized of lime for every pound of nitrogen contained as a highly practical guide to the Albrecht sys- in the anhydrous (22). Anhydrous ammonia tem. ATTRA can provide more information on initially kills many soil microorganisms in the Albrecht Fertility Management Systems. application zone. Bacteria and actinomycetes recover within one to two weeks to levels higher Several firms—many providing backup fertilizer than those prior to treatment (23). Soil fungi, and amendment products—offer a biological- however, may take seven weeks to recover. farming program based on the Albrecht theory. During the recovery time, bacteria are stimu- Typically these firms offer broad-based soil lated to grow more, and decompose more or- analysis and recommend balanced fertilizer ganic matter, by the high soil nitrogen content. materials considered friendly to soil organisms. As a result, their numbers increase after anhy- They avoid the use of some common fertilizers drous applications, then decline as available soil and amendments such as dolomite lime, potas- organic matter is depleted. Farmers commonly sium chloride, anhydrous ammonia, and oxide report that the long-term use of synthetic fertil- forms of trace elements because they are con- izers, especially anhydrous ammonia, leads to sidered harmful to soil life. The publication How soil compaction and poor tilth (23). When bac- to Get Started in Biological Farming presents such terial populations and soil organic matter de- a program. See the Additional Resources sec- crease, aggregation declines, because existing tion for ordering information. For names of com- glues that stick soil particles together are de- panies offering consulting and products, order graded, and no other glues are being produced. the ATTRA publications Alternative Soil Testing PAGE 14 //SUSTAINABLE SOIL MANAGEMENT
  15. 15. Potassium chloride (KCl) (0-0-60 and 0-0-50), Sodium nitrate, also known as Chilean nitrate also known as muriate of potash, contains ap- or nitrate of soda, is another high-salt fertilizer. proximately 50 to 60% potassium and 47.5% Because of the relatively low nitrogen content chloride (24). Muriate of potash is made by re- of sodium nitrate, a high amount of sodium is fining potassium chloride ore, which is a mix- added to the soil when normal applications of ture of potassium and sodium salts and clay nitrogen are made with this material. The con- from the brines of dying lakes and seas. The cern is that excessive sodium acts as a dispers- potential harmful effects from KCl can be sur- ant of soil particles, degrading aggregation. The mised from the salt concentration of the mate- salt index for KCl and sodium nitrate can be rial. Table 7 shows that, pound for pound, KCl seen in Table 7. is surpassed only by table salt on the salt index. Additionally, some Protecting soil from erosion is Top$oil – Your Your plants such as tobacco, potatoes, the first step toward a sustain- peaches, and some legumes are able agriculture. Farm’$ Capital especially sensitive to chloride. High rates of KCl must be avoided Topsoil is the capital reserve of every farm. Ever on such crops. Potassium sulfate, potassium ni- since mankind started agriculture, erosion of trate, sul-po-mag, or organic sources of potas- topsoil has been the single largest threat to a sium may be considered as alternatives to KCl for fertilization. Table 7. Salt index for various fertilizers. Salt index per unit Material Salt Index of plant food Sodium chloride 153 2.9 Potassium chloride 116 1.9 Ammonium nitrate 105 3.0 Sodium nitrate 100 6.1 Urea 75 1.6 Potassium nitrate 74 1.6 Ammonium sulfate 69 3.3 Calcium nitrate 53 4.4 Anhydrous ammonia 47 .06 Sulfate-potash-magnesia 43 2.0 Di-ammonium phosphate 34 1.6 Monammonium phosphate 30 2.5 Gypsum 8 .03 Calcium carbonate 5 .01 PAGE 15 //SUSTAINABLE SOIL MANAGEMENT
  16. 16. soil’s productivity—and, consequently, to farm Water erosion gets started when falling rainwa- profitability. This is still true today. In the U.S., ter collides with bare ground and detaches soil the average acre of cropland is eroding at a rate particles from the parent soil body. After of 7 tons per year (2). To sustain agriculture enough water builds up on the soil surface, fol- means to sustain soil resources, because that’s lowing detachment, overland water flow trans- the source of a farmer’s livelihood. ports suspended soil down-slope (Figure 5). Suspended soil in the runoff water abrades and The major productivity costs to the farm associ- detaches additional soil particles as the water ated with soil erosion come from the replace- travels overland. Preventing detachment is the ment of lost nutrients and reduced water hold- most effective point of erosion control because ing ability, accounting for 50 to 75% of produc- it keeps the soil in place. Other erosion control tivity loss (2). Soil that is removed by erosion practices seek to slow soil particle transport and typically contains about three times more nu- cause soil to be deposited before it reaches trients than the soil left behind and is 1.5 to 5 streams. These methods are less effective at pro- times richer in organic matter (2). This organic tecting the quality of soil within the field. matter loss not only results in reduced water holding capacity and degraded soil aggregation, Commonly implemented practices to slow soil but also loss of plant nutrients, which must then transport include terraces and diversions. Ter- be replaced with nutrient amendments. races, diversions, and many other erosion “con- trol” practices are largely unnecessary if the Five tons of topsoil (the so-called tolerance level) ground stays covered year-round. For erosion can easily contain 100 pounds of nitrogen, 60 prevention, a high percentage of ground cover pounds of phosphate, 45 pounds of potash, 2 is a good indicator of success, while bare ground pounds of calcium, 10 pounds of magnesium, is an “early warning” indicator for a high risk and 8 pounds of sulfur. Table 8 shows the ef- of erosion (27). Muddy runoff water and gul- fect of slight, moderate, and severe erosion on lies are “too-late” indicators. The soil has al- organic matter, soil phosphorus level, and plant- ready eroded by the time it shows up as muddy available water on a silt loam soil in Indiana water, and it’s too late to save soil already sus- (25). pended in the water. Table 8. Effect of erosion on organic matter phosphorus and plant-available water. Organic matter Plant-available water Erosion level Phosphorus Lbs./ac % % 7.4 Slight 3.0 62 6.2 61 Moderate 2.5 3.6 Severe 40 1.9 From Schertz et al., 1984. (24) When erosion by water and wind occurs at a Protecting the soil from erosion is the first step rate of 7.6 tons/acre/year it costs $40 per acre toward a sustainable agriculture. Since water each year to replace the lost nutrients as fertil- erosion is initiated by raindrop impact on bare izer and around $17/acre/year to pump well soil, any management practice that protects the irrigation water to replace the soil water hold- soil from raindrop impact will decrease erosion ing capacity of that lost soil (26). The total cost and increase water entry into the soil. Mulches, of soil and water lost annually from U.S. crop- cover crops, and crop residues serve this pur- land amounts to an on-site productivity loss of pose well. approximately $27 billion each year (2). PAGE 16 //SUSTAINABLE SOIL MANAGEMENT
  17. 17. Figure 5. Raindrops falling on bare ground initiate erosion. Drawing from cropland monitoring guide (27). Additionally, well-aggregated soils resist crust- The researchers commented that subsoil had ing because water-stable aggregates are less been mixed with topsoil in the continuous corn likely to break apart when the raindrop hits plots from plowing, making the real topsoil them. Adequate organic matter with high soil depth less than was apparent. In reality, all the biological activity leads to high soil aggregation. topsoil was lost from the continuous corn plots in only 100 years. The rotation lost about half Many studies have shown that cropping sys- the topsoil over 100 years. How can we feed tems that maintain a soil-protecting plant future generations with this type of farming canopy or residue cover have the least soil ero- practice? sion. This is universally true. Long-term crop- ping studies begun in 1888 at the University of In a study of many different soil types in each Missouri provide dramatic evidence of this. of the major climatic zones of the U.S., research- Gantzer and colleagues (28) examined the ef- ers showed dramatic differences in soil erosion fects of a century of cropping on soil erosion. when comparing row crops to perennial sods. They compared depth of topsoil remaining af- Row crops consisted of cotton or corn, and sod ter 100 years of cropping (Table 9). As the table crops were bluegrass or bermuda grass. On shows, the cropping system that maintained the average, the row crops eroded more than 50 highest amount of permanent ground cover times more soil than did the perennial sod crops. (timothy grass) had the greatest amount of top- The two primary influencing factors are ground soil left. cover and tillage. The results are shown in Table 10. Table 9. Topsoil depth remaining after 100 So, how long do fields have before the topsoil is years of different cropping practices. gone? This depends on where in the country Inches of topsoil the field is located. Some soils naturally have Crop Sequence remaining very thick topsoil, while other soils have thin topsoil over rock or gravel. Roughly 8 tons/ 7.7 Continuous Corn acre/year of soil-erosion loss amounts to the 6-year rotation* 12.2 thickness of a dime spread over an acre. Twenty Continuous timothy grass 17.4 dimes stack up to 1-inch high. So a landscape *Corn, oats, wheat, clover, timothy with an 8-ton erosion rate would lose an inch From: Gantzer et al. (28). of topsoil about every 20 years. On a soil with a thick topsoil, this amount is barely detectable within a person’s lifetime and may not be no- PAGE 17 //SUSTAINABLE SOIL MANAGEMENT
  18. 18. Table 10. Effect of cropping on soil erosion rates Soil type Location Slope Row crop soil loss Sod soil loss State % Tons/ac Tons/ac Silt loam 9 38 .02 Iowa Loam 8 51 .16 Missouri Silt loam 12 99 .02 Ohio Fine sandy 7.7 19 .02 Oklahoma loam Clay loam 10 31 .31 N. Carolina Fine sandy 8.7 24 .08 Texas loam Clay 4 21 .02 Texas Silt loam 16 111 .10 Wisconsin Average 9.4 49 .09 Average Adapted from Shiflet and Darby, 1985 (29). ticed. Soils with naturally thin topsoils or top- systems, such as no-till and cover crops, are our soils that have been previously eroded can be best alternative until perennial systems are de- transformed from productive to degraded land veloped. within a generation. Summar y of Part I Summary Forward-thinking researcher Wes Jackson, of the Land Institute, waxes eloquent about how tillage has become engrained in human culture Soil management involves stewardship of the since we first began farming. Beating our soil livestock herd. The primary factors affect- swords into plowshares surely embodies the tri- ing organic matter content, build-up, and de- umph of good over evil. Someone who creates composition rate in soils are oxygen content, ni- something new is said to have “plowed new trogen content, moisture content, temperature, ground.” “Yet the plowshare may well have and the addition and removal of organic mate- destroyed more options for future generations rials. All these factors work together all the time. than the sword” (30). Any one can limit the others. These are the fac- tors that affect the health and reproductive rate Tillage for the production of annual crops is the of organic matter decomposer organisms. Man- major problem in agriculture, causing soil ero- agers need to be aware of these factors when sion and the loss of soil quality. Any agricul- making decisions about their soils. Let’s take tural practice that creates and maintains bare them one at a time. ground is inherently less sustainable than prac- tices that keep the ground covered throughout Increasing oxygen speeds decomposition of or- the year. Wes Jackson has spent much of his ganic matter. Tillage is the primary way extra career developing perennial grain crops and oxygen enters the soil. Texture also plays a role, cropping systems that mimic the natural prai- with sandy soils having more aeration than rie. Perennial grain crops do not require tillage heavy clay soils. Nitrogen content is influenced to establish year after year, and the ground is by fertilizer additions. Excess nitrogen, with- left covered. Ultimately, this is the future of grain out the addition of carbon, speeds the decom- production and truly represents a new vision position of organic matter. Moisture content af- for how we produce food. The greatest research fects decomposition rates. Soil microbial popu- need in agriculture today is breeding work to lations are most active over cycles of wetting develop perennial crops that will replace annual and drying. Their populations increase follow- crops requiring tillage. Farming practices us- ing wetting, as the soil dries out. After the soil ing annual crops in ways that mimic perennial becomes dry, their activity diminishes. Just like PAGE 18 //SUSTAINABLE SOIL MANAGEMENT
  19. 19. humans, soil organisms are profoundly affected Commercial fertilizers have their place in sus- by temperature. Their activity is highest within tainable agriculture. Some appear harmless to a band of optimum temperature, above and soil livestock and provide nutrients at times of below which their activity is diminished. high nutrient demand from crops. Anhydrous ammonia and potassium chloride cause prob- Adding organic matter provides more food for lems, however. As noted above, anhydrous kills microbes. To achieve an increase of soil organic soil organisms in the injection zone. Bacteria matter, additions must be higher than remov- and actinomycetes recover within a few weeks, als. Over a given year, under average condi- but fungi take longer. The increase in bacteria, tions, 60 to 70 percent of the carbon contained fed by highly available nitrogen from the anhy- in organic residues added to soil is lost as car- drous, speeds the decomposition of organic bon dioxide (20). Five to ten percent is assimi- matter. Potassium chloride has a high salt in- lated into the organisms that decomposed the dex, and some plants and soil organisms are organic residues, and the rest becomes ‘new’ sensitive to chloride. humus. It takes decades for new humus to de- velop into stable humus, which imparts the Topsoil is the farmer’s capital. Sustaining agri- nutrient-holding characteristics humus is culture means sustaining the soil. Maintaining known for (20). The end result of adding a ton ground cover in the form of cover crops, mulch, of residue would be 400 to 700 pounds of new or crop residue for as much of the annual sea- humus. One percent organic matter weighs son as possible achieves the goal of sustaining 20,000 pounds per acre. A 7-inch depth of top- the soil resource. Any time the soil is tilled and soil over an acre weighs 2 million pounds. left bare it is susceptible to erosion. Even small Building organic matter is a slow process. amounts of soil erosion are harmful over time. It is not easy to see the effects of erosion over a It is more feasible to stabilize and maintain the human lifetime; therefore, erosion may go un- humus present, before it is lost, than to try to noticed. Tillage for production of annual crops rebuild it. The value of humus is not fully real- has created most of the erosion associated with ized until it is severely depleted (20). If your agriculture. Perennial grain crops not requir- soils are high in humus now, work hard to pre- ing tillage provide a promising alternative for serve what you have. The formation of new drastically improving the sustainability of future humus is essential to maintaining old humus, grain production. and the decomposition of raw organic matter has many benefits of its own. Increased aera- Summar y of Sustainable Soil Summary tion caused by tillage coupled with the absence of organic carbon in fertilizer materials has Management Principles caused more than a 50% decline in native hu- mus levels on many U.S. farms (20). • Soil livestock cycle nutrients and provide many other benefits. Appropriate mineral nutrition needs to be present for soil organisms and plants to pros- • Organic matter is the food for the soil live- per. Adequate levels of calcium, magnesium, stock herd. potassium, phosphorus, sodium, and the trace elements should be present, but not in excess. • The soil should be covered to protect it from The base saturation theory of soil management erosion and temperature extremes. helps guide decision-making toward achieving optimum levels of these nutrients in the soil. • Tillage speeds the decomposition of organic Several books have been written on balancing matter. soil mineral levels, and several consulting firms provide soil analysis and fertility recommenda- • Excess nitrogen speeds the decomposition of tion services based on this theory. organic matter; insufficient nitrogen slows down organic matter decomposition and starves plants. PAGE 19 //SUSTAINABLE SOIL MANAGEMENT
  20. 20. • Moldboard plowing speeds the decomposi- tion of organic matter, destroys earthworm habitat, and increases erosion. • To build soil organic matter, the produc- tion or addition of organic matter must ex- ceed the decomposition of organic matter. • Soil fertility levels need to be within accept- able ranges before a soil-building program is begun. Photo by USDA NRCS PART II. MANAGEMENT STEPS TO IMPROVE SOIL QUALITY 1. Assess Soil Health and Biological is not as readily available, such as hypodermic Activity on Your Farm needles, latex tubing, a soil thermometer, an electrical conductivity meter, filter paper, and A basic soil audit is the first and sometimes the an EC calibration standard. The Soil Quality only monitoring tool used to assess changes in Test Kit Guide can be ordered from the USDA the soil. Unfortunately, the standard soil test through the Soil Quality Institute’s Web page, done to determine nutrient levels (P, K, Ca, Mg, < etc.) provides no information on soil biology and KitGuideComplete.pdf>. The 88-page on-line physical properties. Yet most of the farmer-rec- version of the guide is available in Adobe Acro- ognized criteria for healthy soils (see p. 2) in- bat Reader format through the above Web page clude, or are created by, soil organisms and soil and may be printed out. A summary of the tests physical properties. A better appreciation of is also available from the Web page. To order a these biological and physical soil properties, and print version, see the Soil Quality Institute ref- how they affect soil management and produc- erence under Additional Resources. tivity, has resulted in the adoption of several new soil health assessment techniques, which A greatly simplified and quick soil quality as- are discussed below. sessment is available at the Soil Quality Institute’s Web page as well, by clicking on “Getting to The USDA Soil Quality Test Kit Know your Soil,” near the bottom of the homepage. This simplified method involves dig- The USDA Soil Quality Institute provides a Soil ging a hole and making some observations. Quality Test Kit Guide developed by Dr. John Here are a few of the procedures shown at this Doran and associates at the Agricultural Re- Web site: Dig a hole 4 to 6 inches below the last search Service’s office in Lincoln, Nebraska. tillage depth and observe how hard the digging Designed for field use, the kit allows the mea- is. Inspect plant roots to see whether there is a surement of water infiltration, water holding lot of branching and fine root hairs or whether capacity, bulk density, pH, soil nitrate, salt con- the roots are balled-up. A lack of fine root hairs centration, aggregate stability, earthworm num- indicates oxygen deprivation, while sideways bers, and soil respiration. Components neces- growth indicates a hardpan. The process goes sary to build a kit include many items commonly on to assess earthworms, soil smell, and aggre- available—such as pop bottles, flat-bladed gation. Another useful, hands-on procedure for knives, a garden trowel, and plastic wrap. Also assessing pasture soils is discussed in the ATTRA necessary to do the tests is some equipment that publication Assessing the Pasture Soil Resource. PAGE 20 //SUSTAINABLE SOIL MANAGEMENT