Texas Hill Country Soils and Erosion Control


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Amanda Bragg, research soil scientist with USDA/NRCS, presents Texas Hill Country Soils and Erosion Control to the 2013 Master Naturalist, Hill Country Chapter training class.

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  • Parent material composition has a direct impact on soil chemistry and fertility. Parent materials rich in soluble ions-calcium, magnesium, potassium, and sodium, are easily dissolved in water and made available to plants. Limestone and basaltic lava both have a high content of soluble bases and produce fertile soil in humid climates. If parent materials are low in soluble ions, water moving through the soil removes the bases and substitutes them with hydrogen ions making the soil acidic and unsuitable for agriculture. Soils developed over sandstone are low in soluble bases and coarse in texture which facilitates leaching. Parent material influence on soil properties tends to decrease with time as it is altered and climate becomes more important.
  • Physical disintegration is the first stage in the transformation of parent material into soil. The freezing of absorbed water, causes the physical splitting of material, along a path toward the center of the rock, while temperature gradients within the rock can cause exfoliation of ‘shells’. Cycles of wetting and drying cause soil particles to grind into finer sizes as does the physical rubbing of material caused by wind, water, and gravity. Organisms also reduce parent material in size through action of plant roots or digging on the part of animals.When minerals are made soluble by water or are changed in the structure.Solubility changesSolution of salts in water results from the action of bipolar water on ionic salt compounds*Hydrolysis – transformation of minerals into polar molecules by the intervention by and splitting of water. Results in soluble acid-base pairs.*Carbonation – reaction carbon dioxide in solution with water forms carbonic acid.Structural changesHydration – inclusion of water in a mineral structure causing it to swell and leaving it more stressed and more easily decomposedOxidation – compound swells and increase its oxidation number leaving it more easily attacked by water or carbonic acidReduction – oxidation number some part of the mineral is reduced when oxygen is scarce. Reduction of minerals leaves them electrically unstable, more soluble and easily decomposed.
  • Soils tend to show a strong geographical correlation with climate, especially at the global scale. Energy and precipitation strongly influence physical and chemical reactions on parent material. Climate also determines vegetation cover which in turn influences soil development. Precipitation also affects horizon development factors like the translocation of dissolved ions through the soil. As time passes, climate tends to be a prime influence on soil properties while the influence of parent material is less.
  • The A and B horizons are considered to be the Solum or ‘True Soil’ - Most of the chemical and biological activity that has formed the soil takes place in those two horizons.
  • increased supply of available nutrients for plants as it is broken downincreased water-holding capacity in sandy soil, because organic matter holds watera greater amount of air in the soil, because the organic matter opens up pore spaces by forming crumbs (especially important in clay soils)raised soil temperature as the dark colour of the organic matter absorbs and holds heat
  • 1. Apply practices that enhance soil organic matterDiverse, high biomass crop rotations Cover crops Reduced tillage Rotational or prescribed grazing 2. Organic matter dynamics changeIncreased surface residue forms a physical barrier to wind and water erosion. Higher residue rotations and cover crops contribute more organic matter and nutrients to the soil. Less soil disturbance means lower organic matter losses. 3. Soil properties change Surface structure becomes more stable and less prone to crusting and erosion. Water infiltration increases and runoff decreases when soil structure improves. Soil organic matter holds 10 to 1,000 times more water and nutrients than the same amount of soil minerals. Beneficial soil organisms become more numerous and active with diverse crop rotations and higher organic matter levels. 4. Air quality, water quality, and agricultural productivity improve Dust, allergens, and pathogens in the air immediately decline. Sediment and nutrient loads decline in surface water as soon as soil aggregation increases and runoff decreases. Ground and surface water quality improve because better structure, infiltration, and biological activity make soil a more effective filter. Crops are better able to withstand drought when infiltration and water holding capacity increase. Organic matter may bind pesticides, making them less active. Soils managed for organic matter may suppress disease organisms, which could reduce pesticide needs. Crop health and vigor increase when soil biological activity and diversity increase. Wildlife habitat improves when residue management improves.
  • Most nutrients, with the exception of the lack of nitrogen in desert soils, are present in the soil but may not be available to plants due to extremes of pH. Most nutrients originate from minerals and are stored in organic material both live and dead on colloidal particles as ions. The actions of microbes on organic matter and minerals may free nutrients for use, sequester them, or cause their loss from soil by their volitalization to gasses or by leaching upon their conversion to soluble forms. Most of the nitrogen available in soils is the result of nitrogen fixation by bacteria. The organic material of the soil has a powerful effect on its development, fertility, and available moisture. Following water, organic material is next in importance to soil’s formation and fertility.
  • Texas Hill Country Soils and Erosion Control

    1. 1. Hill Country Soils and Erosion Control September 12, 2012 Amanda Bragg Resource Soil Scientist San Angelo, Texas The USDA is an equal opportunity provider.
    2. 2. Presentation Overview – How soil forms – Physical soil properties – Soil Biology – Chemical soil properties – Soil vegetation relationships – Web Soil Survey – Soil erosion Source: http://soil.gsfc.nasa.gov
    3. 3. The Five Soil Forming Factors • Parent Material • Climate • Biotic Activity (Living Organisms) • Topography • Time
    4. 4. Parent Material • The material that the soil is weathering or forming from • Includes: – – – – Bedrock Alluvium (water-laid sediments) Colluvium (moved and deposited by gravity) Eolian (wind-blown sediments) • As the parent material is weathered, transported, deposited, and precipitated it is transformed into a soil.
    5. 5. Different Types of Parent Material • Residuum – developed from underlying parent rocks…Found on mesas, plateaus, and plains (~3% of all soils in U.S.) • Aeolian or Eolian – moved and deposited by wind – Because it forms stable aggregates clay is rarely moved by wind. • Alluvium – moved and deposited by flowing water • Lacustrine – sedimentary deposits in lakes • Marine deposits – beds of ancient seas revealed as the land has uplifted • Glacial deposits • Colluvium – moved and deposited by gravity
    6. 6. Parent Material Affects • • • • • • Sand, silt, and clay Chemical content Bulk density Structure Kinds and amounts of rock fragments Soil fertility
    7. 7. Weathering • Physical and chemical decomposition of parent material caused by climatic and biotic activity • Physical Weathering: – Abrasion • Chemical Weathering: – Dissolution of rocks by acid • Carbon dioxide dissolved in rain • Plant produced acids (like pine needles)
    8. 8. Climate • Dominant factor in soil formation • Rainfall / Moisture – Speeds chemical weathering – Runoff speeds physical weathering – Promotes biotic activity – Erosion and deposition • Wind – Abrasion – Erosion and deposition
    9. 9. Texas Precipitation --- 9 to 67”
    10. 10. Climate • Temperature – Heating and cooling cycles cause rocks to expand and contract – Water freezing in cracks expands and breaks up parent material – Warmer conditions promote biotic activity and speed chemical weathering – Decomposition of organic matter increases with temperature
    11. 11. Climate Influences • In areas of low rainfall – shallow accumulation of lime as caliche • In humid areas – formation of acidic soils • Erosion of soils on steep hillsides • Deposition of eroded material • In warm and humid areas – very intensive chemical weathering, leaching, and erosion
    12. 12. Biotic Activity • All activities of living organisms that result in parent material weathering • Includes: – Plant roots – Burrowing animals and insects – Some plants produce rock dissolving acids – Organic matter produced improve conditions for other, higher, organisms – Microorganisms
    13. 13. Wiki- organisms • Animals and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about. • Plant roots open channels in soils • Plants with deep taproots can penetrate many meters through different soil layers to bring up nutrients from deeper in the profile. Plants with fibrous roots that spread out near the soil surface have roots that are easily decomposed, adding orgamic matter. • Microorganisms – effect chemical exchanges between the roots and soil and act as a reservoir of nutrients. Humans – remove vegetative cover -> erosion
    14. 14. Wiki - Vegetation • Prevent erosion • Shade soils – keeping them cooler – slowing the evaporation of soil moisture • Transpiration can cause soil to lose moisture • Form new chemicals that can break down minerals and improve soil structure
    15. 15. Topography • Layout of the landscape • Impact of elevation and slope on water movement and erosion – Soil runoff – Infiltration • Aspect • Temperature – Cooler with higher elevation • Vegetation Source: Soil Survey of McCulloch County, Texas, 1974
    16. 16. Time • Cumulative effects of all other factors over hundreds to thousands of years • Takes about 500-1000 years to form 1” of soil • With age typically: – Soil depth increases – Soil resembles parent material less – Soil becomes more developed • Exceptions – Areas with constant sediment deposition – Areas where erosion is greater than soil formation
    17. 17. Soil Horizons • Distinctive layers within a soil that is parallel to the soil surface and used to describe and classify soils • Surface horizons: – O – Layer of organic matter – A – Mineral layer at soil surface
    18. 18. Soil Horizons • Typical subsurface horizons: – E – zone of eluviation or leaching of clays and minerals – B – more developed than the A horizon; sometimes considered a zone of accumulation of clays and minerals like calcium carbonates – C – unweathered to slightly weathered parent material; little to no soil development – R – Consolidated bedrock
    19. 19. Soil Development O A R O A O A O A B A B E B R C R R C Time
    20. 20. Drainage and Topography
    21. 21. Physical Soil Properties • • • • Texture Structure Density Porosity • Consistency • Color • Temperature
    22. 22. Major Soil Components Typical Soil Composition Mineral Organic Water Air Wet Dry Year Year Pore Space* 50% Pore Space 25% water* 25% air* Solids 50% Solid Material 45% mineral material 1 to 5% organic matter
    23. 23. Soil Texture • How a soil feels • The percent by weight of sand, silt, and clay in the soil • Most important physical soil property • Affects: • Soil structure • Water-holding capacity • Nutrient-holding capacity • Essentially impossible to change unless you remove it or add large amounts to it
    24. 24. Soil Texture – Soil Particle Sizes Separate Name Size in mm Number of particles per gram Surface Area (g/cm2) Shape Sand 0.05 to 2.0 90 – 722k 11-227 Round Silt 0.002 to 0.05 5.8 million 454 Round Clay Less than 0.002 90.3 trillion 8 million Flat From: Henry Foth – Fundamentals of Soil Science, 7th Edition From: Discovery Education Resources
    25. 25. Soil Particle Sizes and Pore Space
    26. 26. Soil Texture
    27. 27. Affect of particle size on soil properties • Sands – Largest in size – Chemically – not very active – Supplies bulk – Porosity • Do not pack closely together (MACROPORES) • Easy root penetration • Allows water flow and exchange of gases
    28. 28. Affect of particle size on soil properties • Silts – In-between size – Express properties of both sands and clays – Chemically – more active than sands, less active than clays – Porosity • Smaller pores than sands • Can retain water against gravity • Water more available to plants than with clays
    29. 29. Affect of particle size on soil properties • Clays – Smallest size – (Microscopic) – Chemically – most active – Negative ionic charge attracts positive ions – Surface holds required minerals (cations) required by plants – Porosity • • • • Smallest pores (Micropores) Can retain water against gravity (Capillary action) Holds the most water cumulatively Not all water is available to plants.
    30. 30. Chemical structure for Smectite
    31. 31. Soil Structure Highest density? Fastest infiltration? Best for roots / plant growth?
    32. 32. Soil Structure
    33. 33. Porosity Volume of soil not occupied by mineral and organic matter. Macropores – typically hold air Micropores – typically hold water Allows for the movement and storage of: Air Water Dissolved nutrients Tillage - increases % of macropores temporarily but results in destruction of soil structure.
    34. 34. SOIL COLOR Write color as: Hue Value/Chroma (10YR 6/3) HUE CHROMA VALUE
    35. 35. Soil Colors
    36. 36. Dark Colors - High Organic Matter
    37. 37. Light Colors - Low organic matter content
    38. 38. Mottles/Redoximorphic Features Mottles or redoximorphic features are caused by oxidation or reduction of iron in the soil. • Red, orange and yellow colors are iron accumulations (oxidized iron) • Grays are iron depletions (reduced iron)
    39. 39. Yellows, Browns, Reds – Well Drained – oxidized iron water-shedding areas
    40. 40. Red Colors Highly weathered –oxidized iron
    41. 41. Gray – wet water accumulating areas
    42. 42. Soil Temperature
    43. 43. Soil Organic Matter (Humus)
    44. 44. Soil Biology • • • • Plant Roots Bacteria Fungi Actinomycetes • • • • Nematodes Protozoa Arthropods Earthworms
    45. 45. Soil BiologyFacts • A single shovelful of soil can contain more species of organisms than live aboveground in the entire Amazon rain forest • One cup of soil may hold as many bacteria as there are people on Earth • The weight of all the bacteria in 1 acre of soil can equal the weight of 1 or 2 cows • Mature trees can have as many as 5 million active root tips. • A teaspoon of forest soil may hold more than 10 miles of fungi. • Almost all freshwater travels over soil or through soil before entering rivers, lakes, and aquifers. • Plants can remove 400 to 2,000 lbs of water for every 2 lbs of plant material produced. • About 85% of the CO2 in the air comes from the actions of soil microbes feeding on organic matter. • Microbes have been found as deep as 10 miles.
    46. 46. Organic Matter
    47. 47. Soil pH • Acidity or Alkalinity • Classes: – Acid – pH < 7 – Neutral – pH = 7 – Alkaline – pH > • Potential acidity is contained on the surfaces of the clay and organic particles • Active acidity is the liquid portion of the soil • High rainfaill climates produce more acid soils
    48. 48. Soil Reaction • Determines solubility and availability of certain elements for plant growth • Nutrient levels are optimum 6.0 to 7.5 for most plants. • Below 6.0: reduced availability for phosphorus, potassium, calcium, magnesium, and sulfur • Above 7.5: reduced availability for copper, zinc, iron, and manganese
    49. 49. Soil Fertility • Ability of the soil to supply nutrients in proper amounts and proportions as needed for establishment, development, and reproduction of the plant species • Mainly based on the cation-exchange capacity – Clay fraction – Organic matter
    50. 50. Salinity • A localized problem on Gulf Coast • Almost all soils have some salinity • Causes soil to become hard • Damages roots and stunts plants • Reduces the water that is available to plants (moisture can be in the soil, but the sodium “ties” it up to where it is not available to plants • Damages steel
    51. 51. Soil Compaction • High traffic areas • When soils are manipulated when wet • Plowing or incorporating organic matter decreases compaction • Minimize hazard of compaction by having a good thick layer of mulch on surface
    52. 52. MLRAs of Texas
    53. 53. MLRA’s Texas Hill Country
    54. 54. Clay Loam
    55. 55. Low Stony Hill
    56. 56. Redland
    57. 57. Adobe Seep Muhly
    58. 58. Steep Adobe
    59. 59. Loamy Bottomland
    60. 60. What happens when it rains? • Infiltration – Texture – Structure – Organic matter – Depth to root limitation – Soil Moisture – Temperature • Runoff
    61. 61. Soil Erosion • Erosion - Wearing away of a substance • Two types of soil erosion – Geologic – slow method by which natures forces try to bring all surfaces into one plane – Accelerated – erosion that occurs because of the effects of man and occurs at a much faster rate • Example – Undisturbed land – 0.02 tons of soil lost per acre per year – Cultivated with mismanagement and dramatic climate conditions – 4 to 50 tons lost per acre per year
    62. 62. Soil Erosion • Reduced productivity • Inability to hold excess rainfall – Overland flow losses and ends up in drainage-ways and rivers – Can cause damage from flooding – Does not recharge aquifers • Formation of gullies • Silted waterways • Turbid water
    63. 63. Detachment – The beginning of soil movement Raindrops break the bond between soil particles and splash them a short distance. The detached particles are vulnerable to water moving over the soil surface
    64. 64. Sheet Erosion When rain falls faster than the soil can absorb it, water will collect and flow over the ground surface. The surface water will begin to carry soil particles that were detached by raindrops.
    65. 65. Rill Erosion Surface flow will soon establish paths. If the soil is unprotected, some of these paths become small eroding channels. Moving further downslope, flow in rills becomes more erosive, causing them to enlarge and join with others.
    66. 66. Concentrated-Flow Erosion (Ephemeral Gullies) The topography of the landscape in many places will allow flow to collect in a few major water courses before it leaves the field. Rills are erased by tillage, but channels eroded by concentrated flow tend to re-form in the same location each year.
    67. 67. Gully Erosion Rapidly moving water may cause severe “head cutting” and erosion of the gully’s sidewalls. Headcutting will continue as the gully advances up slope. Unless treated, it will not stop until it reaches the top of the slope.
    68. 68. Poor land treatment Sedimentation Nutrients attached to sediment Nutrients in solution or in air Effective Land Treatment Reduced sedimentation and nutrient loading
    69. 69. Approximately 1 billion tons of topsoil are lost annually in the U.S. due to wind erosion. 5 million acres of land in the Great Plains Region of the US are damaged each year. NRCS Darren Richardson The loss of fertile topsoil is severe enough in many places to permanently reduce inherent soil productivity.
    70. 70. Abrasion from wind blown soil particles can damage or destroy crops. The quality of many crops, particularly vegetables, may be reduced because of soil deposition or damage. NRCS Darren Richardson Peanuts
    71. 71. Aerosols Some soil from damaged land enters the atmosphere obscuring visibility, polluting the air, causing vehicle accidents, fouling machinery and impairing animal and human health. Approximately 20 infectious diseases, including tuberculosis and anthrax have been associated with dust particles.
    72. 72. Deposition of wind blown soil particles can clog canals, lakes and water courses. Other sediment deposited by wind is susceptible to movement by rainfall into water bodies. This sediment and the associated substances attached to it can significantly deteriorate water quality.
    73. 73. Wind erosion may occur wherever the soil is exposed, loose and dry. In Texas it is most severe in the Southern High Plains, Southern Rolling Plains, Rio Grande Plains and Gulf Coastal Regions.
    74. 74. How Wind Moves the Soil Under field conditions, soil begins to move when the wind velocity reaches about 13 miles per hour at 1 foot above the ground surface. People tend to notice dust high in the air. Soil particles carried high in the air are the smaller particles that include the most valuable portions of topsoil – clay and organic matter. However, most soil movement occurs within a foot of the ground. Only a small percentage is carried high in the air. ARS-WERU
    75. 75. Wind Erosion Processes Saltation Surface Creep Suspension
    76. 76. SALTATION • Fine and medium sand and sand sized particles move in this manner. • The wind lifts them a short distance into the air and they “bounce” to the soil surface. Most saltation occurs within 12” of the soil surface. • The “bouncing” impact detaches soil aggregates that can be carried by wind. • Saltation destroys protective surface crusts, making the soil more erodible. • Saltation accounts for 50 – 80% of total soil movement.
    77. 77. Suspension • Very fine soil particles are lifted by the impact of saltation. • They are carried high into the air and suspended for long distances. • Suspension generally accounts for only a small part of the total soil moved by wind. • Small suspended particles are the most fertile parts of eroded soils. • Movement of fine particles off-site tend to make sandy soils sandier.
    78. 78. Surface Creep • The movement of larger, sand sized soil particles along the surface. • Too large to be lifted by the wind, these particles move along the surface in a rolling motion. • Surface creep can account for as much as 25% of the soil moved by wind.
    79. 79. Not all sand sized particles are sand. Certain clay and clay loam soils will form stable sand sized aggregates. Calcareous soils particularly exhibit this type of aggregate stability.
    80. 80. Preventing Erosion Stubble Mulch No-Till Contour Farming Strip Cropping Terracing Prescribed Grazing Secondary Benefits: - Moisture Conservation Increased Crop Yield Prevents Loss of Fertilizer Saves Fuel Improved Structure & Rooting Depth - Sustainability P. 67 Brady
    81. 81. Preventing erosion helps in that… • Moisture Conservation leads to Increased Yields (especially in drier years) and makes money. • Improved Soil Structure leads to Moisture Conservation (see above). • Preventing Fertilizer and PesticideLoss leads to Increased Yields and saves money. • Takes less horse power to use a spray rig than tillage (saves money). • Reduced pollution from fertilizer and pesticides
    82. 82. References • The Nature and Properties of Soils, by Brady • Essentials of Physical Geography, by Gabler, Sager, Brazier and Wise • Field Book for Describing and Sampling Soils, by Schoeneberger, Wysocki, Benham and Broderson • http://munsell.com/ • http://soilcrop.tamu.edu/ • http://soils.usda.gov/ • http://www.tx.nrcs.usda.gov/
    83. 83. USDA Nondiscriminatory Policy... The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, sex, religion, age, disability, political beliefs, sexual orientation, or marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative means for communication of program information (Braille, large print, audiotape, etc.) should contact USDA’s TARGET Center at 202-720-2600. To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington, DC 202509410, or call (202) 720-5964