Plain Talk About Clay Soils


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In plain English, the presentation explains what can and cannot be done to mitigate the effects of clay content in soil. It includes some basic irrigation terms and their definitions, within the context of soil texture.

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  • Kaolin Clay
  • Think about taking the clay soil model to the library?
  • I’m going to show you a series of slides that show how water moves through coarsely and finely textured soils. I filled mylar “ant farms” (hold one up) with sand, and started water slowly dripping into the sandwich. I traced the path of the water onto the plastic surface with a marker – this is what you’re seeing here.
  • Cation Exchange is trading: Plants trade what they cannot use for what they can. The soil acts as like the old drive-in movie theater.
  • Plain Talk About Clay Soils

    1. 1. Plain Talk About Clay Soils Serenity, courage, wisdom Linda J. Brewer Extension Horticulture Oregon State University
    2. 2. God grant me the serenity to accept the things I cannot change; 9/29/05
    3. 3. courage to change the things I can;
    4. 4. and wisdom to know the difference.
    5. 5. Why Does Clay Crack ? <ul><li>Shrink-swell clay minerals </li></ul><ul><li>Precise molecular structures </li></ul><ul><li>Water out – shrink and crack </li></ul><ul><li>Water in – swell and close </li></ul>H 2 O in or out
    6. 6. Fact #1: Soil includes minerals, organic matter, air and water. Minerals 45% Solid stuff Fluid stuff Air 20-30% Water 20-30% Organic matter 5%
    7. 7. Fact #2: Soil texture is the percentage of sand, silt and clay. Soil texture cannot be changed. 
    8. 9. Fact #3: Soil minerals exist as sand, silt and clay particles. Sand particles are big. Clay particles are small.
    9. 10. Sand: big particles, big pores. Clay: little particles, tiny pores.
    10. 11. <ul><li>SAND - Large </li></ul><ul><li>SILT </li></ul><ul><li>CLAY - Small </li></ul>
    11. 12. Used with permission: Bradshaw, M and R. Weaver. 1995. Foundations of Physical Geography. W.C. Brown Communications, Inc. Dubuque, IA. Subduction zone
    12. 13. Fact #4: Same volume, different surface area . Surface area = 2X2X6X8 = 192 sf Do the math and prove it to yourself! 4’ 4’ Surface area = 4X4X6 = 96 sf
    13. 14. Small particles, enormous surface area. Big particles, small surface area.
    14. 15. Soil Texture and Surface Area <ul><li>One gram of sand: seven square feet </li></ul><ul><li>One gram of clay: 48,000 square feet </li></ul>
    15. 16. Fact #5: Two major forces control water movement through soil. “ capillarity” gravity <ul><li>Capillary force draws water from wet to dry zones. </li></ul><ul><li>Soil texture dictates which is in control. </li></ul>
    16. 17. Gravity Coarse texture <5 minutes
    17. 18. Fine texture 20 minutes Capillarity and gravity.
    18. 19. Capillarity and gravity. 60% Clay >90 minutes
    19. 20. Serenity Message: <ul><li>Clay soils gain strength as they dry. </li></ul><ul><li>Add water slowly to clay soils. </li></ul><ul><li>Add water regularly to clay soils used for garden beds – don’t let them dry out. </li></ul>
    20. 21. Fine texture over coarse texture 7 minutes 2 minutes
    21. 22. Coarse texture over fine texture
    22. 23. Fact #6: A plant taking up water is playing tug of war with the soil. Clayey soils hold water tightly. Sandy soils have limited storage capacity. Image used with permission from the BBC’s Schools Website Evaporation
    23. 24. Transpiration drives water flow toward roots. Roots explore for water. root growth Plant root diffusion
    24. 25. Saturation = Soil Soup
    25. 26. Plant Available Water capillarity gravity evaporation
    26. 27. Fact #7: Unavailable Water As soil dries out, it holds on to water more tightly. capillarity not connected
    27. 28. Fact #8: Soil’s water storage depends on texture. stored water more less clay sandy loam silt loam sand loam clay loam soup water available to plants unavailable water field capacity permanent wilting point
    28. 29. Fact #9: Like diamonds, soil texture is forever. 
    29. 30. Serenity Message <ul><li>Soil texture will not change over the time scale of a human life. </li></ul><ul><li>Accept this. </li></ul>
    30. 31. Serenity Message: <ul><li>Gypsum may improve clay soils. </li></ul><ul><li>Sand will not improve clay soils. </li></ul><ul><li>Sand reduces drainage in clay soils. </li></ul>
    31. 32. Fact #10: Organic matter + clay: sow’s ear = silk purse <ul><li>Organic matter absorbs and stores water. </li></ul><ul><li>Organic matter stores plant nutrients. </li></ul><ul><li>Organic matter feeds soil biology. </li></ul><ul><li>Organic matter improves soil structure. </li></ul>
    32. 33. Fact #10 continued: <ul><li>Organic matter acts like a blanket to slow surface water loss. </li></ul><ul><li>Surface mulch protects soil aggregates. </li></ul><ul><li>Organic matter does not change soil texture! </li></ul>
    33. 34. Serenity Message: <ul><li>Organic matter does not change soil texture! </li></ul><ul><li>Accept this. </li></ul><ul><li>Organic matter does improve clay soil characteristics. </li></ul>
    34. 35. Working with Organic Matter <ul><li>Till in organic matter once – or twice. </li></ul><ul><li>Till in at depth. </li></ul><ul><li>Renew surface layer annually. </li></ul>
    35. 36. Advantages of Cracking Clays <ul><li>“ Self tilling” </li></ul><ul><li>Organic matter deposited at depth </li></ul><ul><li>Cracks drain early fall rains </li></ul><ul><li>Cracks increase soil drying by 50-100% </li></ul><ul><li>Cracks deposit water at depth </li></ul>soil surface
    36. 37. More Tips for Succeeding <ul><li>Alter the micro-climate </li></ul><ul><li>Don’t dig pots in clay soils </li></ul><ul><li>Choose clay tolerant plants: </li></ul>
    37. 38. Linda J. Brewer Extension Horticulture Extension Small Farms [email_address] 541-737-1408