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PHYSICAL PROPERTIES OF SOIL

PHYSICAL PROPERTIES OF SOIL. INTRO TO SOILS CH 4. SOIL TEXTURE. Describes the proportion of soil particle sizes: Sand Silt Clay Soil Texture influences other traits such as: Water Holding Capacity Aeration. EFFECT OF PARTICLE SIZE.

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PHYSICAL PROPERTIES OF SOIL

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  1. PHYSICAL PROPERTIES OF SOIL INTRO TO SOILS CH 4

  2. SOIL TEXTURE Describes the proportion of soil particle sizes: Sand Silt Clay Soil Texture influences other traits such as: Water Holding Capacity Aeration

  3. EFFECT OF PARTICLE SIZE Smaller particles – larger internal surface area

  4. EFFECT OF PARTICLE SIZE Morepores – micropores Larger pores – macropores

  5. SOIL SEPARATES SAND 2.00 – 0.05 mm SILT 0.05 – 0.002 mm CLAY < 0.002 mm

  6. TEXTURAL CLASSIFICATION Twelve Textural Classes Textural Triangle

  7. SOIL DENSITY AND PERMEABILITY DENSITY: MASS per VOLUME D = M V

  8. SOIL DENSITY AND PERMEABILITY Two densities in Soil: Particle Density – PD Bulk Density - BD

  9. SOIL DENSITY AND PERMEABILITY PD average soils ~ 2.65 gm/cu cm BDaverage range from 1.0 – 1.8 gm/cu cm Depends on amount of pore space

  10. SOIL POROSITY Usually expressed as a percentage; e.g. 50% Two means determining porosity: • Calculate ratio water volume to total core volume • Calculate from bulk density and particle density

  11. SOIL POROSITYExamples Water Volume to Core Volume Porosity = wet weight (g) – dry weight (g) soil volume (cu cm) x 100%

  12. SOIL POROSITYExamples An oven-dry soil core, volume 500 cu cm, weighs 650g. When wet, it weighs 900g. Find it’s % porosity. Porosity = 900g – 650g x 100% = 250g x100% = 50% 500 cu cm 500 cu cm Note: the unit cancellation is made possible by the metric system which defines 1 cu cm of water as weighing 1g.

  13. SOIL POROSITYExamples Bulk Density to Particle Density Defines the percentage of the soil that is solid matter The percent solid matter is subtracted from 100% to give percent porosity: Porosity = 100% - (BD/PD x 100%)

  14. SOIL POROSITYExamples An undisturbed oven-dry soil, BD of 1.3 g/cu cm, consists of average mineral composition (PD 2.65 g/cu cm). Find its % Porosity: Porosity = 100% - (1.3 g/cu cm/2.65 g/cu cm x 100%) Porosity = 100% - (0.49 x 100%) = 100% - 49% = 51%

  15. SOIL POROSITY Question: Which has greater porosity, Sand or Clay? Answer: Clay at about 50%; Sand is lower at about 30% Why?

  16. TEXTURE AND SOIL PORES

  17. Why, then, does water move through sandy soil very rapidly, but moves s l o w l y into clay ?

  18. This is explained by the physical property Permeability

  19. Permeability – the ease with which air, water, and roots move through soil - Depends on number, size, and continuity of pores - Liken to a maze PERMEABILITY

  20. Fine-textured soils would be impermeable if not for: Soil Structure

  21. STRUCTURE Structure – the way soil particles clump together into large units called aggregates or peds

  22. STRUCTURE Structure can alter the effects of texture • e.g. a fine-textured silty clay with good structure can be permeable!

  23. STRUCTURE • Structure is classified by three groups of traits: • Type – refers to shape of aggregates e.g. Granular, Platy, Blocky, Prismatic, Columnar • Class – refers to size of peds e.g. very fine, fine, medium, coarse, very coarse • Grade – refers to strength and distinction of peds e.g. weak/not visible vs. strong/easily distinguished

  24. FORMATION OF SOIL STRUCTURE Two-step formation: • Individual soil particles loosely aggregate • Weak aggregates are cemented to strengthen - clay - iron oxides - organic matter - microorganism gums

  25. ADDNL. SOIL PHYSICAL PROPERTIES Soil Consistence – the behavior of soil when pressure is applied; measured at three different moisture levels: Wet, Moist, Dry (fig 4-15, p. 59) Soil Tilth – ease of tillage, seedbed preparation, and seedling/root movement Compaction – results from pressure applied at the soil surface

  26. ADDNL. SOIL PHYSICAL PROPERTIES Puddling, Clods, and Crusts: Puddling occurs when pressure is applied to very wet soils (esp. plowing) Crusts occur when bare soil is struck by raindrops; disperses soil then dries to a hardened crust

  27. ADDNL. SOIL PHYSICAL PROPERTIES Improving Tilth: Best accomplished by improving structure - tilth relates to texture, structure, permeability, and consistence; however, texture and consistence cannot be altered easily therefore, improve tilth by improving structure and avoiding compaction

  28. ADDNL. SOIL PHYSICAL PROPERTIES Soil Channels – continuous macropores leading from surface to deep subsoil Soil Pans – any layer of hardened soil; includes: - claypans - fragipans (clays) - plinthite (tropics) - caliche (Ca cemented)

  29. ADDNL. SOIL PHYSICAL PROPERTIES Soil Temperature (varies w/color, texture, O.M.) Soil Color Munsell soil color chart Hue, Value, Chroma e.g. 10YR 3/6

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