Understanding Soil Properties: Texture, Adhesive-Cohesive Features, Structure, Density, Porosity, and Color

1. Soil properties A. Texture B. Adhesive-Cohesive properties (Plasticity/Stickiness) C. Structure D. Density E. Porosity F. Color

2. A. Texture • Relative proportion of sand, silt, clay sized particles in a soil • Does not change (in human lifetime) • Important for agricultural and engineering uses of soil

3. Fine earth fraction only • Does not include coarse fragments: • Boulders: > 600 mm • Stones: 250 – 600 • Cobbles: 75 – 250 • Gravels : 2 - 75

4. USDA fine earth fraction (“soil separates”): • Sand 0.05 – 2.0 mm • Very coarse 1.0 – 2.0 • Coarse 0.5 – 1.0 • Medium 0.25 – 0.5 • Fine 0.1 – 0.25 • Very fine 0.05 – 0.1 • Silt 0.05 – 0.002 • Clay <0.002

5. sand • Naked eye • Gritty • Predominantly quartz • Round

6. silt • Light microscope • Cannot feel individual grains; slippery • Predominantly quartz and other primary minerals • In between round and flat

7. clay • Electron microscope • Wide variety of minerals • Flat

9. Properties that vary with particle size: • Surface area • Geometry of pore spaces • Adhesive / Cohesive properties; Plasticity / Stickiness

10. Surface area • (site of water adsorption, gas adsorption, mineral weathering, nutrients) • Very coarse sand: • Particles per gram = 90 • Surface area = 11 cm2 / gm • Clay : • Particles per gram = 90,260,853,000 • Surface area = 8,000,000 cm2 /gm

12. Pore space geometry • Sand has large pores between grains • Highly permeable • Silt has relatively small pores • Less permeable • Clay has very small pore spaces • Least permeable

13. B.Adhesive/Cohesive properties • Adhesion: force with which something clings to other surfaces • Soil and water • Cohesion: force with which something clings to itself • Soil particles

14. Plasticity/Stickiness • Plasticity is ability to be molded; force required to deform soil in wet pliable condition • Make a “worm” of soil; see how thin the worm can be and still support its own weight on end • Indicates cohesiveness

16. Stickiness is force required to pull soil apart when wetted (beyond plastic) • Press moist soil between thumb and forefinger and see how much sticks to fingers • Indicates adhesiveness

18. Non-sticky

19. Slightly sticky

20. Very sticky

21. Shape governs extent of contact between adhering and cohering surfaces • Greatest contact occurs when flat surfaces lie parallel to one another (as in clay) • e.g. cohesiveness makes some clays turn into hard clods when dry and become very sticky when wet

22. Sand has a large particle size and round shape • Limited contact with other surfaces • not sticky, not plastic • Silt is more cohesive and adhesive than sand, but has only limited plasticity and stickiness • Can be crushed when dry

23. C. Structure • Way in which soil particles are assembled in aggregate form • Results from pedogenic processes • Structural unit is ped • e.g., blocky soil has blocks as peds • Ped: < cm to several cm

24. Structures: • Platy: flat horizontal units; diverse sizes

27. 2. Prismlike: tall peds with flat sides Prismatic: flat tops Columnar: rounded tops

31. 3. Blocky Angular: flat faces, sharp corners Subangular: faces and corners are rounded

33. Subangular blocky Angular blocky columnar prismatic

35. 4. Granular: roughly spherical; porous

38. 5. wedge-shaped peds form in clays where cracking and swelling cause soils to slide along planes

40. 6. Structureless single-grained massive

41. Importance of structure • Movement of air and water • Root penetration

42. What gives structure to soil? • Organic gums (HUMUS!) • Decay products • Shrink and crack on drying • Shrink-swell clays • Roots • Freeze/thaw cycles • Soil animals • Fungal hyphae

43. compaction

44. Pan structures • Dense layers, diverse origins

45. Clay pan Clay accumulation; usually B Duripan cemented by ppt silica , iron oxides, and/or CaCO3 Fragipan hard, brittle dense and compact, but breaks apart when taken out Caliche white layer of CaCO3 (soft or hard); arid near surface

46. 5. Plinthite (laterite) sesquioxides, usually B tropical, weathered soft when wet; brick hard when dry 6. Plowpan compaction from weight of implements

47. Clay pan

48. duripan

49. fragipan

50. caliche