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Lab 5 – Soil Physics

Lab 5 – Soil Physics . 1. Mechanical Analysis: Textural Class in the Lab %Sand , Silt, and Clay 2. Bulk Density: 3 Methods 3. Aggregation: Wet-Stability of Soil Aggregates 4. Particle Density and Sand Mineralogy. Mechanical Analysis. Soil Textural Class - Laboratory Method

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Lab 5 – Soil Physics

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  1. Lab 5 – Soil Physics 1. Mechanical Analysis: Textural Class in the Lab %Sand, Silt, and Clay 2. Bulk Density: 3 Methods 3. Aggregation: Wet-Stability of Soil Aggregates 4. Particle Density and Sand Mineralogy

  2. Mechanical Analysis • Soil Textural Class - Laboratory Method • Soil Particles dispersed in 1 L H2O to separate • Stoke’s Law determines suspended particles • Hydrometer measures suspended particles (g/L)

  3. Mechanical Analysis Sand Silt Clay 50 g soil added Dispersed Particles

  4. Mechanical Analysis Sand Silt Clay 50 g soil added After 40 sec Silt + Clay in suspension

  5. Mechanical Analysis Sand Silt Clay 50 g soil added After 8 hours Clay only in suspension

  6. Mechanical Analysis • Stokes Law (simplified): v = 8950·r^2 Where r = particle radius, v = settling velocity Since r^2 (r squared), as r , v exponentially i.e. Sand settles quickly, Clay settles extremely slowly

  7. Calculating Particle Size • 40 second reading: • Gives si + cl = 20 g • Since sa + si + cl = 50 g • Sa = 50 – (si+cl) = 50 – 20 = 30 g • So % sa = 30/50 x 100 = 60% • 2 hour reading • Gives suspended clay = 6 g • So, % cl = 6g/50g x 100 = 12% • Since %sa + %si + %clay = 100% • % si = 100% - %sa - %cl = 100 – 60 – 12 = 28%

  8. Bulk density and Porosity • Bulk Density: g/cm3 • Inversely related to porosity • Porosity: % pore space (non-solid) • Non-solid = macro/micro-pores filled with air and/or water • %Porosity=100 x (1-BD/PD)

  9. Bulk density and Porosity • Three Methods: 1. Cylinder – best for tilled soils; subjective volume 2. Ring – in situ method; retains soil structure 3. Clod – plastic coated clod; allows for irregular shapes

  10. Archimedes’ Principle

  11. “The Crown of Syracuse” https://www.youtube.com/watch?v=wEvtahSn_ms

  12. Aggregate Stability • Wet-Sieving Soil Aggregates: How erodible? • Stability: humus, macro-pores, roots • Slaking: aggregate breakdown due to rapid wetting • %WSA (Water Stable Aggregates): Proportion of retained (wet-stable) aggregates

  13. Sand Mineralogy Display • Two sands; one dark, one light-colored • Why? • Particle Density • Black mineral, white mineral: what are they?

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