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 Soil Particles dispersed in 1 L H2O to separate Stoke’s Law determines suspended particles Hydrometer measures suspended particles (g/L)

Mechanical Analysis Sand Silt Clay 50 g soil added Dispersed Particles

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

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

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

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%

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)

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

Archimedes’ Principle

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

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

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