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 50 g soil added Silt Clay After 40 sec
in suspension

5. Mechanical Analysis Sand 50 g soil added Silt Clay 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%

9. 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)

10. 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

11. Archimedes’ Principle

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

13. 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

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