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 MethodSoil Particles dispersed in 1 L H2O to

separateStoke’s Law determines suspended particlesHydrometer measures suspended particles

(g/L)

Mechanical Analysis

50 g soil

added

SandSiltClayDispersed

Particles

Mechanical Analysis

50 g soil

added

SandSiltClayAfter 40 sec

Silt + Clayin suspension

Mechanical Analysis

50 g soil

added

SandSiltClayAfter 8

hoursClay only

in suspension

Mechanical AnalysisStokes 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 Size40 second reading:

Gives si + cl = 20 gSince sa + si + cl = 50 gSa = 50 – (si+cl) = 50 – 20 = 30 gSo % sa = 30/50 x 100 = 60%

2 hour reading Gives suspended clay = 6 gSo, % 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 DisplayTwo sands; one dark, one light-coloredWhy?

Particle DensityBlack mineral, white mineral: what are

they?