1. Soil Physical Properties – Colour, Texture, Structure
LECTURE 5
Soil Physical Properties –
Colour, Texture, Structure

2. What is meant by physical properties?
Characteristics of soil which can be measured by physical means and expressed in physical terms, such as colour, density, porosity, hydraulic conductivity, structure, texture and depth.

3. Soil colour…
Colour does not have a great effect on the behaviour of soils
Exception is the fact that darker soils absorb more solar radiation.
Colours give us clues about other soil properties.
Standard system for the description of soil colour = the Munsell colour chart system.

4. Soil colour described by referring to 3 components:
Chroma (intensity/brightness)
0 = neutral grey
Hue (redness or yellowness)
Value (lightness or darkness)
0 = black

5. What causes soil colour?
Different coatings of individual soil particles
Organic coatings = darker colours
Iron oxides = yellow, red, brown
Manganese oxide = black
Glauconite = green
Carbonates = whitish colour

6. Why do we need to interpret soil colour?
Classification
Info. about hydrologic regime/drainage
Bright colours (high chroma) = well-drained
Anaerobic conditions = greys

7. Soil Texture…
Soil texture = proportions of different sized particles in a soil.
We can draw many conclusions about a soil from its texture
Soil texture is not readily subject to change

8. What is meant by the term “soil separates”?
Division into sand, silt and clay fractions.
SAND:
Diameter smaller than 2mm, larger than 0.05mm
May be rounded or angular
Usually consist of a single mineral (often quartz)
Large particles = large pore spaces = free drainage & entry of air
Low specific surface area
Prone to drought

9. SILT: Particle diameter smaller than 0.05mm, larger than 0.002mm
Particles not visible to naked eye
Quartz generally the dominant mineral
Releases significant amount of plant nutrients
Pores between particles are smaller and more numerous
Higher water retention
Low “stickiness” means silt is washed away easily

10. CLAY: Particles smaller than 0.002 in diameter
Very large specific surface area = high water retention
A spoonful of clay might have the same surface area as a FOOTBALL FIELD!!!
Generally shaped like tiny flakes or platelets
When wet, clay is sticky, when dry, it forms a hard mass
Amount and type of clay largely dictates shrink-swell behaviour, plasticity, water-holding capacity, soil strength

11. Why is surface area important?
Water retention
Capacity to retain nutrients and other chemicals
Weathering and release of plant nutrients
Ability of soil particles to combine with one another
Microbial reactions

12. Classification of soils into “textural classes” requires the use of the textural triangle…

14. Soil structure… Definition:-
“The combination or arrangement of primary soil particles into secondary particles, units or peds. Secondary units are characterized and classified on the basis of size, shape and degree of distinctiveness” – Brady & Weil (1999)

15. How do we define soil structure?
Size of secondary units
Divided into classes from very fine to very coarse
Shape (types)
Granular, blocky, columnar, prismatic, platy
Degree of distinctness (grade)
Refers to both inter- & intra-aggregate adhesion
0 = structureless
1 = weakly durable
2 = moderately durable
3 = strongly durable