LECTURE 5

Soil Physical Properties –

Colour, Texture, Structure

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.

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.

• Soil colour described by referring to 3 components:

• Chroma (intensity/brightness)» 0 = neutral grey

• Hue (redness or yellowness)• Value (lightness or darkness)

» 0 = black

• 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

• Why do we need to interpret soil colour?• Classification• Info. about hydrologic regime/drainage

» Bright colours (high chroma) = well-drained» Anaerobic conditions = greys

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

• 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

• 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

• 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

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

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

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)

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