Soil Physical Properties – Colour, Texture, Structure 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