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