An introduction to soils

Andrew Biggs

What’s in a soil?

• Mineral particles (inorganic fraction) – small particles of rock and other minerals, produced from weathering of rocks

• Organic materials – humus and the dead and decaying parts of plants and soil animals

• Water – the ‘soil solution’ in which nutrients for plants are dissolved

• Air – which fills the spaces between the soil particles not filled by soil solution

• Living organisms – ranging in size from small animals to viruses

+ living organisms

Soil components

Mineral45%

Water 25%

Air 25%

Organic matter

Clay soil

Air 25%Mineral64%Water

10%

Sandy soil

Why do soils differ?

• Different proportions of the main components

• Components are grouped together in different ways

• Many different types of minerals in the inorganic fraction, and each soil

has different proportions of them

Different disciplines

• Morphology – description of the soil

• Chemistry (a key aspect)

• Physics (includes geomechanics)

• Biology (the latest rage)

How do you describe these things?

• Depends on who you are

• AS1726 – Geotechnical site investigations• AS1289 – Testing for engineering properties of soils• AS4119 – Soils for landscaping and garden use• AS4439, 4479, 4482 – Contaminated land• AS4454 – Compost/Manufactured soils

How did the soil get there?

• Know your geology

• Topography

• Is the soil unrelated to the underlying rock/material?

• Understanding the formation of a soil helps understand how it will

behave

Soil forming factors

• Parent material (geology, rock type)

– granite = often sandy, infertile

– basalt = clayey, fertile

• Climate (rainfall, temperature, wind)

– influences rate of weathering, plant growth

• Topography (shape, length, grade of slope, aspect)

• Organisms

– plants, bacteria, fungi, animals, worms, insects

• Time

Morphology

• Colour (including mottles)

• Field texture

• Structure

• Presence/absence of segregations, coarse fragments, pans

• Field tests (pH, EC, dispersion, carbonates, peroxide etc)

• Horizons

• And the landscape in which the profile sits (landform, geology,

vegetation, etc)

Texture

• The proportion of sand, silt and clay sized particles that make up the mineral matter of the soil

• How a soil ‘feels’

Sand: 2.0 – 0.02 mm

Silt: 0.02 – 0.002 mm

Clay: <0.002 mm

Texture influences:

• Amount of water that can be stored in the soil (water holding capacity)

• The rate of water and air movement through the soil (drainage, permeability, aeration)

• The soil’s nutrient supply (amount and availability)

• Ease of root growth

• Workability, trafficability (potential for compaction)

• Resistance to erosion

• Ability of a soil to maintain a stable pH

Sand Clay

Sand Loam Clay

Drainage High Medium Poor

Water holding capacity Low Medium High

Aeration Good Medium/good Poor

Compaction potential Low Medium High

Resistant to pH change (buffering capacity)

Low Medium High

Nutrient supply (cation exchange capacity)

Low Medium High

Ability to retain chemicals and nutrients

Very low Low Medium-high

Ease of cultivation High Medium Low

Root penetration Good Good Low

Texture

Structure

• Soil particles (sand, silt, clay) are usually arranged into larger units (called aggregates or peds)

• Soil structure refers to the size and arrangement of the aggregates, and the pore space between them

Structure influences

• Water entry into the soil

• Runoff of water

• Permeability (ease of movement) of water and air in the soil

• Root penetration

• Seedling emergence

• Resistance to erosion

• Workability

• Drainage

Colour

• Colour may be due to soil forming processes, or inherited from the

parent material

• In general, soil colour is determined by the amount and state of

organic matter and iron oxides

Colour gives an indirect measure of other soil attributes:

• Presence/amount of organic matter

• Drainage/waterlogging potential

• Degree of weathering/leaching

Red Yellow Grey/blue-grey

Colour

Black/dark Pale Red/brown Yellow Grey/blue grey

Drainage Often slow Well drained Well drained Less well drained

Poorly drained

Waterlogging potential

Medium Low Low Low-medium High

Organic matter accumulation

High Low Medium-high Medium-low Low

Leaching of nutrients

Low High Medium Medium Low

Horizons

• Layers with differing properties

• Why describe them?

– A common language for communication – an A2e has known

characteristics

• O – organic material

• A – surface

– A1 is zone of organic matter accumulation

– A2 may/may not be present. Often bleached

• B is zone of maximum accumulation (colour, texture, chem)

– Usually a B2 (B21, B22)

– Various suffixes (g, h, k, y, s etc) e.g B22k

• C – parent material

• D – unrelated to the overlying horizons – common in alluvial soils

• R - Rock

• Understanding horizons demonstrates an understanding of how the

soil got there and what its properties are

• You don’t have to use all the letters!!

A1

B21k

B22y Coarse lenticular peds

Coarse prismatic peds

40% clay

50% clay

70% clay

Medium blocky peds

Texture

contrast soil

Hard clay - columnar

Loamy sand - massive

Hard clay - massive

Loamy sand - bleached

A1

A2e

B21t

B22t

5% clay

35% clay

Organic Matter

• Biological origin (dead plant and animal material)

• Has a strong influence on soil properties

- reservoir of nutrients (esp N, P, S)

- contributes to cation exchange capacity of the soil

- improves water holding capacity

- improves structural stability of the soil

• Measured as ‘organic carbon’

Organic matter pools

Turnover time

1. Passive (inert) eg charcoal 50,000 years

2. Slow eg humus 100 years

3. Active eg plant debris and fungal hyphae 1-20 years

‘Active’ or labile OM is an indicator of OM quality

Soil water (and air)

• Total porosity = all the air spaces

• A proportion is generally filled with water

• The amount of water varies

• Plants can extract varying amounts of water

– Crop lower limit (Permanent wilting point, 15 bars suction)

• Saturation = full of water

• After drainage has reached “zero’

– Drained upper limit (Field capacity, 1/3 bar suction)

De

pth

Soil water content

Saturation

DUL

Lower limit

Air dry (45o C)

Oven dry (105o C)

Soil water movement

• Deep drainage = water moving out of the profile (downwards)

• Lateral flow = water moving out of the profile (sideways)

• Proportion of each varies with soil type and landform

• There is a lot more to it!!

Soil classification

• Description is different to classification• Different classification schemes for different purposes

• Northcote factual key: Ug5.15• Stace et al. (1968) Great Soil Groups: Black Earth• Australian Soil Classification: Self-mulching Black Vertosol• Unified soil classification: CH

• Classify what you have described!

Available from CSIRO Publications