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 Mineral 45% Water 25% Air 25% Organic matter Clay soil Air 25% Mineral 64% 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 – 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

SandClay

SandLoamClay DrainageHighMediumPoor Water holding capacityLowMediumHigh AerationGoodMedium/goodPoor Compaction potentialLowMediumHigh Resistant to pH change (buffering capacity) LowMediumHigh Nutrient supply (cation exchange capacity) LowMediumHigh Ability to retain chemicals and nutrients Very lowLowMedium-high Ease of cultivationHighMediumLow Root penetrationGood 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/darkPaleRed/brownYellowGrey/blue grey DrainageOften slowWell drained Less well drained Poorly drained Waterlogging potential MediumLow Low-mediumHigh Organic matter accumulation HighLowMedium-highMedium-lowLow Leaching of nutrients LowHighMedium 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)

Depth Soil water content Saturation DUL Lower limit Air dry (45 o C) Oven dry (105 o 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