Soil properties

Soils vary in their suitability for specific purposes. For example, in Queensland a deep, fertile clay soil is suitable for intensive agriculture but a shallow, sandy soil is better suited to grazing and growing native trees.

The suitability of a soil for a particular purpose can often be determined by looking at some of the easily recognisable features and carrying out simple tests. The most common properties used to compare and recognise soil are:

Other important soil properties include dispersibility, organic matter and soil carbon sequestration, permeability and porosity, salinity and water holding capacity.

Soil depth

An important feature of a soil is that it changes with depth. To properly analyse a soil, it should be examined from the surface to the parent material.

Soil texture

Soil texture (e.g. loam, sandy loam or clay) refers to the proportion of sand, silt and clay sized particles that make up the mineral fraction of the soil. For example, light soil refers to a soil high in sand relative to clay, and heavy soils are made up largely of clay.

Texture is important because it influences the amount of water that the soil can hold, the rate of water movement through the soil as well as its workability and fertility. For example, sand is well aerated but does not hold much water and is low in nutrients. Clay soils generally hold more water, and are better at supplying nutrients.

Texture often changes with depth so that roots have to cope with different conditions as they penetrate the soil. A soil can be classified according to the manner in which the texture changes with depth. The three profile types are:

Structure

Soil structure refers to the way soil particles group together to form aggregates (or peds). These aggregates vary in size and shape from small crumbs through to large blocks.

Very sandy soils are structureless because sand grains do not cling together. Some soils resemble a large solid, featureless mass—referred to as massive and have little or no structure. Good soils fit in between the two extremes. A well-structured soil breaks up easily into aggregates or peds with a definite shape (e.g. granular or blocky) and size (1-60 millimetres). Organic matter helps give a soil good structure by binding soil particles together.

Good structure is important, as it allows water to soak into the soil and excess water to drain away. It also allows air movement through the soil. Soil, air and water are vital for healthy plant growth and continued nutrient supply.

Colour

Soil colour is strongly influenced by humic (organic) materials which are brown or black, iron oxides (red or yellow) and features of the parent material. Poorly drained soils may contain blue, grey and green colours.

Soil pH level

Soil pH is the measure of the acidity or alkalinity level of the soil. It affects plant growth, as it determines the availability of plant nutrients in the soil.

Soil pH is measured on a scale from 0-14, with 7 being neutral. A highly acidic soil can have as low as pH 3, while a highly alkaline soil can be close to pH 10. Most soils have a pH 6-8 range and plant growth is usually best in a soil of pH 6-7.

Nutrients

For plants to be healthy, they need a steady supply of nutrients from the soil. Nitrogen (N), phosphorus (P), potassium (K), sulfur (S), calcium (Ca) and magnesium (Mg), are required in relatively large quantities (macronutrients). Others are required in small quantities (micronutrients or trace elements), eg. copper (Cu), zinc (Zn) and manganese (Mn).

A shortage or absence of any one of these essential nutrients can severely retard plant growth. Too many nutrients can be as bad as too few. The availability of nutrients is affected by the pH level of the soil. For example, in very acid soils, manganese and aluminium may be present in toxic concentrations. The nutrient status of a soil can be determined by a laboratory analysis of the soil or the plants that grow in it.

Dispersibility

This is characteristic of some clay rich soils that have a high concentration of sodium or magnesium in the clay fraction. A ‘sodic’ soil has a high sodium ion concentration. When these soils come into contact with water, they may become unstable and disperse.

Dispersion in the surface soil leads to crusting and surface sealing, dispersion in the subsoil accelerates erosion and may lead to the formation of gullies and tunnels.

Soil organic matter and soil carbon sequestration

Soil organic matter is the component of soil derived from all biological sources—whether living or nonliving. Soil organic matter is a vital indicator of soil health because of its impact on a variety of soil functions and properties. It provides the energy source for micro-organisms in the soil, is a reservoir of nutrients (especially N,P & S) and improves the structural stability, water holding capacity and pH buffering capacity of the soil.

Soil organic matter content is difficult to measure directly. Laboratory tests actually measure soil organic carbon (SOC), which makes up about 58 per cent of total soil organic matter. Soil organic matter is made up of several pools that vary in their contribution to soil functions and their longevity in soil systems. Organic residue deposited in or on the soil is the most active pool, but may be rapidly lost (has low stability). Humus (made up of resistant compounds derived from decayed organic residues) is a slow, more stable pool. Charcoal is very stable, but is not biologically active, and therefore is an inert or passive pool.

Soil cultivation and soil degradation result in losses of organic carbon which is released as CO2 into the atmosphere. Land clearing and overgrazing also contribute to the loss of soil carbon.

Improved soil management strategies such as crop stubble retention on the soil surface and reduced grazing pressure have the potential to increase the store of soil carbon, thereby acting as sinks for atmospheric carbon.

Permeability and porosity

Permeability is a measure of how easily water moves through a soil. At the surface, it affects the rate at which water can enter a soil, called the infiltration rate. It is affected by soil structure and texture.

Porosity is the amount of space around mineral grains that can be filled by water or air, which contribute to a soil’s permeability. Particularly large pores that are visible are called macropores.

Water holding capacity

Field capacity (upper limit of available water) is a term used to describe the amount of water held in the soil after free water drains from the macropores.

Permanent wilting point (lower limit of available water) is the stage when soil water is held too strongly for the plant to extract it and the plant wilts beyond recovery.

Plant available water capacity (PAWC) is the quantity of water between field capacity and permanent wilting point which is available for plant-use.

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Last reviewed 10 April 2012
Last updated 6 January 2010

Soil and land resources