Note Number: AG1060
Published: September 2003
Reviewed: September 2006
How is soil formed?
Soils are dynamic, forming continuously over a long period of time. Soil types differ, depending on the parent materials from which they came and from the surrounding environment. The way in which soil forms depends on:
- parent material
- living organisms
Soil is a natural medium made up of five major components:
- mineral particles: clay, silt, sand and gravel
- organic matter: decaying plant and animal material
- living organisms (soil biota): ranging from bacteria, fungi and earthworms
A healthy soil should have a balance of these components.
How does the soil profile form?
A soil profile is made up of parallel layers or horizons of different soil and rock materials (Figure 1).
A soil profile develops over a long period of time and is quite a complex process. The major points are:
- Soil forming factors interact to form the soil profile, including climate, topography, parent rock material and vegetation cover.
- Soil profiles may be formed by weathering or deposition processes.
- Parent material sitting on top of bedrock may be weathered to form part of the soil profile.
- Addition of organic matter on the surface or deposits carried by wind and rain form duplex soil types.
- Fine clays and mineral salts wash down through the topsoil to create the B horizon.
- In cultivated soil, fine particles can cement together to form hardpans in the profile.
- Soil biota break down organic matter and form pores and clusters in the soil.
The depth of the topsoil varies due to factors such as erosion, vegetation cover over time and climate. The way we manage the soil on our farms can determine the depth of topsoil.
Why classify soils? There are many different soil types that can vary in their physical and chemical characteristics across a relatively short distance on the ground. By classifying soils we can describe them in a way that is universally understood across Australia.
Classifying soils enables us to:
- organise soil descriptions into a taxonomic framework, reducing the complex nature of describing soils
- describe soils to others
- form a basis for soil survey and field research.
Brief description of the major soil classification systems
- Great Soil Groups system: developed by Prescottin 1931-44 where he classified soils into 18 groups which were related to vegetation and climate (e.g. Mallee soils, red-brown earth);
- Northcote Factual Key: Northcote developed this system in 1968, releasing the "Atlas of Australian Soils". This system used descriptive class names such as red friable earths;
- Australian Soil Classification System: developed in the late 1980's by R. Isbell (CSIRO Division of Soils). This new system is a general purpose classification based on Australian soil data and makes more use of physical and engineering properties.
Some examples of the Australian Soil Classification groups:
|Soil order||Major features|
|Chromosols||Soils with strong texture contrast between A and B horizons, most widely used for agriculture in Australia. The B horizon is not strongly acid or sodic.|
|Kurosols||Strong texture contrast between A & B horizon but B horizon is strongly acid with high aluminium content.|
|Vertosols||Clay soils with shrink-swell properties that exhibit strong cracking when dry.|
|Sodosols||Strong texture contrast between A & B horizon but B horizon is not strongly acidic but is sodic.|
This information Note was developed by Carole Hollier, Rutherglen. September 2003.