As rock is broken down through weathering processes, it forms smaller and smaller particles that become one of the five components of soil. It is these components that determine the type of plants that can survive in a particular area. If the proportion of any one of these components is changed then the soil will be different and the types of plants that will grow there will be different.
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Organic material |
Dead and decaying plants and animals, and animal manure, add nutrients to the soil and hold water. The sponge-like properties of organic materials help the soils retain moisture, which is particularly important to plant survival in drought affected areas. |
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Minerals |
These are inorganic materials derived mainly from rocks. The three main mineral parts of soil are considered to be sand, silt and clay. These give soil its texture. |
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Living organisms |
A huge range of organisms aerate, mix and add nutrients to the soil. |
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Water |
This is an essential nutrient for plants. It also allows plants to absorb other nutrients. |
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Air |
Provides drainage to prevent plants drowning. |
As you move down through a cross-section of a soil you will often notice that there are layers, bands or horizons in the soil. What you are seeing is a change in the characteristics of the soil. This cross-section of soil is called a soil horizon, and it is used to describe the type of soil found in a particular area.
Each layer is given a symbol as a name to describe it. Use the table below to read the characteristics of each layer.
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O Horizon | If this layer is present then it will contain organic materials, such as leaves and branches, at various levels of decomposition. |
| A Horizon(Topsoil) | This is a surface mineral layer containing some organic material. It is usually darker than lower layers. The minerals that make up this layer are in the form of soil particles that can range from fine clay or silt to a coarser mix of sand or gravel. The size of the particles determines the texture of the soil. | |
| B Horizon(Subsoil) | The makeup of this layer will vary according to the amount of clay, iron and aluminium it contains. Its strong colouring, such as reds or yellows, often identifies this layer. | |
| C Horizon | This layer is characterised by partly weathered or decomposed rock. | |
| R Horizon | This is the solid rock layer from which the layers above develop. |
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As you dig deeper into the soil the structure of the soil may change. These changes, in amounts of sand, silt and clay, are described as a profile made up of a number of levels or horizons. For some soils it is easy to identify the boundaries in different soil profiles. For others the layers are less distinct. The three main profiles in Australia are:
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Uniform |
Profiles that show very little difference in horizons throughout. |
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Gradational |
Soils in which each following horizon passes gradually from the one above to the one below, and gets a larger clay component as you go deeper. |
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Duplex |
Profiles that show a dramatic change between horizons. |
These profiles lead to different soil types that are identified by their texture or proportions of sand, silt and clay. A soil texture triangle can be used to describe soil types.
Click on the Play button to see how a moist soil sample could be classified into different soil types according to soil texture.
You can also take a look at how the proportion of particles (sand, silt and clay) in a soil sample could be plotted on the texture triangle and used to identify the type of soil you have.
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