Guideline Index

Chapter 4: Soil Properties

4.5 The soil profile

A soil profile describes the various layers within the soil and can be seen as a vertical section through the soil – See Figure 4.6. Each of the layers in the profile can affect plant growth due to differences in soil physical, chemical and biological properties. A soil profile can be created by digging a hole with a shovel, excavator or with an auger. Where there are distinctive layers within the soil, the profile can be divided into horizons.

4.5.1 Soil depth

The depth of soil or ‘soil depth’ is the material that favours plant growth. Physical and chemical barriers and high water tables can restrict rooting depth which can affect plant growth. For example hard pans, or gravel layers. Pastures and crops therefore prefer deep well drained soils with good texture and structure.

Each horizon is a layer within the soil profile that has distinct characteristics, such as colour, texture or structure that are different from the layer above or below it – See Figure 4.6. Where there are no distinct horizons, this soil is referred to as a gradational soil. Please refer to Chapter 6 for further information.

 

Figure 4.6   An example duplex soil profile
Figure 4.6 An example duplex soil profile

4.5.2 Soil profile descriptions

Soils are called gradational, duplex or uniform based on how the soil texture changes from the A to the B horizon. In a gradational soil, the clay content gradually increases, so that the change from the A horizon to the B horizon is indistinct – See Figure 4.7a. In a duplex soil, a sharp contrast in texture occurs between the A and B horizons, and the two horizons are easily distinguished – See Figure 4.7b. In a uniform soil, the texture change throughout the profile is very small or nonexistent; in general, no textural boundaries can be found in the profile. Figure 4.7 shows two very different soil profiles which have formed through differing geological processes and weathering.

Soil profiles can also be used to classify soil types. In Australia the Australian Soil classification is the method used for determining soil type – Refer to Chapter 6 for more information.

Figure 4.7   Examples of different soil profiles
Figure 4.7 Examples of different soil profiles