Soil physical properties: Texture and structure
Note number: AG1061
Published September 2003
Reviewed: September 2006
Soil is made of varying amounts of silt, sand and clay. The proportion of these components determines if a soil is a sand, loam or clay or any combination of these. Soil texture has a number of implications for management because it effects the ability of the soil to hold water and its ability to withstand cultivation and compaction.
It is easy to assess texture in the paddock by mixing a small amount of soil with water in your hand, just enough to form a slightly sticky ball. The way the sample feels in your hand and the way it forms a ribbon, allows you to determine the texture.
Use the following table to assess your soil:
|Texture class||Properties of moist soil ball|
|Sands||Won't form a ball. Forms a ribbon less than 10 mm. Feels very sandy and not sticky at all. Clay content 0 to 10%.|
|Sandy loams||Able to form a ball. Forms a ribbon 15-25 mm long. Feels sandy and slightly sticky. Clay content 10 to 20%.|
|Loams||Forms a smooth ball, ribbons to 25-40 mm. Feels slightly sandy and moderately sticky. Clay content 20-30%.|
|Clay loams||Forms a smooth, plastic ball, ribbons to 40-50 mm. Almost no sandy feel. Distinctly sticky. Clay content 30-35%.|
|Light clays||Forms a smooth, plastic ball, ribboning to 50-75 mm. Very sticky. Clay content 35-45%.|
|Medium to heavy clays||Forms a smooth, extremely plastic ball. Ribbon more than 75 mm. Feels very sticky with no sand. It is more difficult to mould than light clay. Clay content greater than 45%.|
Soil is comprised of three-dimensional arrangements of solid particles and pores. Soil structure is determined by the distribution and the size of these soil aggregates and pore spaces.
Soil structure is influenced by its physical, chemical and biological characteristics. Good soil structure is vital, as it can affect the availability of air, water and nutrients for plant growth. Agricultural practices can significantly alter soil structure. Poor soil structure can greatly reduce plant growth, making it difficult for plants to obtain water, air and nutrients and also impeding seedling emergence due to surface crusting.
The structure and texture of soil affects the soil's ability to hold or drain water and withstand cultivation and compaction by machinery and stock. For example, sandy soils have low water holding capacity and are easily damaged. On the other hand, heavy clay soils are very dense, do not drain water very well and have small pore spaces.
Sand is weakly structured because the sand grains are only weakly bonded together. A very heavy dispersive clay which sets hard into large sheets when dry has a massive structure. Most soil types fall in between these two structures.
An ideal soil has well formed, loose aggregates which hold water but have adequate drainage and are not easily broken down by machinery and stock.
Soil structure and organic matter
- Organic matter is the remains of living things or products of living things in the soil. Organic matter is important for soil structure.
- Organic matter on the soil surface (such as wheat stubble residues) protects the surface from the action of raindrops, reducing surface compaction and hardsetting. Organic matter also helps to bind sandy and silty soils together and also improves water infiltration through the soil. Organic matter also acts as a buffer against the forces of compaction.
- Continuous cropping and cultivation can diminish organic matter in the soil very quickly, leading to soil structural decline.
The chemical make-up of the soil will also determine structure. When high amounts of sodium are present (>6ESP%) clay particles separate and move freely about in wet soil conditions.
When sodic soils come into contact with water, the water turns milky as the clay disperses. When the soil dries out a crust forms on the surface.
The effects of high levels of sodium on soil structure can be overcome through the application of gypsum. Gypsum contains calcium which stops clay from dispersing when it is wet. Calcium is able to overcome the repulsion of the negative charges which cause the clay particles to separate. It helps the clay particles to clump together through a process known as "flocculation".
Slaking is another problem that causes damage to soil structure. When intense rainfall hits dry soil, the surface of porous aggregates rapidly absorb the water and air is trapped internally. With further wetting the force of the air escaping can cause weak aggregates to disintegrate. This process of aggregates breaking into small particles is known as slaking and can block up pore spaces. When the soil surface dries, a crust will form. Slaking occurs within minutes, whereas dispersion may take hours. Slaking also causes water infiltration and seedling emergence problems.
A simple way of checking for sodic, or dispersive soils, is to take two or three pea-sized samples of clay and put them in a shallow container of rainwater. If the soil is sodic, a cloudy appearance will develop in the water around the clay. The quicker this happens, the more sodic the soil.
Slaking can also be checked using this method. When the clay sample is put into the shallow dish of water, it will crumble if prone to slaking (note that no cloudiness of the water occurs with slaking).
This Information note was developed by Carole Hollier, Rutherglen, September 2003.