Soils and Water

A soil survey is needed to determine how suitable a crop is for a particular soil type. It is also needed to determine how much water to apply at each irrigation.

A soil survey is made up of a series of sites where soil samples are taken, on a grid basis. They are selected on the basis of topography (land forms - slopes, rises, crests, flats), crop type and irrigation systems. At each site a vertical picture of a soil profile can be obtained by using an auger to remove a core of soil or by using a back hoe to dig a soil pit.

Soil Texture
Soils are like big sponges - they can only soak up so much water. When soils are fully wet there is little benefit in applying more water as this only causes waterlogging, drainage problems and loss of valuable fertilisers. How much water the soil can hold depends on the soil texture.

Soil Profiles
Soils from a vertical point of view are like a layered cake. At the top of the profile you have topsoil where most soil, water and fertiliser is extracted by the plant's roots.

Soils (depending on where you are located) may consist of several bands with varying characteristics and varying textures, and can be identified by the variation in colour. Eg see Tatchera sandy loam profile.

For instance, within the Mallee you will find underneath the topsoil a layer of carbonate (known locally as the lime layer). The carbonate layer can restrict root growth, however some crops such as vines can partially penetrate the carbonate layer into the subsoil below. Defining your layers of soil and the type of carbonate layer will help to determine your plant's rootzone. It is important when extracting soil for your sample from an auger hole that the soil is laid down in a line in the same order that it came from the hole. It is also very important that when digging the hole that you only go down 5 to 10 cm at a time. This avoids mixing the soils and provides greater accuracy when doing measurements and samples. When using a hand auger it is easier to get information on topsoil depth rather than rootzone depth because it is often hard to find roots when sampling such a small volume of soil. In this case rootzone depth is estimated from topsoil depth. A back hoe pit makes locating the rootzone much easier. Impermeable layers can be missed if you only sample to a depth of 1 metre, so it is wise to use a longer extension every few holes to sample down to a greater depth. The depth of the rootzone and the depth of each layer of soil are very important for determining the soil's capacity for holding moisture.

Diagram: Showing soil layers

(Source: Swan Hill Irrigation Management Course, Book 1, Chapter 1,B and C.)

Soil Texture

What are soils made of?
The texture of a soil is determined by the relative amounts of sand, (large particles) silt and clay (small particles) that it contains. Texture can be defined as the coarseness or fineness of a soil. The texture of each layer will determine how much water it can hold and how much is available to plants.

• Clay - the very small particles less than 0.002 mm,
• Loam - the particles in between 0.002 and 0.05 mm and
• Sand - the coarse particles between 0.05 mm to 2.0 mm