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Soils are highly variable within regions and considerable variation in soil type often exists on the one property. A knowledge of what soil types are present greatly assists irrigation management. Soil type governs at what rate irrigation can be applied without puddling and run-off and how much water can be held within the trees root zone.

Survey

A soil survey maps the types and distribution of soils on a property. Assessment of the soil data reveals the maximum depth of the crops root zone and the amount of water that the soil can hold within this root zone.

Normally soil assessments are made at sites located on a 75 m by 75 m grid pattern. This sampling grid may be altered according to topography, crop type, and layout of an existing irrigation system.

At each site a vertical picture (profile) of the soil is obtained by extracting soil with an auger or preferably assessing soil in the side of a pit dug with a backhoe.

A soil profile consists of layers of varying soil types. Colour of the soil is normally a good indication of the boundaries between each soil layer.

Each layer is assessed for the presence of tree roots, for soil texture and impermeable layers such as carbonate and clay. Other information such as salinity, pH and structure can be collected.

Plant available soil water

The potential amount of water available to a tree depends on the depth to which tree roots can grow in the soil type present and the texture of soil within this depth.

Depth of root zone

Effective rooting depth is determined by the crop type and the presence of impeding layers of soil to root growth. Rooting depth is generally regarded as that to the deepest easily observed roots. Where root growth is restricted by an impeding layer, (eg. heavy clay, carbonate, sandstone, ironstone), the effective rooting depth is the depth to this layer.

Soil texture

Soil texture is a description of the relative amounts of clay, sand and silt present in the soil. Soil texture determines the amount of water a soil can make available for plant growth.Soil texture of each layer in the soil profile must be assessed individually.Soil texture is assessed by taking a small portion of soil which will fit comfortably into the palm of your hand. The soil is moistened with water, a little at a time, (the soil must be damp not sodden), and kneed into the shape of a ball (bolus) until the soil just starts to stick to your fingers. The feel of the soil and the length of ribbon which can be produced from it provide an indication of the proportions of clay, sand and silt present, that is, the texture of the soil. Table 1 lists the feel and ribboning characteristics which allow texture of a soil to be identified. Experience is required to determine the texture of soils properly.

Water holding capacity
Soil types differ in their ability to hold water, and how much of this water that can be easily extracted by plant roots. Readily available water (RAW) is defined as the millimetres of water that tree roots can utilise per centimetre of soil depth. RAW varies with texture of the soil. Typical readily available water values for soils in the Riverland are listed in Table 2.

The amount of readily available water in the root zone of a tree crop (millimetres) is the cumulative total of the depth in centimetres of each soil layer multiplied by the appropriate RAW value for the soil texture of that layer, refer to example in figure 1. The number calculated represents the water holding capacity of soil in the tree crops root zone, that is, the amount of irrigation water in millimetres that it takes to fill the soil profile.
Water holding capacity of soils used for stone fruit production in the Riverland vary from 30 - 70 millimetres. This represents the quantity of water that a soil loses when it dries from field capacity (-8kPa) to its refill point (approximately -60kPa).

The quantity of water applied in one irrigation should not exceed this value, otherwise water will be lost below the root zone and/or add to the water table if one exists. The exception is where leaching irrigations to remove salt from the root zone are applied.

Figure 1.

Infiltration rate

Infiltration rate is the speed at which water can move through a soil profile. Infiltration rate is related to soil texture. A number of other factors such as bulk density, organic matter, surface soil stability and ground cover also affect infiltration rate.
The infiltration rate of a soil determines the maximum rate at which irrigation should be applied. Applying irrigation at a higher rate results in puddling and run-off. Table 3 provides indicative upper limits for irrigation application rates on various soil textures in the Riverland. More accurate readings can be obtained by field measurement of actual infiltration rates as they are known to vary between soils of similar texture.