Soils and Irrigation

2. How much water can my soils hold? (SO2 - 0801)

Soil Water:

In order to understand how much water is held in a soil, there are a number of terms that must be explained;

• Full point is the maximum amount of water a soil is capable of holding. Once the soil is full, additional water applied will be lost through runoff and deep drainage.

• Refill Point is when soil moisture has declined to a point where the cane becomes stressed. Active growth stops as the plant finds it harder to extract the remaining soil moisture. Irrigation should occur when the soil moisture reaches this point.

• Permanent Wilting Point is the soil moisture content at which the plant has wilted and died. The crop cannot be revived by an irrigation or rainfall event.

The amount of water in the soil between Full point and Permanent Wilting Point is termed Plant Available Water (PAW). This is the total amount of water available to plants.

The amount of soil water between Full point and the Refill Point is called Readily Available Water (RAW). This is the amount of water the plant can readily use before growth rates become affected. Irrigation should aim to replace this volume of water.

How can I use this to manage irrigations?

The less RAW a soil holds, the more frequently it needs to be irrigated. For example, 80% of PAW in sandy soils is readily available. In clay soils, only 45-50% of PAW is readily available. But, because clay soils have approximately twice the amount of RAW as sandy soils, they should require less frequent irrigations .

Irrigations should aim to replace the RAW content of the soil. By applying only this amount, over- and under-watering can be avoided. A list of typical RAW amounts for a range of soil types is shown below.

The Rural Water Use Efficiency Initiative is a joint venture between BSES, CANEGROWERS and the Queensland Government through the Department of Natural Resources and Mines. For more information, please ring your local Rural Water Use Efficiency Officer.