When this approach is followed, the irrigation amount exceeds the measured profile or root zone deficit by a certain fraction. A leaching fraction is usually added to the irrigation amount to leach salts out of the root zone or when water of poor quality (high salt content) is used for irrigation. A build-up of salt in the root zone is detrimental to crop production in the long-term, and if present, should be leached from the root zone from time to time, but preferably not when the profile is loaded with nutrients. In the context of the water balance framework, over-irrigation according to a leaching fraction is, therefore, beneficial. The desired leaching fraction is usually determined by factors such as the quality of irrigation water, salt tolerance of the crop, long-term rainfall and the ability of the soil profile to transmit water. For the example illustrated in the figure below, the allowable depletion level is 24 mm and an ‘over irrigation’ (leaching fraction of 20%, equivalent to 6 mm) is applied with each irrigation event. It is important to realise that one may choose to leach the profile only occasionally, and not with each irrigation event, as it is in no one’s interest to leach when expensive nutrients have been applied but not yet taken up by the crop.
Applying a leaching fraction.
Irrigation scheduling where a leaching fraction of 20% (6 mm) is applied to leach salts from the root zone. Arrows indicate irrigation (black) and rainfall (blue) events.