Many soils in NSW have sodic clay subsoils of low permeability. These soils waterlog easily after heavy rain or irrigation. Under these circumstances subsoil oxygen levels remain low for long periods during which root development is seriously restricted. Waterlogging also favours the development of root diseases, including crown rot and Phyrophihora. and the loss of nitrogen by denitrification.
Gypsum can improve these soils by reducing swelling and, possibly. clay dispersion. This increases permeability and reduces the time taken for the oxygen content to return to levels satisfactory for root growth.
An important step in developing land for surface irrigation is landforming. which ofteninvolves cutting and filling. This reduces the depth of the topsoil in some places. frequently exposing sodic clay or bringing it closer to the surface. These cut areas are notorious for their poor production and often require gypsum, increased fertiliser application and extra organic matter to improve soil fertility.
The Agriculture Western Australia, tested the residual effects of deep ripping, gypsum and nutrients on grain yields and soil physical properties at Merredin and Northam. Deep ripping (DR), gypsum (G) and nutrient (N) treatments applied in 1997 continued to increase grain yield in 1998.
Deep ripping alone did not improve structure of compacted soil. It did increase the yield in the first year through increasing water infiltration and temporarily increasing soil water storage. In the absence of a binding agent which helps soil to aggregate and form a structure, soil infiltration and storage are likely to be reduced with time and become worse than before ripping. Gypsum, which plays an important role as a flocculation and aggregation agent, must be used (with or without ripping) to improve soil physical properties. The best result of 30% increased yield came from a combination of deep ripping, gypsum and complete nutrient.
Reference: NSW Agfacts AC.10, http://www.agric.wa.gov.au
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