Rebuilding the soil after 60 years of losses
Sheep'n wheat didn't come together by accident. Now the tried and true practice of cereal-pasture rotation is getting a renewed boost with the work of Ram Dalai and Wayne Strong of the Queensland Wheat Research Institute and Errol Weston of the Queensland Department of Primary Industries. They are trialing the most common thods of restoring soil fertility. The work is based on conditions in northern cereal-growing areas but the lessons apply to most crop lands.
The long-term research trial at Warra in the north-eastern Darling Downs started in 1986. It was established on a soil which had been continuously cropped for about 60 years, making it very nitrogen-deficient.
The trials are mapping the available management choices for restoring soil fertility while achieving sustained high yields of Prime Hard wheat.
The project is testing:
- reduced or zero tillage and fertiliser application
- grain legume-cereal rotation (2-year rotation)
- pasture legume-cereal rotation (2-year rotation)
- pasture grasses and legumes-cereal rotation (eight-year rotation).
"We're finding there are substantial long-term nitrogen benefits to the soil om pastures, moderate benefits from growing chickpeas and small benefits from residual fertiliser," said Dr Dalai (see Table 1).
Table 2 shows the relative benefits in wheat grain yield and protein following the various practices for restoring soil fertility.
Advantage of long term
Restoring soil fertility over the longer haul ensures that grain yields and protein levels also hold up over a long period. However, good crop productivity can be maintained using non-pasture options under certain environments and on some soils.
Dr Dalai said the increasingly popular method of zero tillage accompanied by nitrogen fertiliser application may maintain but is unlikely to restore the fertility of depleted soils.
Similarly, growing legumes may maintain fertility but is less likely to restore it than mixed legume-grass pastures.
Mixed pasture rotations appear to be the best long-term option for restoring soil fertility because:
- organic residues are returned to the soil in excess of the rate of soil organic matter decomposition;
- reduced cultivation (or zero cultivation) means there is reduced decomposition of organic residues; and
- pastures use water effectively, reducing leaching, runoff and soil erosion.
Pastures also increase nitrogen fixation, provide better crop protection, and improve soil structure.
Pastures need less reliable rainfall
Dr Dalai said the pasture leys are particularly useful to restore soil fertility on medium to coarse-textured soils which require inputs of larger amounts of organic matter than fine-textured soils. "The relative benefits are likely to be greater in western districts," he said. The pasture option is considered appropriate for areas of uncertain and variable rainfall on all soils. Mixed pasture growth synchronises more easily with available water from rainfall than single crops. And applied fertilisers require rainfall to be effective.