Scientists from the SA Research and Development Institute have been trying to get more productivity from farms in heavy soil areas with compacted subsoil layers — resulting from machinery traffic, grazing and regular cultivations at the same depth.
David Malinda, Jeff Schultz and Rick Darling found, in this GRDC-supported project in the mid-north of SA, that just one change of method made a difference. If cultivation depth was varied, it resulted in reduced penetration resistance in the soil, increased aeration and more biological activity to deeper depths.
These improved soil conditions in turn allowed more water into the soil and encouraged early root penetration, resulting in better exploitation of water and nutrients from deeper depths, and higher grain yields.
Mr Malinda said other techniques such as deep-ripping, trenching and the use of soil ameliorates like gypsum have been used to encourage root development through hard pans.
"On-farm adoption of these practices has been limited by high costs relative to certainty of outcomes. "We believe a practical alternative is tillage rotation where a shallow hard pan of say 8-15 cm is progressively ameliorated using specific tillage tools at varying depths and at optimum soil moisture levels. Costs are spread over time but increased returns are generated in the short term."
Comparing cultivation, no-till and tillage rotation
Their trial compared conventional cultivation (full soil disturbance to 5 cm at least twice before sowing with 17.5 cm points) with no-till (depth of cut 7 cm but sowing 5 cm deep into uncultivated soil with 1.5 cm points) and a tillage rotation.
The latter involved the use of 'super seeder' points which have an 8 mm leading edge, 5 cm wing width and 4 cm wing depth. In the three years of the research, the soil was cultivated to 12 cm, 15 cm and 12 cm. While allowing for normal seed placement, the points shatter layers below the normal cultivated depth.
The tillage rotation gave good results, reducing density and penetration resistance and increasing air porosity at depth. Rainfall infiltration increased as did the production of roots that penetrated below 300 mm depth. This in turn appeared responsible for higher uptake of water and nutrients leading to higher grain yield.
In 1999 wheat on the tillage rotation plot's yielded 17 per cent more than wheat on the conventional treatment. The researchers say they expect that the improvements in soil condition and productivity will continue to increase, reflecting the effect of both crop rotation and the tillage regime.
Contact: Mr David Malinda or Mr Jeff Schultz 08 8303 9400
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