AUSTRALIAN FARMERS can overcome soil compaction and increase gross margins by gradually increasing theirtillage depth over three or four years, according to a SA researcher who is at the cutting edge of world tillage research.
Work done by David Malinda and his group at the SA Research and Development Institute suggests better management of tillage in no-till cropping systems can significantly increase gross margins across the estimated 5 million hectares of compacted cropping soils in southern Australia.
In trials at Halbury, in the Mid North of SA, the two-part concept, which Mr Malinda has labelled progressive tillage and tillage rotation, produced successive yield increases in continuous cereals over four years. The work has been supported by growers and the Federal Government through the GRDC.
The technique is simple and the benefits many, according to Mr Malinda who has been studying soil compaction for more than a decade. Progressive tillage (PT) involves progressively increasing the depth of tillage over several years until the layer of compacted subsoil is broken up and crop roots are able to penetrate deeper into the soil.
Trial results indicate a period of progressive tillage (PT) followed by tillage rotation (TR) is an achievable, low-cost, sustainable solution to'much of Australia's compacted soil./p>
The successive yield increases, from around 10 per cent in the first year to around 20 per cent in the fourth year, lifted gross margins by an average of around 16 per cent - $50 a hectare - a year.
Other apparent benefits include improved soil condition, an 18-fold increase in water infiltration and a 116 per cent increase in water uptake by crops.
"The level of nitrogen in wheat shoots in 2000 was 64 per cent higher in crops on the treated area than in the conventional control, shoot dry matter at tillering is 68 per cent higher and the number of roots that went below 30 cm is up by an average 145 per cent," Mr Malinda said.
"Organic carbon in the top 20 em of the soil has increased by 46 per cent over the four years of the trial and crops on the treated area consistently contain twice the amount of nutrients applied as fertiliser, which indicates they are accessing nutrients leached into the subsoil in previous years."
All of which was achieved with standard tillage equipment and minimal additional energy use.
Any other subsoil constraints, such as boron toxicity, sodicity, salinity or similar, will need to be addressed at the same time as compaction to gain the full benefit.
Maintaining the system
Tillage rotation (TR), the maintenance version of progressive tillage, involves varying tillage depth from year to year to ensure compaction does not redevelop.
It is more effective, and more cost- and energy-efficient, than deep ripping because it is done with the farmer's standard tillage equipment and capitalises on plant and soil biology to maximise the benefits.
It does not disrupt current no-tillage farming systems and is tailor-made for continuous cropping because it is based on a single annual working.
Mr Malinda and his team have previously shown that soil compaction will gradually reduce under a no-till system, but it can take more than a decade for soil to return to an acceptable bulk density - a measure of compaction - naturally.
Wheat and barley will grow quite well in soils with bulk densities - one of the measures of compaction - up to 1.3 tonnes per cubic metre, but cereal roots are unable to penetrate soils with bulk densities of more than about 1.5 tonnes per cubic metre.
In a long-term trial at Halbury, in soil with a compacted layer which started 7 cm below the surface and was 8 cm deep, it took 16 years of no-till farming practices to reduce the bulk density of a compacted red-brown earth from 1.86 tonnes per cubic metre to 1.4.
It took just four years to reduce the bulk density of a similar soil from 1.86 to 1.34 tonnes per cubic metre using two years of PT and two year's of TR.
How deep is deep?
In the first year the research team set the seeder up to cut to a depth of 12 cm so it broke up the top 5 cm of the compacted layer, providing the crop with access to 12 cm of loosened topsoil.
The following year the seeder was set to cut to a depth of 15 cm, which broke through the remainder of the compacted layer.
They are now into the ongoing PT phase, working to 12 cm one year, 15 the next, then back to 12, and so on.
"In the first year of the process we had most of the root mass in the top 12 cm of the soil," Mr Malinda said. If we had gone to 15 cm and through the compacted layer, we would have had a similar volume of roots in a much larger volume of soil, so the amount of organic matter, and subsequent biological activity, would have been much less.
"With the initial shallow cut the organic matter levels, which are important in preventing re-compaction, built up to quite high levels before we further expanded the volume of accessible soil the following year."
Increasing the depth of loose soil to 12 cm effectively doubled the volume of soil the crop could access.
Extending the depth of loosened soil to 15 cm at least trebled the volume of soil available to the roots and opened the way for them to penetrate even further into the subsoil to access deeper moisture and nutrients leached out of the root zone in previous years.
All the trial work used deep-working Super Seeder points, but Mr Malinda and his team are now involved in another GRDC-supported trial to determine whether or not other points have the same or a similar effect in different soils.
Program 4 Contact: Mr David Malinda 08 8303 9350
Nutrient content of wheat shoots (kg/ha) for each tillage regime
TR = tillage rotation
NT = no-tillage systems
CC = conventional cultivation
North, South, West