A simple change in tillage depth from one year to the next can reduce soil compaction and improve crop yields by up to 20 per cent in soils where hard pans are a problem.
According to SARDI researchers, breaking up the hard pan allows plant roots to penetrate deeper into the soil profile and tap into additional stores of water and nutrients.
In the SA trials on a red-brown earth site where the compacted layer was at a depth between 80 and 150mm, tillage depth was changed from 120mm in the first year to 150mm in the second. This procedure, known as tillage rotation (TR), was repeated in the following two years.
When compared to conventional cultivation (CC) and no-tillage systems (NT), the TR treatment improved soil porosity, decreased bulk density and resistance to penetration and increased organic carbon levels.
The three tillage regimes were tested in three rotations: continuous cereal (WBWW), wheat-pasture-pasture-wheat (WPaPaW) and wheat-grain legume-canola-wheat (WGlCaW).
When all rotations were in wheat in 2000, the TR system on WPaPaW, WGlCaW
Water infiltration improved
The Tillage Rotation (TR) system increased water infiltration capacity by 18 times at 50mm, by 10 times at 100mm and capacity was doubled at 150mm.
The improved soil conditions also increased root mass every year. For example in the fourth year of the trial, root numbers were measured at 300mm with the TR system averaging 22 roots per 50 square centimetres compared to 11 roots for the NT and CC systems.
Table 1: Nutrient content of wheat shoots (kg/ha) for each tillage regime
The trial was sown using controlled traffic principles so improvements in bulk density and porosity were also observed in NT and CC over the original measurements.
The NT treatment was sown with knife points (15mm) at 70mm depth and the CC had two passes (175mm points) before seeding at 50mm. The TR treatment was a form of no-till using narrow leading edge winged points (8mm leading edge, 50mm wing width and 40mm wing depth) with a sowing depth of 50mm.
Overcoming the Compaction Problem
Research has shown that cultivating at the same depth each year can result in the formation of a zone of compacted soil often called the hard pan. The depth of this hard pan depends on soil texture, moisture content and the machinery used.
However, it is important to accurately assess whether you have a compaction problem and at what depth. The use of a penetrometer is recommended to determine soil density at various depths.
Most of the SA trial work at Halbury was carried out on a red-brown earth with an average annual rainfall over the trial period of 495 mm. Compacted layers below 200mm are likely to be very costly to repair.
Researchers expect in this type of red brown earth it should be possible to rehabilitate compacted layers 50 and 200 mm below the surface within a few years.
According to researchers, using tillage rotation to solve subsoil compaction will only work at the right moisture content.
If you take a shovel of soil from the deepest point of cut and the soil breaks into smaller pieces, the soil is at the correct moisture for sowing.
A Big Economic Gain
The trial results at Halbury were assessed to determine the comparative financial return from the different tillage treatments.
Fuel use was recorded for all operations (Table 2) with the extra workings of the CC using far more fuel than the deeper working of the TR treatment.
Table 2: Recorded fuel use (litres/ha)
Results in Table 3 indicate that TR provides the highest average gross margin for each of the three rotations. The impact of the different rotations is also clearly shown with the highest one year gross margins from wheat following two years of pasture.
Table 3: Yields (t/ha) and gross margins ($/ha) for rotations and tillage regimes
Other trial results
Researchers in NSW found similar results in trial sites on duplex soils at Harden. Coming out of the pasture phase, using reduced tillage, they cultivated at different depths with sweep and ripper shares, which reduced compaction in the 100-250mm soil layer. There were no yield increases in the canola that year but recorded savings in time and energy consumption.
Deep ripping has been trialed in the Wimmera and Mallee of Victoria, but was not successful as it brought sodic subsoils to the surface. Researchers suggest ripping may only be effective on these soil types if gypsum is slotted into the rip.
Much work has also been done on hard pans in sandy soils. Pure sands tend to pack to maximum density very quickly. Researchers on southern Yorke Peninsula gained 100% dry matter increases after ripping a sandy site and injecting organic pellets to reduce or delay recompaction.