Bringing alkaline, sodic clay to the surface and mixing it with non-wetting, acidic sands is transforming the productive capacity of the soils on a mixed farming property run by Roger and Sue Groocock at Lowan Vale, north-west of Bordertown in SA.
The alkaline clay, at a pH(w) of 9-9.5, has changed the surface sand from dangerously acid (pH(w) of 5.5) to an ideal pH of 6-6.5. The dispersive clay also overcomes the non-wetting nature of the soil by improving moisture and nutrient retention, trafficability and stability against erosion.
The result — a highly productive soil, and all in one year.
Soil acidity is regarded as a major threat to primary production in several states and the prevention and remediation of it is the target for various GRDC-supported projects.
Monitoring by CSIRO and other agencies indicates that pH levels in many soils are falling. Normally lime or dolomite is applied to overcome or prevent soil acidity, but in some areas, such as the upper south-east of SA, alkaline clays occur beneath the topsoil.
The surface application of these clays has an equivalent effect to lime with the added benefits of improving overall soil health.
How it's done
Mr Groocock has used a delving machine on the better sand-over-clay country where the clay is about 15-30 cm below the surface. On the deeper sands he uses a Lehmann scraper to extract the alkaline subsoil clay from a potential dam site and spread it, usually to a depth of 10 cm, on the surface sands elsewhere on the property.
However, he is exploring ways to make these technologies work even better.
"I like to determine the principles of soil transformation in our area and then develop the machinery and techniques necessary for those principles to work for me," Mr Groocock said.
His technique is to lift the clay to the surface in spring at 1.5 m tine spacings and to a depth of 0.5 m. This leaves large, football-sized clods of moist clay on the surface to break down naturally over summer. Around the end of February he goes over the delved area with an offset disc machine in the same direction as the rip lines. Finally, a single pass with a piece of railway track dragged at 45° over the surface returns the area to farming condition.
In the process some of the sand drops down the rip lines as the clay is brought up, mixing the sand and clay to depth in the profile.
Tips for success
A secret to success is to get below the sodic clay to a calcareous marl layer. This both brings the high-pH clay to the surface to combat acidity and breaks through the dense sodic clay which restricts root development in these soils.
By lifting moist clay in spring, less power is required for machinery and there is more effective natural breakdown of the clay clods than an equivalent autumn operation.
The first crop following the delving operation is canola, followed by wheat and then barley in subsequent years.
Results have been impressive. Canola yield has increased by 15 per cent to about 2.2 t/ha and wheat yields have so far increased by 20 per cent to about 4 t/ha with protein levels of 12.5-13 per cent.
A sound nutrition program based on 100 kg DAP/ha with pre- and post-seeding urea, plus trace elements, supports the production levels. The canola also gets some gypsum to supply sulphur.
Last year his wheat crop, grown on a paddock renovated by clay delving two years ago, won the light soil section of the district cropping competition.
"Our aim is to improve the productivity of our farm without having to buy more land," he said.
Mr and Mrs Groocock started soil modification with trials run with the local Landcare group in association with University of SA engineers and Primary Industries and Resources agronomists. They are now determined to take it to another level through their own experiments.
Contact: Mr Roger Groocock 08 8754 6025
According to a recent study commissioned by the CRC for Soil and Land Management, acidic soils cover about 40 per cent of Australia. The 92 million hectares that occur in high-rainfall agricultural lands are costing about $633 million a year in lost agricultural production and, by inducing low water-use efficiency in plants, an increase in salt higher in the soil profile.
Soil pH levels are falling and acidity is increasing because of:
- removal of nutrients such as calcium, magnesium, potassium and sodium in agricultural produce
- leaching of nitrates
- addition of ammonium-based fertiliser
- accumulation of soil organic matter.
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