Future lies in the (sub) soil by Denys Slee

Nigel Wilhelm

Summer crops: sunflowers and corn growing without irrigation in a traditional wheat/sheep district of the Eyre Peninsula, South Australia.
Summer crops: sunflowers and corn growing without irrigation in a traditional wheat/sheep district of the Eyre Peninsula, South Australia

A South Australian crop specialist believes the next big leap in yield increases will come when we fix some major subsoil problems.

Nigel Wilhelm points to a record of spectacular increases in wheat yields in Australia over the past 10 years and sees no reason why these increases, for wheat and other crops, cannot continue.

Dr Wilhelm, of the SA Research and Development Institute, said state records for total grain harvests had been regularly broken in the southern states — yet there had not been any significant increases in the area of crops being sown and rainfall patterns had not been at record levels either.

He illustrates his point with a graph (right) showing wheat yield trends since 1860, originally produced by Colin Donald of the Waite Institute in 1960, and updated by John Angus of CSIRO Plant Industry in 2001.

The graph shows, among other things, that since 1990 average wheat yields have increased by about 0.5 t/ha. Two of the principal drivers of this increase were the widespread growing of canola in southern Australia (and lupins in WA), which acted as a cereal disease break, and increased use of nitrogen fertilisers.

Dr Wilhelm believes the current level of water use efficiencies still allow plenty of room for improvement. However, “while there are obviously further gains to be made by continuing adaptation of our crop genetics to local climates and disease pressures, I believe that our major constraints are soil-based — particularly subsoilbased”.

Major subsoil barriers

Fixing three subsoil conditions could provide the launch pads for further large yield increases.

Sodicity — sodic soils are hard-setting, prone to waterlogging and estimated to be affecting yields over 12 million hectares of cropping land. Because much of the sodicity is deep in the soil profile and beyond the reach of current tillage or deep-ripping techniques, solutions are long-term in nature.

The most direct corrective method is to apply finely divided and active gypsum to the soil surface, which then moves slowly down the soil profile. However, for subsoil sodicity, high rates and a lot of patience are required for the technique to be fully effective.

Because sodicity most often impacts on plant growth by causing waterlogging in the root zone, any treatments that improve crop growth (and thus increase water use which, in turn, keeps the soil drier) will also reduce the impact of sodicity.

Sound nutrition, eliminating root diseases and seeding early will all help in sodic soils. Sodic soils also tend to collapse under repeated cultivation, so minimum tillage and stubble retention (especially with controlled-traffic systems) will help keep good condition in sodic soils.

Infertility — some innovative research in the 1980s, and more recent work, showed that if nutrients could be economically placed deep in the soil profile, say 30-40 cm below the soil surface, it could lead to large yield increases.

In the more recent work, a deep-ripping plough was modified to allow addition of fluid fertilisers deep into the soil profile. In a deep sand-over-clay profile, this technique more than doubled wheat yields despite unexceptional rainfall conditions.

Developments with nutrient seed dressings, fluid fertilisers, foliar applications and more efficient crop varieties all point towards more vigorous and more robust crops in the future, despite our impoverished soils.

Wheat yield trends since 1860

Boron toxicity — said to be reducing crop and pasture production across 10 million hectares of the southern grains belt. Plant breeders are producing more tolerant genotypes and there is scope to exploit germplasm with even higher tolerances.

Two other ‘soil-friendly’ techniques stand out from current research.

Controlled traffic

“Controlled traffic reduces wastage of inputs, improves timeliness of operations and reduces soil compaction,” said Dr Wilhelm. Experiences with controlled-traffic systems from northern Australia (where soils are prone to compaction) show savings in inputs of $40-80/ha plus increased yields of 10-20 per cent (despite a drop in the sown area in a paddock).

Practitioners have also noticed substantial improvements in the timeliness of operations (night spraying is possible and trafficability is much better) and reduced risks from water erosion.

Summer crops

Summer crops for southern Australia offer exciting prospects for increased water usage, reduced weed and disease impacts, and increased rooting depth in soil profiles. They should also help decrease the leakiness of current systems.

It appears that a lot of the water that currently escapes from farms (by moving down the soil profile, past the root zone, and into groundwaters) does so in the intermittent but intense rain storms that occur in summer when there are no growing plants to use the water; with a summer crop in place, much of this storm water will be trapped by the crop.

Another advantage of summer crops is that they will spread risk. Currently, adverse and extreme weather events (e.g. frost in late spring, rain storms during harvest) can jeopardise the total farm income because all the crop (and hence potential income) is grown at the same time. Summer crops are insurance against such events.

Program 4.1.1 Contact: Dr Nigel Wilhelm 08 8303 9353