Water repellent soils symptom of decline

CEREAL YIELDS of five tonnes per hectare can slip to less than 0.5 t/ha in the same paddock in WA.

A project team, headed by John Blake of the Department of Agriculture, with support from growers and the Federal Government through the GRDC, conducted a grower survey of 28 local grower groups from throughout the WA wheatbelt to determine the crop growth limitations they faced in poorer-performing paddocks and the economic impact of those limitations.

Water-repellent soils emerged as a symptom of declining physical, chemical and biological condition, which is limiting yield and quality across paddocks.

Water repellence can also cause other system challenges. For instance, herbicides don't travel well through dry soils and so provide limited control on non-wetting sands because they are left isolated away from the weed root zone.

While treatments to overcome these problems have been the exception on-farm, in the next decade they could become the rule.

The solution is clay

Depositing clay and digging it into the soil can help overcome water repellence. However, incorporating 100 t/ha of clay can cost up to $165/hectare, so growers first need to evaluate the cost-benefit equation to ensure investment in paddock improvement is recaptured through higher productivity and profits.

That's why the project monitored changes in productivity and returns from renovative techniques to establish their economic viability.

Mr Blake's team and the grower groups trialed farm-scale applications of clay at 100 t/ha and 200 t/ha at high and low incorporation rates.

Best results came from high incorporation (high incorporation means the clay was vigorously dug into the soil rather than being left on top) of 100 t/ha, which lifted barley yields by 45 percent, to 3.0 t/ha on medium sandy duplex soils and 78 percent, to 3.0 t/ha on coarse sandy duplex soils from the previous yield. The control yields were 2.1 t and 1.7 t respectively.

Importantly, this treatment also benefited quality, with barley screenings dropping from an average of 11 percent to less than 6 percent in treated areas, while grain more consistently achieved the protein window for malting.

These results are consistent with those from a project that investigated claying on the south coast.

Dividing paddocks into management zones, depending on soil limitations, proved valuable because it showed up the significant variation in the returns from investment in corrective programs.

Weed removal a bonus

An associated advantage of claying is the damage it does to the weed seed bank, with background ryegrass and brome grass counts plummeting by up to 90 percent over two seasons.

Mr Blake and Graeme McConnell are_developing a decision support system known as IVC - invest, vary or cull- to make better decisions on where to invest in amelioration, where to vary annual crop inputs or where to cull areas from the cropping program. IVC is being tested by five collaborating grower groups in 2002.

Program 4 Contact: Mr Ian Pritchard 08 9622 6100 Mr John Blake 08 9690 2143 email blake@agric.wa.gov.au

Region West