Graingrowers have been urged to use this winter cropping season to identify paddocks – or parts of them – that are consistently low yielding.

After ruling out other factors like nematodes and crown rot, they should soil test for sodicity and salinity, down to at least a metre, and check rooting depth when the crop flowers.

That’s the message from senior principal scientist Ram Dalal, who is leading a $2.2 million study targeting these sub-soils constraints in northern NSW and Queensland.

The study – supported by The Grains Research & Development Corporation – links Queensland’s Departments of Natural Resources and Mines and Primary Industries, the NSW Departments of Agriculture and Land and Water Conservation, the Agricultural Production Systems Research Unit and the Universities of Western Sydney and Queensland. .

The aim is to help graingrowers identify and manage sodic, saline and acid sub-soils, estimated in 1999 to have caused yield losses of $90 a hectare – a total $36 million – across 400,000 hectares of marginal cropping land.

"Identification of the presence of sodicity and/or salinity in sub-soils of cropping paddocks is the first step to doing something positive about them," Dr Dalal says.

"High sodicity and salinity in the sub-soil limit rooting depth and make marginal cropping areas even more marginal by reducing plant available water (PAW).

"Soils with minimum subsoil constraints may have a rooting depth of 100 centimetres and more, while roots may only be able to penetrate 50 cm, or even less. The former may have more than 200 millimetres of PAW and the latter as little as 80 mm even when the profile is full.

"Because grain yield and rooting depth are highly correlated, the yield in such shallow rooting depth soils in a good season can be as much as $90/ha less than soils with minimum subsoil constraints."