NSW DPI soil agronomist, Lindsay Evans, in a high-yielding irrigated wheat crop near Finley just before harvest last year. The stubble can now be used to boost soil health and subsequent crop yields.

Two research projects in the Murray Valley have shown that stubble can be used to improve soil health and productivity on farms under continuous irrigated cropping.

NSW Department of Primary Industries (DPI) soil agronomist, Lindsay Evans, said a joint project with the Victorian DPI found practices which conserved or boosted soil organic matter (OM) increased soil organic carbon (SOC), and microbial activity which improved soil structure.

“Soil productivity improved where the soil biota functioned properly and this was achieved by stubble retention, minimisation of cultivation, maintenance of adequate moisture levels and the presence of major elements – nitrogen, phosphorus and sulphur,” Mr Evans said.

“Other key practices which helped increase soil OM were the elimination of long fallows and the adoption of no-till or minimum till farming.

“The project confirmed that optimal soil structure is achieved at an SOC level of about two per cent in the top 10 centimetres and there is little benefit from increasing levels further.”

The Grains Research and Development Corporation project also showed stubble can be occasionally removed, as long as subsequent cultivation is minimised.

Excessive cultivation caused greater losses of SOC than infrequent stubble burning or baling.

Soils were compared on 16 irrigation farms in NSW and Victoria where some included a pasture phase, but most were cropped continuously with winter cereals and maize or rice in summer.

Another Murray Valley project has seen consultant, Clive Kirkby, with assistance from NSW DPI and a group of Moulamein farmers, monitor soil OC levels and biological activity in heavy soils.

Mr Evans said key soil properties could be improved more quickly if the heavy stubble loads produced by high-yielding irrigated crops were retained.

“Heavy stubble should be regarded as an asset,” he said.

“The greater the stubble load, the faster the improvements in soil properties.

“Carbon will be sequestered into the soil if stubble is retained.”

An important finding was the need for adequate contact between soil, moisture and stubble to speed up decomposition when stubble loads exceeded seven tonnes per hectare.

“Incorporating heavy stubble to at least 10cm was beneficial in boosting SOC and microbial biomass,” Mr Evans said.

“For light stubble surface, retention may still be the best option as it poses fewer problems to direct drilling and produces smaller volumes of SOC.”

The project has concentrated on investigating the population size and activity of the free-living nitrogen- fixing bacteria in the soils.

“Their ability to decompose freshly added OM is enhanced if their population is diverse.

“A range of bacterial eco-types allows any of them to dominate and perform decomposing functions as environmental conditions change.”

Research has shown the benefits of soil nitrogen, phosphorous and sulphur in hastening the build up of SOC, as well as optimising crop production.

Both projects have received funding from Murray Land and Water Management Plans Research and Development Budget.

Contact Lindsay Evans, Deniliquin, (03) 5881 9906.