Farmer-supported trials are showing low levels of microbial activity in mallee soils sampled from three states — an indicator that these soils are ailing.
The sampling and testing come from nine sites under the five-year Mallee Sustainable Farming Project supported by farmers through the GRDC and the Natural Heritage Trust.
Project research leaders Vadakattu Gupta and David Roget of CSIRO Land and Water say the farmer-driven project is all about increasing sustainable farming practices in the mallee areas of NSW, Victoria and SA.
"The levels of microbial activity in these soils are lower than elsewhere in Australia where measurements have been done," Mr Roget said. Microbial activity in soil is associated with residue breakdown and nutrient release (mineralisation).
"The majority of activity is close to the surface — at me 2.5 cm level. This may be due to long-term soil loss through erosion or inherent restrictions.
"The upshot is that any further erosion will reduce microbial activity even further and so we should be looking at ways to reduce erosion and get build-up further down in the profile by increased dry-matter production and stubble retention, and possibly by placing fertiliser deeper to encourage deeper root growth."
Carbon/nitrogen 'Catch 22'
Mr Roget said all soil samples came up deficient in carbon, which micro-organisms need for energy. When carbon in the form of wheat stubble was added to the samples, renewed activity was evident. However, it was limited by a shortage of nitrogen needed to build up microbial populations.
Countermeasures are to return as much stubble as possible to the soil and try to grow pulses, such as vetch, whose stubbles are high in nitrogen.
Erratic soil moisture
Another major finding concerned the wide range of nutrient and moisture levels when soil tests were taken down to one metre. This suggested that in some paddocks something was stopping full plant access to nutrients and moisture. "It could be a range of things including root disease, boron toxicity, salinity or sodicity," said Mr Roget. The group is investigating these possible causes.
Project coordinator Marion Murphy said the farmer-driven project is also examining many other aspects of profitable and sustainable farming.
For example, a soil erosion assessment has yielded data at the three core research sites — Waikerie (SA), Werrimul (Victoria) and Euston (NSW). The group confirmed that soil cover was the key to erosion control with soil aggregation (clods) adding additional protection when it was available.
Results showed there is less erosion under direct-drill farming systems on mallee soils than under conventional systems.
"At the core sites there were yield responses to increased levels of MAP (N and P fertiliser), urea and zinc. We have developed a model to assess the economics of trial rotations to ensure our results are not only sustainable but also profitable for farmers," Ms Murphy said.
"We are also monitoring a range of commercial farming systems in 46 focus paddocks spread across the three states. We're looking at continuous cropping, conventional long fallows and alternative break crops.
"The focus paddocks give farmers the chance to compare the profitability and water-use efficiency of various systems and rotations by swapping information at post-harvest TOPCROP group meetings."
Program 3.5.2 Contact: Ms Marion Murphy 03 5021 9413