Soil organisms and plants have a dynamic relationship. So it’s not really surprising that, with greater frequency of cropping and retained stubble, soil organism activity increases. This increase is even greater when inputs of fertiliser rise above district practice. But add a long fallow phase to the rotation and soil organism numbers and activity fall off.
These findings from Mallee Sustainable Farming Project sites have encouraged Vadakattu Gupta of CSIRO Land and Water to look further at changes in soil organism populations and their activity under intensive and less intensive rotations on lighter soils.
Dr Gupta said the Mallee research, supported by growers and the Federal Government through the GRDC, was showing that fallowing caused a major decline in soil organism levels, and their activity.
“We are getting more than a 50 per cent decline in microbial biomass in fallow versus non-fallow treatments,” he said.
He says the increase in soil organisms during the cropping phase is partly related to increased root growth.
Roots release carbon into the soil both as they grow and when they die. This carbon provides the energy source for many soil organisms.
“In the intensively cropped plots more plant matter is returned to the soil not only through the roots, but also as stubble. Both of these provide a good source of carbon and nutrients for the organisms and help increase their population size.”
As the soil organisms access the carbon they release nutrients such as nitrogen and phosphorus and make this available to the growing crop.
Another knock-on effect has been the increase in stable soil aggregates under the intensively cropped plots. "Soil bacteria and fungi produce sticky mucus compounds and, together with the networks of fungal filaments, help to bind the soil particles together to form stable aggregates.” (See photos this page.)
The increase in soil aggregation along with increased stubble cover has been shown by John Leys of NSW Land and Water Conservation to make the soil less susceptible to erosion.
“Seventy-five per cent of the microbes are in the top 5 cm of soil and if you lose this soil, you are just exporting them to someone else’s farm,” said Dr Gupta.
Take home: closer rotations don't degrade soils
These results have been in line with findings in other climatic districts. However, an important part of Dr Gupta’s work has been to quantify the activity of soil organisms important for plant essential functions in lowrainfall environments.
“We know that the availability of carbon, nutrients and moisture are key limiting factors to soil organisms. Here we have shown that, even in these drier cropping districts, carbon and nutrients are more likely to limit soil organism populations than the availability of moisture.”
A key outcome from this work is to show that soils (even in low-rainfall Mallee areas) are not degraded by more intensive rotation but can, in fact, be improved as long as fertiliser inputs are adequate and stubble retention is practised.
In future, Dr Gupta will try to determine the time of year when the effects from micro-organisms are maximised and which farming systems are most supportive of microorganisms which help improve soil structure.
Program 3.4.2 Contact: Dr Vadakattu Gupta 08 8303 8579 email firstname.lastname@example.org
Left: A network of fungal hyphal filaments binding and covering soil particles onto wheat stubble. Middle: Fungal hyphae with their mucilage-covered hyphae help physical binding of soil particles. Right: Large populations of bacteria and polysaccharide-like compounds on the decomposing canola residue.