The war below: is there a way to speed it up?
The good and bad news on stubble retention is sometimes confusing. Goodfor the soil, but badfor disease promotion? Long-term trials are providing a growing body of evidence that if growers stick it out, the 'white hat' bugs in the soil will overcome their disease-causing adversaries. Alex Nicol reports.
RETAIN STUBBLE for long enough and fungal diseases like Rhizoctonia, take-all and Fusarium crown rot will become a thing of the past. The longer stubble retention continues, the greater the build-up in the soil of microbes, which counter the fungi . But there's a price to pay.
Disease suppression doesn't kick in until about year four or five, and the first three to four years of the practice can result in very high levels of Rhizoctonia. That, at least, is the experience on a range of calcareous sandy soils, including those at Avon in South Australia. David Rogel's work at CSIRO's Land and Water suggests that the same process is at work in red-brown soi ls in other parts of the southern wheatbelt but, wherever this natural disease suppression works, the question remains: can farmers afford the time it takes?
At CSIRO Land and Water, Steve Barnett looked at the bacteria responsible. Now, at SARDI, he is working on ways of speeding up the process. "Three groups of bacteria have a role to play," he says. One, a group of microbacteria that live in the soil, works by suppre ssing the infection of the Rhizoctonia. The other two, associated with the roots, work by promoting growth, even in infected roots. And there's a bonus - the groups associated with the root system may also fix nitrogen.
Dr Barnett says all three groups must be present for the suppression of the disease and he believes that there may be other organisms in the soil that help. He's identifying farming systems that might speed up the process of bringing these organisms to effective levels. Inoculation is a possibility but the aim of current work is to more effectively manage whal's there naturally.
"Once we've got them to a suitable level, then we know that Rhizoctonia at least ceases to be a problem and that an occasional stubble burn doesn' t upset the balance. The problem is that there's a high risk of disease development in the first three or four years of stubble retention.
Balance of cultivation, rotation, carbon and nitrogen
"We've found that cultivation helps, possibly by breaking up the mycelia of the fungi, so cultivation in the first couple of years could help. There's also a suggestion that other crops in the rotation may have a role to play. There's speculation, for example, that canol a may host the beneficial organisms."
But Dr Barnett believes that the relationship between carbon and nitrogen in the soil will probably hold the key. "That's the focus of our work at present. Certainly the build-up of organic matter in the soil encourages the development of these organisms and the relationship between soil carbon and nitrogen is probably the crucial element."
There are other benefits associated with a policy of long-term stubble retention. Dr Barnett says that crops in such a system seem to have the ability to make better use of available phosphorus and that, at the Avon site, there is an estimated 15- 20 kg of extra nitrogen in the system. "We're interested to determine whether or not free-living bacteria may contribute to this extra N.
" None of our current varieties show resistance to Rhizoctonia and the need to plan break crops in a rotation is restrictive. We need to be able to manage weeds and diseases including nematodes, but a soil with the level of microbes needed to suppress disease is a soil that promises a more flexible rotation."