Snuffing out soil bad guys
"Suppress"- quench, blowout, put out, snuff out (Roget's Thesaurus)
"Suppressive soils" - soils that contain sufficient levels of beneficial organisms able to suppress the activity of pathogenic organisms, for example take-all or Rhizoctonia fungi (David Roget, CSIRO Land and Water)
Ground Cover has been updating the fascinating story of how beneficial soil organisms appear to overcome or 'suppress ' pathogens under long-term conservation management. Denys Slee reports on the latest trial analysis.
THE ROLE of carbon inputs, nitrogen placement and such optional management tools as an occasional burn are becoming clearer as CSIRO scientist David Roget and his colleagues continue to refine an understanding of how soil organisms balance themselves under conservation farming.
Farming systems that involve frequent cropping, high production levels and stubble retention are under the spotlight for their apparent ability to control pathogenic root-damaging organisms naturally. "All soils have some level of natural disease suppression and our management of those soils can strongly influence that suppression," says Mr Rogel.
Feeding the good guys
"Suppression of diseases increases because, as the level of carbon inputs (residues) increases, so do the numbers, diversity and activity of organisms that can suppress the incidence of disease from plant pathogens - carbon being the food source for most soil organisms, including the benelicial organisms.
"Development of disease suppression is dependent not only on the increased carbon inputs but also on an increase in the rate of turnover of those inputs. It is this aspect that regulates the rate of development of suppression. Improved soil suppression of disease is a natural outcome and long-term benefit of intensive conservation farming practices."
Mr Roget and his CSIRO colleagues have identilied the "suppression" factor in long-term studies of inputs and cropping intensity on alkaline sandy loam soils but say there is strong circumstantial evidence for significant improvements in disease suppression in a wide range of soils.
Occasional autumn burn on suppressive soils may be OK
They also say that an occasional stubble bum to control snails, weeds or mice is unlikely to significantly lessen the beneficial activities occurring in suppressive soils.
After autumn burning of selected plots in the first year at the Avon trial site in the lower north of SA, no Rhizoctonia incidence was measured; after autumn burning in the second year, there were low levels present on plant roots; and after three successive years of burning, typical Rhizoctonia patches were seen in the crop.
"This indicates that once good suppression levels have been achieved, the benefits are not easily lost and that an occasional burn, say one year in three, is unlikely to adversely affect disease control ," Mr Roget said.
Nitrogen placement matters
"We have also found that placing nitrogen near the seed benelits disease-causing organisms and so we advise banding the nitrogen below the seed, or side-banding it, so that crop roots can grow in an environment dominated by the benelicial organisms."
Mr Roget said farmers considering a move into a frequent cropping/high input/stubble-retained farming system needed to be aware that suppression did not occur overnight.
Noticeable changes in crop root disease occurrence tended to appear in three to five years with further benelits occurring over time. Because of the lag, those embarking on this system needed to continue to employ standard take-all and Rhizoctonia reduction techniques in the lirst few years. With take-all, for example, these included the use of a break crop in a rotation, and removal of grass hosts.
Carbon the critical factor
He also noted that the soils in which disease suppression can be improved are those where carbon inputs are currently limiting soil microbial activity. In soils where other factors are limiting microbial activity, such as in non-wetting soils, the potential to improve disease suppressive ability is limited.
"Soils with effective disease suppression increase the flexibility of rotational options," Mr Roget said. "Farmers could, for example, consider increasing the GROUND COVER intensity of cereals whereas previously they needed to use a cereal disease break crop, which is generally less profitable than cereals, particularly in lower-rainfall areas.
"A farming system that involves increases in disease suppression also improves other soil functions including nutrient turnover and soil aggregate stability which, in combination with the disease suppression changes, provides strong evidence for the long-term sustainability of soils under intensive conservation farming."
Contact: Mr David Roget 08 8393 8528