Returning nutrients to the soil: canola, vetch stubble extra tasty to microbes

Dr Gupta and interested farmers check on Mallee soil microbial activity under the microscope. Total microbial activity in soil after both vetch and canola was 16 per cent greater than after wheat.

Canola's ability to suppress certain plant pathogenic fungi has been well documented. Now research by scientists and growers in the Mallee Sustainable Farming Systems group is showing relatively high levels of soil microbial activity following crops of canola and vetch.

Soils following canola, vetch and wheat were measured in terms of total microbial activity, and in regard to the types of microbes present in the soil. The surprise to scientists and growers was that total microbial activity in soil after both vetch and canola was 16 per cent greater than after wheat.

"This extra activity was sustained for the first seven weeks after harvest (see the graph), giving soil microbes quite a long time to break down the stubble, and so convert carbon and other nutrients into a form that plants can use," says CSIRO Land and Water scientist Vadakattu Gupta.

For grower Luke Follett, of 'Benington', near the trial site at Euston, NSW, it's something that all growers should know a lot more about.

"I had never thought about how important to our crops the life in the soil is, and in fact that there was that much there at all," says Mr Follett.

The work also found that canola changed the ratio of bacteria to fungi, when compared to wheat. This favours more short-term microbial activity, and so more nutrients such as nitrogen can be mineralised to a form that is available to plants. An added bonus was the increase in microbial predator activity (such as protozoa) in soils after canola crops.

"Although it is only early days, the implications are that in years to come growers may be reviewing their fertiliser rates following a canola crop," says Dr Gupta. "At the least, it means that there is more microbial activity in these lowfertility Mallee soils and that is bound to give growers additional benefits."

Note: Post-harvest microbial activity increases dramatically after rainfall. Microbial activity in all post-harvest soils
Note: Post-harvest microbial activity increases dramatically after rainfall. Microbial activity in all post-harvest soils decreases over time as organic matter from stubble is consumed.

The effects of three different crops (vetch, wheat and canola) on microbial activity in soils after harvest. Microbial activity in soils after harvesting vetch and canola is greater than microbial activity in soils after harvesting wheat for up to seven

Program 3.4.2 Contact: Dr Vadakattu Gupta 08 8303 8579