WithTheGrain: Soil biology proves resilient to herbicide inputs

Author: | Date: 12 Jan 2018

Research into the biological activity within soils across Australia as part of a joint New South Wales Department of Primary Industries (NSW DPI) and GRDC investment has found there are limited impacts on soil biology from herbicide residues.

However, there is the potential for herbicide residues to cause crop damage if they are not used correctly.

NSW DPI soil scientist Dr Michael Rose says research conducted at the Wollongbar Primary Industries Institute has found soil biology is generally quite resilient to herbicide inputs.

“The key finding that’s come out of this project is that when herbicides are used at the recommended label rates and directions, they generally have very little impact on soil biological activity,” he said.

“The soil biological functions are what help to break down most of the herbicides, so as long as a soil is healthy, has adequate organic matter, and is at the right temperature and moisture to promote microbial activity, the impact of the herbicide is negligible.

“There are times when the environmental conditions can slow the rate of herbicide degradation, for example, when there has been a long dry cool period, or low organic matter, or a low or high pH.”

Dr Rose says despite an increase in herbicide applications following the adoption of low or no-till farming, farmer management strategies such as diverse crop rotations, liming, applying organic amendments or correcting nutrient deficiencies helped to maintain healthy soil biological activity.

“We do know that there are two chemical groups that target enzyme pathways found in microorganisms as well as plants – these are glyphosate and the Group B herbicides, e.g. sulfonylureas (SUs) and imidazolinones (IMIs),” he says.

“However, just as weeds evolve herbicide-resistance, so too do the microbes.

“To have any measureable effect on the soil microbial function, the dose of glyphosate that’s needed is more than 10 times the recommended label dose rate and more than five times for the Group B herbicides.

“At label rates, we haven’t seen changes to the functional biological activity of soil greater than plus or minus 10 per cent.”


Mick Rose

NSW DPI soil scientist Dr Michael Rose says research conducted at the Wollongbar Primary Industries Institute has found soil biology is generally quite resilient to herbicide inputs. Photo: Rebecca Jennings

Dr Rose says growers need to be aware herbicide residues can directly adversely impact crops, which is more likely to happen after a dry summer or fallow sprays, or if the soil has low biological activity.

The complicating factor is there are more than 50 different active ingredients registered for use in the Australian grains industry.

“Each of these chemicals will behave differently from paddock to paddock depending on the climate, soil type, crop type,” Dr Rose says.

“We are now looking at developing a model which will give growers and consultants an in-field tool to assess the persistence of different herbicides in individual soil types.

“Growers would input spray rate and timing data as well as data from normal soil tests including soil pH, organic matter and texture. The tool would then geo-locate the paddock to get a reading of rainfall and temperatures since the spray application from the nearest weather station.

“By combining all those elements, we aim to get a good prediction of how much herbicide is left at a given point in time.

“The model framework is in place, we are working on developing this to the next level into a tool for growers to use. We still need further data on what these herbicide residues actually mean for the subsequent crop.

“While we could tell you what the level of the herbicide residue in the soil may be at any given time, we are not yet at a stage where farmers can interpret that data to inform management decisions.”

GRDC research codes: DAN00180, DAW00213

More information

Dr Michael Rose, NSW DPI
02 6626 1123

Dr Lukas Van Zwieten, NSW DPI
02 6626 1126

Useful resources

GRDC Update Paper: Herbicide residues in soils – are they an issue? (Northern), 23 Feb 2016

GRDC Project code: DAN00180, DAW00213