Numerous factors that influence the length of herbicide persistence in soil need to be taken into consideration by growers when using herbicides as part of their farming operation.

Senior consultant with Independent Consultants Australian Network (ICAN) Mark Congreve has been facilitating GRDC workshops on the soil behaviour of pre-emergent herbicides. He says the various factors for herbicide breakdown made predicting their persistence difficult, sometimes causing frustration when it comes to the following season’s planting opportunity.

“Unfortunately, there are not just one or two simple factors that influence the length of herbicide persistence,” he says.

“The main factors include soil properties, climatic conditions in particular temperature and moisture, the chemistry of the herbicide and how it is degraded, for example by microbes or chemical hydrolysis and, in most situations, soil organic matter and pH.”

Soil microbe activity

Soil microbial activity is a major degradation pathway for many of the main herbicides and happens fastest when the conditions are right to support microbial populations.

“Oxygen, warm soil temperature, an adequate level of organic matter, soil pH and good soil moisture are the key requirements,” Mr Congreve says.

“As a result, microbial populations are usually at their highest levels in the top 10 centimetres of soil, with populations rapidly declining below 15cm where conditions are often less suitable to sustain microbial activity.”

Typically, the most limiting factor slowing microbial activity in the topsoil is the lack of soil moisture.

“As a rule of thumb, if the top 10cm of soil is not moist, then little herbicide degradation is occurring, regardless of how many months have passed,” Mr Congreve says.

“Rainfall in summer, when conditions are warm, will lead to much higher microbial populations than rainfall in the colder months.

“In our workshops, we encourage growers to consider how the rainfall has fallen over the summer months since the herbicide application, rather than just looking at the rainfall total. Many labels will have a plantback period specifying the number of months and a rainfall requirement.

“Rather than considering the rainfall in total, it is better to think in terms of the number of weeks of moist topsoil. A single rainfall event with months of dry topsoil is less effective at sustaining microbial populations compared to the same rainfall occurring as a number of events that keep the topsoil wet for longer.”

While rainfall is a major factor, several other factors also substantially impact herbicide persistence.