Soil & Land

Soil pH, SU herbicide and legumes


Have your legumes been struggling? Charles Sturt University research into the interaction of soil pH and the breakdown of sulfonylurea (SU) herbicide may hold a clue.

The research by Charles Sturt Bachelor of Agricultural Science (Honours) student, Ms Anne-Maree Farley aimed to quantify the influence of soil pH on the degradation of herbicides and study the interaction of soil pH and SU herbicide concentration on legume growth and nodulation.

“I found Soil pH effects triasulfuron and chlorsulfuron degradation,” Ms Farley said.

“Degradation slowed as soil pH increased resulting in elevated herbicide residues under higher soil pH.

“In the plant sensitivity study, under the higher pH soils the lupin growth and nodulation were affected by lower concentrations of herbicide.”

Anne Maree Farley, photo courtesy GRDC Toni SomesThese findings are important as we know that agricultural soils have stratified soil pH, with the surface few centimetres having higher pH over an acidic layer a few centimetres below that.

Testing in 10 cm intervals hides the presence of pH stratification.

This means that you may not know that the pH at the surface few centimetres, where the herbicide is, could be at a pH that will slow the breakdown of the herbicide and make it more toxic to plants.

“The take home message is that it is important to measure soil pH in 5 centimetre intervals to accurately identify pH of the profile and allow for the best management of crop rotations and herbicide use,” Ms Farley said.

Ms Farley’s research was supported by a Graham Centre Honours scholarship.

The research was presented at the Grains Research and Development Corporation Update (GRDC) in Wagga Wagga in February 2020 and you can read the paper online.

Source: CSU