GLYPHOSATE promotes some complex reactions among soil organisms, American scientist Bob Kremer says, but not enough is understood about these interactions to provide specific management advice.
Dr Kremer, a microbiologist with the USDA’s Agricultural Research Service (ARS), instead suggests that there are some general principles that farmers can apply if they are regular users of the chemical.
Occasional studies over more than two decades, mostly in the laboratory, have shown that glyphosate has a complicated relationship with soil life.
In the soil, the use of glyphosate can promote certain strains of the fusarium fungus, possibly because the chemical also stimulates some plants to release more sugars into the soil through root exudates.
Glyphosate has also been shown to inhibit beneficial forms of bacteria, including some of the rhizobium that live synergistically with legumes to produce nitrogen nodulation on roots.
In the plant, the chemical suppresses phytoalexins, the amino acids that plants produce in response to infection, and binds up and makes unavailable some of the key minerals the plant needs to maintain health.
Dr Kremer suspects that all these factors combined could encourage colonisation of crop plants by hostile organisms, like fusarium.
Problem is, very little field research has been done to follow up on laboratory observations, which appear also vary between crops, crop varieties and soil types.
“At field days, I tell farmers that we really haven’t linked big problems to the use of glyphosate,” Dr Kremer said.
“But we’re saying that if all these effects are happening in the field, there may be potential for problems with diseases and decreases in yield.”
If farmers want to manage for this possibility, they should be looking at minimising the possibility of extreme conditions under which glyphosate residues could tip the balance against the crop.
That includes maintaining good levels of all essential soil nutrients.