- Glyphosate herbicide enables farmers to use no-till practices, which have been shown to benefit soil health and minimize greenhouse gas emissions from agricultural soils. Much of the benefit to soil health is mediated by microbial communities.
- Many of the allegations that glyphosate suppresses the growth of microbes come from studies in which pure cultures were exposed to glyphosate with sensitivity observed only at glyphosate concentrations well above normal field application rates.
- Glyphosate application has not been found to result in increased incidences of disease or yield loss.
Glyphosate herbicide enables farmers to use no-till practices, which have been shown to benefit soil health and minimize greenhouse gas emissions from agricultural soils. Much of the benefit to soil health is mediated by microbial communities. Many studies have investigated the effect of glyphosate and Roundup Ready (RR) crops on these microbial communities, including rhizobia and fungi.
First, I’ll address the findings on rhizobia, which are important members of the microbial soil community because of their role in converting atmospheric nitrogen to an organic form plants can use (this is known as nitrogen fixation). Many of the allegations that glyphosate suppresses the growth of rhizobia come from studies in which pure cultures of these bacteria were exposed to glyphosate. In these studies, differences were found in the susceptibility of rhizobia to glyphosate, with sensitivity observed only at glyphosate concentrations well above normal field application rates. Under agronomically relevant field testing conditions, studies looking at nodule formation, known as nodulation, and nitrogen fixation in RR soybeans treated with glyphosate have shown that many factors can affect the levels of nodulation seen in soybeans, including the genetic background of the soybean cultivar. This type of field testing has shown no consistent effects of glyphosate and RR crops on nitrogen fixation or rhizobia. Most important, numerous studies and farmers’ preference for growing RR soy indicate that glyphosate does not affect crop yield or content/yields of protein and oil for RR soy. Failure to have adequate nodulation would impact yields or nutrient composition.
To address your question on fungi, the fungal members of the soil microbial community vary in the roles they play when interacting with plants. Some cause disease, and others — such as arbuscular mycorrhizal fungi (AMF) — form beneficial, symbiotic relationships and deliver nutrients and water to their plant hosts. Studies on RR cotton, corn and soybeans have shown that glyphosate application does not affect AMF colonization of plant roots and indicated that plant mineral nutrition was not adversely affected. In regard to disease-causing fungi, Dr. Robert Kremer has reported that the number of root-colonizing fungi, such as Fusarium, increased on RR soybean and corn roots after glyphosate application but glyphosate application has not been found to result in increased incidences of disease or yield loss.
Numerous studies investigating the effects of glyphosate on plant disease in RR crops looked at disease-causing fungi, as well as bacteria. Fungal diseases studied include Sclerotinia stem rot (white mold disease); sudden death syndrome (SDS) on soybeans, caused by Fusarium; and Rhizoctonia root and stem rot. Bacterial blight, Goss’s wilt and leaf blight are bacterial diseases that were studied. Overall, it was found that the major contributor to plant disease susceptibility in these studies was the genetic background of the host plant, not the RR crop or treatment with glyphosate. (If you would like a more detailed summary of the information presented here, please take a look at a recent review of the scientific literature on glyphosate, performed by multiple public-sector scientists.)
In conclusion, I have not seen any information linking fungal colonization of RR plant roots with a slowing of plant nutrient absorption. However, there have been claims that glyphosate is a strong chelator that will bind to micronutrients in soil, adversely affecting the mineral nutrition of RR crops and leading to increased potential for diseases to occur. In fact, the review of the scientific literature on glyphosate by Duke et al. that I referred to above notes that glyphosate is actually a weak chelator. If you would like further information on this, I’d recommend reading a response from my colleague Marian Bleeke and a previous response by me.