We have observed many soils that do not deliver manganese well in spite of having large manganese reserves in the soil profile. In most cases, this is a result of manganese oxidation. Manganese oxidation can result from chemistry interactions, but a great deal of manganese oxidation occurs as a result of fusarium overgrowth from accumulated glyphosate applications.
Here are some thoughts Robert Kremer shared in our conversation on our podcast interview:
John: Coming back to the conversation about glyphosate and AMPA, how does glyphosate—and the accumulation of glyphosate and AMPA over extended periods—impact overall soil health?
Robert: We know that there are some indirect effects of continuous use of glyphosate on soil health, because we usually measure a lot of the biological parameters when we set up soil assessments. We see that there are some effects on some of the beneficial bacteria that are involved in plant growth promotion, such as producing plant growth regulators that stimulate root growth and other beneficial bacteria that will produce pathogen-suppressive compounds. We’re noticing that glyphosate tends to suppress those beneficial groups of bacteria, so that has an effect on subsequent plant growth as well. So we feel that there’s a problem there.
I briefly mentioned the effect on some of these microorganisms that are known to cause certain micronutrients to be immobilized, and therefore not available for plant uptake. One of these is manganese. And manganese, of course, is very important for the activity of many enzymes that are involved in many of our metabolic pathways. If it’s tied up, you may have poor photosynthesis. You may have poor amino acid formation because you don’t have enough of it to satisfy the needs of the enzyme.
We’ve found, for example, that fusarium that will colonize the roots of plants that are treated with glyphosate. Fusarium is a manganese oxidizer, so it will immobilize manganese. If it’s on the root system, manganese is not going to be taken up. And if it’s built up in the soil—whether there’s a genetically modified crop there or not—it’s going to remain in the soil. It’s going to continue to immobilize manganese. If you don’t have a lot of available manganese, that’s going to affect the overall soil health as well.
There are a lot of other things. Glyphosate may exchange with phosphorus in the soil, and then you have problems with either excess phosphorus, or, if phosphorus isn’t being taken up by the plants, it can become an environmental problem. We discussed the quality of the organic matter, because we basically just use two crops as the source of the organic residues being returned to the soil. If we don’t have the microbes there to decompose them, or if there’s not a diverse enough quantity of organic substances to help build up soil organic matter, then that will affect soil health as well, because—like I mentioned before—organic matter is one of the key indicators for good soil health.