The popular narrative is that healthy soils produce healthy plants.
This is correct but incomplete.
We need to ask the question, what creates healthy soils? “Healthy plants”, is the right answer.
Without the contribution of plants, soil is just decomposed rock particles; sand, silt, clay.
Plants contribute the carbon, the sugars, the energy that serves as a fuel source, and substrate to develop microbial populations that build organic matter and mineralize nutrients and make them available to plants. The humic substances and humus clay complex are the result of plant contributions to the ecosystem.
Healthy plants create healthy soil.
The key adjective in this statement is ‘healthy’. Unhealthy plants do not create healthy soil. In fact, the opposite.
Unhealthy plants create unhealthy soil.
In this post a few days ago, Robert Kremer described how the root exudates of GMO crops can increase the virulence of soil-borne pathogens. But wait, root exudates are supposed to be a good thing, no?
The influence of root exudates on soil microbial communities is determined by the complexity and quality of the compounds they transmit through the root system, not only the quantity of exudates.
Unhealthy plants will transmit simple carbohydrates, non-reducing sugars, amino acids, and other compounds in ratios that enhance the virulence of pathogens, by providing them with a ready food source.
Healthy plants at higher levels on the plant health pyramid transmit more complex carbohydrates, reducing sugars, polysaccharides, enzymes, and complete proteins, as well as plant secondary metabolites.
Unhealthy plants may also transmit some of these compounds, but in different ratios from healthy plants.
The different ratios of complex carbohydrates, enzymes and secondary metabolites produce a different microbial community response in the rhizosphere.
Unhealthy plants that transmit a lot of simple sugars favor the development of a disease enhancing soil microbial community. They increase the virulence of disease pathogens present in the soil.
Healthy plants that transmit more complex compounds favor the development of a disease suppressive soil microbial community. They decrease the virulence of disease pathogens in the soil, and actually convert them to have a symbiotic relationship with the plant instead of a pathogenic one.
While healthy plants create healthy soils, unhealthy plants create unhealthy soil. This is why focusing on optimizing plant health in the current growing season provides such big soil health rewards.
GMO crops generally have different carbohydrate and amino acid profiles from their non-GM counterparts, which produces a different soil microbial community.
On several occasions, we have observed GMO corn crops and GMO corn stalk mulch produce a soil environment that enhanced disease, sometimes dramatically. Why would it be the case that GMO crops produce a disease enhancing soil environment, where non-GMO corn produces a disease suppressive environment?
Other research has identified that GM plants have altered carbohydrate and amino acid profiles in the root exudates, which seems to be a probable mechanism for producing an altered rhizosphere microbiome.
John: Earlier you mentioned the impact of genetically modified plants themselves, apart from glyphosate and AMPA. How do GMOs impact the soil’s microbial community?
Robert: Well, there’s not a lot of information. We found with soybean, for example, that genetic modification can have what are called pleiotropic effects—indirect effects due to the genetic modification that are in addition to the intended effect. In other words, effects that are in addition to the effect of making the plant resistant to glyphosate. And so there are things that can happen in the root system—with some of the early genetically modified soybean varieties, anyway—that even without being treated with glyphosate, the roots seemed to release a lot more carbohydrates or soluble carbon and amino acids. This is problematic because it attracts a lot of microbes that readily use this material, and many of those can be potential pathogens. So you have a potential problem not only with some root pathology, but it’s also possible to build up these segments of the microbial population and carry them over from year to year.
Another situation where we find these effects is in corn. Not in all varieties, but in many varieties that had been genetically modified to be resistant to insects using Bt, there was a side effect where some of the corn stocks would have a lot more lignin than others. Lignin is very difficult to decompose. That’s one of the reasons we sometimes see a lot of that residue being carried over for two or three years in the field—there’s so much lignin that it can’t decompose very fast.
And I think there are other situations that can occur. I had a Brazilian student here who looked at some of the nutrient composition. Some of the omega fatty acid ratios were changed in soybeans due to the genetic modification; that kind of thing. Now, I can’t say for sure if that has changed with some of the more recent cultivars, because I haven’t been looking at that very closely over the last few years. But, as you know, in our commodity agriculture, these varieties change almost from year to year. Some of the varieties that we were using fifteen years ago are not available anymore. So that’s always another problem. You just don’t know whether the effects of these newer varieties are any better or any worse unless somebody has a research program that’s addressing it.
John: Your first point is very intriguing. In essence, what you’re describing is that these crops and these plants may have the capacity to actually develop a disease-enhancing soil profile—which is interesting when you consider the long-term implications.
Robert: Right. That was a completely unexpected result that we had. And we were comparing it to some of the old non-GMO varieties like Williams 82 and Maverick, and they had much lower soluble carbon and amino acid release. So it was quite interesting, to say the least.
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.
Most agricultural soils today do not supply adequate levels of manganese to a crop. This is a foundational problem, because of the need for manganese in the water hydrolysis process at the beginning of photosynthesis.
When water is absorbed from the soil, and used for photosynthesis, the first step in the process is that the water molecule needs to split into H and OH, hydrogen and hydroxyl. This process is called water hydrolysis. Without this crucial first step, the photosynthesis process is blocked or greatly reduced. The water hydrolysis process is completely dependent on manganese to function. The macro ingredients needed for photosynthesis are chlorophyll, sunlight, water, carbon dioxide, and manganese. Even if we have generous levels of the first four, when manganese is low, it becomes the bottleneck that slows down the photosynthesis process.
We observe inadequate manganese levels in plants almost universally. You can observe it visually, quite easily in most plant species. The leaf vein should be at least as dark green as the area between the veins. When the veins are lighter in color than the area between the veins, this is an indicator of a low-level ‘hidden hunger’ manganese deficiency, and manganese being a limiting factor in the photosynthesis process. You can observe this easily on most plants, crops, cover crops, and so-called ‘weeds’.
I have learned from experienced agronomists that this systemic challenge with manganese has not been present historically. We now know that glyphosate, AMPA, some other pesticides, and oxidizing microbial communities all contribute to manganese being chelated and oxidized in the soil, and not available for plants to absorb. Many soil have generous levels of manganese in the profile, it only needs to be released. I will be hosting a webinar on Friday June 19th at 11 AM EDT describing the cultural management practices and tools that can be used to release the locked up manganese in the soil profile. You can sign up to attend here.
John: In our experience working with many different farms, I would say, just off the top of my head, that greater than 90 percent of the farms we work with have experienced severe manganese deficiencies. I wonder about the long-term effects of this. When you have glyphosate accumulation, you have this shift in the fusarium population, and you have oxidizing organisms that are immobilizing manganese. What are the long-term implications of that manganese immobility in the soil profile? How long does it take before that manganese might be released and converted back into a form that the plants can actually utilize?
Robert: Yeah, that is a concern. I think there are several issues here. You have the effect of the shift in the balance of the microbial diversity. It’s shifted toward a lot of manganese oxidizers, causing manganese not to be available. I think that over time, if one were to alter the management to include, let’s say, cover crops—or at least different crops within the rotation—this could stimulate other types of microorganisms that will help free that manganese, or that will at least compete with those oxidizers to reduce their impact. So that would be one possibility. And how long that will take, it’s hard to say. It may take a couple of seasons, or maybe less. That’s something that really needs to be looked into.
The other issue is something we mentioned previously: how much manganese can be immobilized or chelated by the residual glyphosate and the residual AMPA? That, I think, is a very serious issue—especially in soils where the texture is such, or the level of phosphorus is so low, that you don’t have any competition with the glyphosate or AMPA. They will obviously chelate or immobilize manganese as well. I don’t think we have any real good information on how long that can happen or what the extent of that situation is as far as tying up manganese over the long term. Taking all that together—the shift in the microbes, the residual glyphosate and AMPA, and the basically continuous corn-soybean rotation—if that continues, the manganese problem may persist.
John: When you speak of bringing crops into a rotation to help reduce some of that manganese and to increase its availability, what are some crops that are really effective at having a reducing effect and shifting the biology and the availability of manganese in the soil profile?
Robert: I don’t have any specific ones in mind, but certainly when you have a diversity of cover crops in a mix, there will be some that will support different microbial communities that are able to mobilize these micronutrients, and others that can actually mobilize the nutrients themselves. A common example is the use of buckwheat, or some of the brassica crops, which can mobilize phosphorus or neutralize nitrates.
And then let’s say you add something like sorghum to the rotation—grain sorghum or sweet sorghum. From my experience, sorghum has a keen ability to host a lot of mycorrhizal fungi in its root system. And mycorrhizae are very adept at mobilizing many nutrients—not just phosphorus. If you could add a crop like that, or other crops that can host mycorrhizae, that would be a very good way to get around the manganese problem, to improve regrowth, and to improve the overall diversity of the microbial community.