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.
Robert Kremer discussed these interactions in our fascinating podcast interview:
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.
Low level manganese deficiency in peaches: