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Managing microbes

Taking a page from pulse production, beneficial soil microbes are under the microscope

One of the biostimulant areas receiving the most attention is soil microbes, something that already has a long history in agriculture.

The most obvious example is soybean and other pulse crops that are inoculated with rhizobia to ensure they efficiently fix nitrogen from the atmosphere.

It’s an area that’s growing fast thanks to advances in DNA technology that allow scientists to analyze the soil microbiome.

“We need to learn more about that community in the soil,” said Carl Rosen, a professor and extension soil scientist with the University of Minnesota.

“There is interest in more sustainable practices so maybe this is something that’s going to be beneficial in the long run, maybe not the short term, but as we learn more about these microbial activities.”

What some companies are currently doing is going to very high-yielding fields, and isolating the soil from the rhizosphere, (soil very close to the root system) where there is a lot of activity due to all the compounds that are being exuded by the roots as food for soil microbes. They are then going through a long, complicated and very expensive process to isolate and purify those microbes, test them, and turn them into a commercial product.

Beneficial fungi have been co-evolving with plants for many millions of years. Most people have heard of mycorrhizae, which is associated with about 90 per cent of all terrestrial plants. Mycorrhizae are associated with increased P and zinc uptake, protection from some diseases and increased drought tolerance.

“It’s a way of extending the root system, and make available some of those nutrients like phosphorus and zinc that aren’t that mobile,” said Rosen.

Is it beneficial in the field? It depends on the situation. Most studies under field conditions find no effect on yield from using a mycorrhizal product because most soils have very high populations of native mycorrhizae. In addition, if producers use high levels of P fertilizer, it will inhibit mycorrhizal infection.

There are two types of bacteria that have been isolated but the main focus right now is on plant growth-promoting rhizobacteria that are present in the root zone.

“The mechanisms for stimulating plant growth and uptake are due to N fixation, so you can have N fixation by these bacteria that are just free living in the soil. It’s not an association like soybeans or legumes so non-legume plants can benefit from that in some cases,” said Rosen. “You can have some bacteria that solubilize some of these unavailable phosphorus compounds, to make that phosphorus more available. Some produce chelating agents, some produce volatile organic compounds that have growth-stimulating effects, and then you have some that actually produce growth hormones. So, some of these can stimulate growth in different ways.”

An example of these beneficial bacteria is aspergillum, which is found in close contact with roots and it’s estimated, based on the N-fixing capacity of that bacteria, that it can fix about seven to 12 per cent of the N for wheat. “But these are native in most soils so they’re already there, so does it make any difference if you’re inoculating with more?” said Rosen. “That’s a question that needs to be answered.”

What are the results?

Is inoculation with bacteria beneficial? Field studies show a lot of inconsistency. One thing to consider, said Rosen, is that there are over a billion bacteria in a teaspoon of soil.

“If you are inoculating you might have a few hundred thousand that you’re adding on an acre basis, but there’s a lot of competition in the soil, so they often lose out.”

There are lots of different modes of action reported for biostimulants in scientific literature but, based on field studies, what happens in the lab may not necessarily translate to the field, said Rosen.

“You often get inconsistencies; you might see a response one year and no response the second year,” he said. “In my opinion, we really need to have a more systemic approach to define situations where biostimulants may be beneficial. I think with some metagenomic techniques coming out, that’s something we’ll see more of in the future. The main thing, don’t rely on testimonials; just because somebody else did it, doesn’t mean it’s going to be working in your field.”

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