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Microbes and microbial inoculants

One of the fundamental reasons to grow organically is the preservation and vitality of the soil’s microbial community. We can preserve the integrity of our soil’s microbes by minimizing tillage and eschewing chemical pesticides and fertilizers. Then this diverse, invisible population of fungi, bacteria, protozoa, and others, can get to work contributing to our agricultural production. They suppress plant disease and pests; they mineralize nutrients; they improve soil structure and quality.

We get asked regularly about how to contribute microbes to a field, or how to determine if their levels are “adequate” or ideal. Which lab? What test? What thresholds? Which species? When we passed these questions on to OSU, we were informed that, unfortunately, research has not caught up with popular interest: there’s not much data on established thresholds. The bacterial domain is large and very diverse—and then there’s the fungal kingdom pertaining to the mycorrhizae. And the protozoa. We come to our understanding of valuable species just one-at-a-time.

For instance, Rhizobium spp. are well-understood to promote—actually, to make possible—the nitrogen fixation which accounts for why legumes are such useful cover crops. Legumes provide no nitrogen if grown in a soil without Rhizobium bacteria. This is why the seed is often sold inoculated with Rhizobium.

For another example, there’s the bacterial Pseudomonas spp., which WSU has shown will promote plant growth, apparently via the suppression of certain diseases and root parasites. There’s Azotobacter, another genus known to Cornell to fix nitrogen. Plus, it synthesizes cytokinins and other plant growth regulators, and helps to solubilize phosphorus. In fact, Colombian farmers already inoculate using Azotobacter for their vegetable crops, cotton, and stevia.

And Bacillus is a genus you may already know, for well-known for Bacillus thuringiensis, or B.t. B.t. is already widely used as a kind of pesticide, since its introduction in the 60’s, because this naturally-occurring, soil-borne bacterium produces insecticidal poisons. In fact, certain synthetic pesticides are based on the compounds that B.t. forms. There are organic fungicides on the market based on the subtilis species of Bacillus. And Bacillus amyloliquefaciens helps with fighting root pathogens, including Fusarium, Alternaria, and Phytophthora.

And then in the fungal kingdom, there’s Trichoderma and Scleroderma. Cornell has done research on this versatile fungus, which is already in use by textile industries (it softens denim), and among poultry feed manufacturers (it increases digestibility of certain minerals). In terms of soil health, we know that Trichoderma promotes resistance to plant disease (especially among solanaceous crops) and, in its symbiotic interaction with plant roots, reduces the need for nitrogen fertilizer, even up to 40% in some cases. Fungal hyphae are instrumental in healthy soil aggregation.

None of this gets into the amoebas, ciliates, flagellates, or protists–the protozoa–who feed on pathogens, mineralize nutrients, and “prune” the bacterial population by grazing on them. OSU recommends growing an “alfalfa alley” in orchards to help increase their population.

Healthy soils may or may not already contain populations of these fungal and bacterial species. Just because a field has been out of production for some time, or was recently in woodland, does not guarantee– or eliminate the possibility of– a suitable balance of these species for agricultural production. The species listed above– along with numerous other species not as well understood– are available from different companies in the form of water-soluble inoculants, generally applied as a soil drench, sometimes to foliage. For those curious about introducing them to their field operations, the easiest and most economical method may be to dunk seedlings at transplant. Many of us already apply kelp- and fish-emulsion fertilizers before setting transplants in the field, so it doesn’t complicate the process to add a bacterial or mycorrhizal inoculant at this point. And you are more likely to ensure good contact with roots, and use less liquid.

And finally, some of our customers are enthusiastic about compost tea. They suspend worm castings, molasses, and other fertilizing components in a mesh bag in a bucket of water, and bubble it with an oxygen pump or aquarium pump. The aeration promotes the growth of certain obligate aerobic bacterial species (including Azotobacter), which propagate readily in this environment if well-fed and warm enough. This is then applied as described above—a soil drench, maybe as fertigation or to your starts. Details on equipment and assembly can readily be found on other webpages, including from OSU.

But note that the usefulness of compost tea is not clear. In 2012, Rutgers reported that “reputable researchers, including the Rodale Institute, have not been able to substantiate the disease suppression claims made by proponents of Aerated Compost Tea.” In fact, depending on the nature of the compost you brew with, the tea process can propagate Salmonella bacteria. (Hence, we don’t advise brewing with dairy-manure based composts.) And yet, anecdotal evidence from our customers suggests significant positive outcomes, even if they remain unconfirmed by the professional researchers.


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