No serious grower will plant legumes of any sort without inoculating the seed with the proper strain of rhizobia, the bacteria that infects root nodules of legumes and live in symbiosis with them, secreting mineral nitrogen that the plant uses for food while consuming some of the carbohydrates the plant produces using those nitrates. Rhizobia are pervasive in the soil but having the right strain at the right time assures peak performance, so the seed is often sold with a coating of the proper bacteria.

Raw seed is also sold as are bags of dry rhizobia, a powder that can be used by a grower to do his own seed coating. Dump a bag of seed and a small bag of rhizobia spores into a common half bag cement mixer, add water and something sticky like a bit of honey, and you can do your own seed coating. There are more seeds per pound in uncoated seed since the coating increases both the size and weight of seeds, but it is often worth it since doing your own coating can be a pain in the arse. You not only have to coat the seed but also dry it so that you don't end up with clumps of seed that are hard to broadcast or drill.

It is becoming more common to use fungal inoculants too since the discovery of the beneficial effects of mycorrhizal fungi on plants, doing something analogous to what rhizobia does for legumes, for non-legumes too. Like rhizobia mycorrhizae barter nutrients to plant root hairs in exchange for carbs, but they don't infect the roots hairs and nodulate in the same way. Instead they do something that I find even more interesting. They grow threadlike multi-branching hyphae, far thinner than a plant root hair, that spread out through the soil for long distances. The tiny hyphae can penetrate into spaces where root hairs can't go, and so can access nutrients that plants can't reach. They have a longer reach than root hairs and are more penetrating. The nutrients they access - phosphorous, nitrogen and carbon - are transported along the network of hyphae and bartered to customers on the route for sugar.

But that's not the only interesting attribute of mycorrhizal fungi. The network of hyphae improves soil structure, tilth, since it has a sticky coating to which particles adhere, forming aggregates, and so allows better penetration of soil by water, gases and plant root hairs between the aggregates while harboring nutrients and organic matter within the aggregates. Better yet, the hyphae live and die like all things and the residue of dead hyphae is a substance discovered in 1996 by USDA ARS scientist Sara Wright, now retired, which she named glomalin. Many earlier posts here discuss her and glomalin. Glomalin is a durable material containing much carbon that benefits soil much like true humus (the residue of bacterial decomposition of organic matter which endures since it can not be further digested by bacteria). It is stable for many decades, perhaps 100 years, and so is thought of in these parlous climate times as a form of carbon sequestration more durable than organic matter.

This brief article in the USDA ARS magazine discusses some recent work by Kristine Nichols, a microbiologist, who began her career with ARS working with soil scientist Sara Wright.

soils under native grasses—switchgrass, blue grama, big bluestem, and indiangrass—have higher levels of glomalin than soils planted to nonnative grasses, such as Russian wildrye, intermediate wheatgrass, crested wheatgrass, and western wheatgrass. . .

She found that the amount of glomalin in the soil increased as the degree of interdependence increased between plants and arbuscular mycorrhizal fungi. . .

The plant-fungus interdependence is greatest in warm-season native grasses, such as switchgrass, blue grama, big bluestem, and indiangrass. . .

Nichols uses glomalin measurements as a quick guide to evaluate how soil friendly farming or rangeland practices actually are. The amount of glomalin present is also a measure of how much carbon is being stored under various practices, so quantifying glomalin could be used in conjunction with carbon-credit-trading programs.

Nichols has done studies on cropland as well as on rangeland. On cropland, she found that both tillage and fallowing—as is common in arid regions such as those in the Northern Plains—lower glomalin levels by destroying living hyphal networks. The networks are physically torn by tillage or broken down due to starvation during fallowing.

It isn't news that mycorrhizae can be choosy about which plants it will associate with, and that a well established multi-species community can develop, if undisturbed by cultivation, that is highly productive and interdependent. There is even a name for unsociable plants: ruderals. Unfortunately most crop species valued by humans and grown for food are ruderals.

Or so it seems. It is estimated that there are perhaps 2500 different species of endo-mycorrhizae. There are also ecto-mycorrhizae. Some seem more beneficial than others, and some do better with some plant species than others. The science is young compared to that of rhizobia but I suspect we will get better at identifying good strains of mycorrhizae for each plant species and variety, and begin to inoculate them at planting just as we do with legumes now. There are some companies that market such inoculants, often as a multi-species inoculum containing some variety of mycorrhizae strains that have proven to be highly beneficial. As we learn more it is my expectation that even better, more specific, mixes will be produced and that their use will become more common.

More speculatively, I wonder if it is true that our main crop species are in fact unable to associate with mycorrhizal communities - if they are truly ruderals? Since crop lands are often cultivated, or have been so in the past, such communities could not survive. That's one of the reasons cited for the belated discovery of glomalin. Scientists mostly studied cultivated land in service of agriculture, and so never encountered such networks. It is my hope that we will find or even breed varieties of crop plants and mycorrhizae that do associate. Even if that fails it may be possible to do more aggressive plant modification to enable such behavior, just as we are investigating ways to develop non-legume plant varieties that can associate with rhizobia.

Wouldn't that be grand? Crops that could, with the help of rhizobia and mycorrhizae, in effect fertilize themselves to a great extent while improving soil structure and sequestering carbon. In soils amended with biochar - which seems to stimulate rhizobia and mycorrhizae - the effects would be magnified.

I think we are really just beginners at agriculture. Though we have been during it for eons we still have a very great deal to learn.