David R. Montgomery on Symbioses in the Soil

Published on May 24, 2016

David Montgomery, co-author of The Hidden Half of Nature and Professor of Geomorphology at the University of Washington, describes the amazing symbioses between plants and microbes in the soil. To watch more interviews visit: soilsolution.org/interviews/ Transcript: Over the last 20 or 30 years we’ve learned a lot about the role of soil life in soil fertility. Particularly the role that microbial life plays in helping to make nutrients that are in that mineral part of the soil available to plants that can take them up as nutrients. The arguments about sort of what frames soil fertility go way back through history. Obviously people have long thought about the mystery of fertility. Early on in our history we deified fertility, ascribed it to the workings of the gods.

Today we’ve come almost to the opposite end of the spectrum in thinking of microbial life as the great engines driving fertility in the soil, helping to facilitate the breakdown of organic matter—dead things in the soil—that contain the nutrients that used to be alive that can be recycled into new life if only they could be unlocked from that organic matter. And, also from the mineral matter. Now, we can’t eat rocks, right? But if you look at what makes up our bodies, other than the carbon, the nitrogen, and the water, all the other sort of minor elements that are so critical to our health ultimately all are derived from rocks.

Plants can’t eat rocks either. What does? Microbes. The microbes are incredibly important. That soil life, the invisible part, the hidden half of nature we can’t see with our own senses is the part of soil life that really helps bring out the fertility in natural soils and facilitates that with plants. One of the truly amazing things that’s been speculated about for over a century but has really been documented in the last couple decades is the degree to which microbial life forms partnerships with plants. True symbioses between the microbial life living in the root zone, or the rhizosphere of the soil—sort of, close to plant roots—how those microbes are exchanging nutrients with plants for the benefit of both. Plants of course have a monopoly on photosynthesis.

They can take sunlight and turn it into complex organic molecules. Turns out that they’ll pump a surprising amount of that stuff out of their roots into the soil. I was trained to think of soil, or roots, as straws—things that draw material out of the soil for the benefit of plant nutrition. But it turns out they’re two-way streets. They’re putting out material into the soil. Why would they do that? Why would they waste all that energy? Well they’re not wasting it. It’s to feed the microbes that are actually providing the plants with things in return. Things like phosphorous, zinc, manganese, the micronutrients that help facilitate plant health. But they’re also producing things like plant-growth promoting hormones.

Why would microbes do that? Well, in exchange for sugars and other exudates that plants put out through their roots. And that partnership, the partnership between mycorrhizal fungi and bacteria and plants goes back to the very first plants that colonized the continents. The first fossils that we know of from some 450 million years ago, of plants on land, actually have mycorrhizal fungi entangled with the roots. The microbes colonized the continents first and helped the plants come ashore.

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