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What Do Plant Roots Do for Soil Health?

By   /  March 15, 2021  /  Comments Off on What Do Plant Roots Do for Soil Health?

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Maximizing Living Roots” is the third principle of soil health and it’s focused on feeding the soil. When I first saw this phrase, I confess–it struck me as a bit odd:  since living roots can’t exist without whole living plants, why not just say “maximize living plants?” After a while it dawned on me—if we had a monoculture of a single species of living plants with either short, fibrous roots or just deep taproots, we could potentially maximize living plants, but not necessarily roots.  In order to maximize living roots, we need the widest variety of plants growing together that the site can maintain (more on this next week when we cover diversity).

Here’s a nice illustration of the differences in plant roots among various types of plants.  When we have the maximum concentration and variety of plant roots, then we’ve truly maximized living roots:

Illustration courtesy of the National Park Service

 

So, what’s the big deal about roots, anyway?  Well, we’ve always known that roots are helpful in forming soil aggregates and weaving them together, and we knew that when roots decompose after a grazing event or a crop harvest, they provide food for all the organisms that live in the soil.  But we’re learning that roots are important because of the amazing things they do while a plant is actually growing. You see, plants were never designed to live as isolated, individual organisms—they require a great variety of microbes that perform various functions for them to stay healthy and vigorous.  Plant roots secrete sugars, proteins, and other substances (root exudates) into the soil that recruit and nourish these organisms.  Sometimes people say that plants “leak” these compounds into the soil, but that’s really an incorrect way of describing the process.  Plants govern the types of substances they exude, and the type and amount of exudates varies according to the life stage and needs of the plant.

Some examples of the interactions plants have with microbes are things we’ve known about for a while—symbiotic biological nitrogen fixation by rhizobial bacteria and actinomycetes is one good example. We’ve also known about the huge role mycorrhizal fungi play in plant nutrition.  But we’re learning new things, too,  like how important free living bacteria and archaea are in providing nitrogen to plants that aren’t hosts for the symbiotic bacteria. We’re also learning how mycorrhizae can link different species of plants together– not only for nutrition, but to help the plants protect themselves and each other from insects and pathogens. An example of a game-changing discovery is the rhizophagy cycle, where plants actually “farm” bacteria that they use for food, defense, and physiological function. (We’ve covered some of these in earlier emails, and I hope to re-visit these in more depth later on.) These are all examples of interactions that are initiated and governed by the plants and illustrate how soil functions best as a complete community.

By maximizing living roots, we maximize the volume of soil that the roots occupy and we maximize the amount and types of “liquid carbon” exudates that go into the soil. This maximizes the types and populations of the microbes that live in the soil, helping all the plants to thrive. This also maximizes the substances and organisms that will eventually become the types of stable organic matter (“humus” – like substances) that have all those great properties (water and nutrient holding capacity, aggregate stabilization, etc. ) that we’ve discussed in earlier emails.

These few thoughts barely begin to address the amazing world of roots, but I think you’ll agree that this principle of soil health touches on some amazing interactions. Next week, we’ll continue our journey around the Soil Health Circle with a stop at Maximizing Diversity, the other part of the “feeding” half of the diagram. Stay tuned!

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About the author

Mark is the Soils Agronomist for Organic Valley and CROPP Cooperative. He has spent most of his life working in agriculture. After serving in the Marine Corps, Mark attended the University of Wisconsin-River Falls where he earned a Bachelor of Science in soil science and a minor in agronomy. He worked two years as a soil scientist with the Soil Conservation Service and then attended graduate school at UW-Madison where he completed a master’s degree in soil science. Mark worked for over 24 years an agriculture agent for UW-Extension providing education to farmers throughout Wisconsin, focusing on crop and soil management. In 2012, he accepted his current position, where he helps cooperative members and others to improve their crop and soil management skills. He and his family have operated a small grass based dairy farm in northern Wisconsin for over 20 years and have been members of Organic Valley since 2007.

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