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Home & Garden: Plants need a fiberous diet for root health

Curt Swift
CURT’S CORNER
Free Press Gardening Columnist

I recently read an article in Scientific American about the community of microbes, called the microbiome, in our guts. What piqued my curiosity was the information about microbes and their effect on determining our mind and health. We have all heard about probiotics and how some microbes increase our health while others contribute to health problems. Whether our guts have healthy or disease-causing microbes depends a lot on what we eat including the amount of fiber in our diet.

Fiber is also necessary for soil health as it directly affects the composition and activity of soil microbes. Like our guts, the soil’s microbe diversity and population determines if our plants are susceptible to root diseases or if that population can suppress disease contributing to the health of plants. We talk about soils that increase root diseases as being conducive to disease. A suppressive soil on the other hand is the ability of a soil to prevent or suppress disease even if the pathogens are present.

The type of organic matter we feed our soils has a direct impact on whether the soil is conducive or suppressive. When we use compost made from fibrous plant material we increase the suppressive nature of the soil. Such compost contains lignin and cellulose, the fibrous material that supports the structure of trees and shrubs, and stems of grasses, alfalfa, and other plants. For that reason alone, the compost you use to amend the soil should consist of this fibrous material. Two disease-preventing fungi requiring fibrous compost important in suppressing soil-inhabiting root pathogens are Trichoderma and Gliocladium.



Rhizoctonia, Fusarium, Gaeumannomyces, and other soil-inhabiting fungi that are root rot organisms are common to soil. These soil-inhabiting plant pathogen ultimately influence plant health and productivity if not suppressed. When Trichoderma hamatum comes in contact with one of these pathogens, it attacks producing enzymes that break down the pathogen’s cell wall, preventing it from infecting plant roots. Trichoderma has also been shown to reduce plant damage due to saline soil conditions and drought stress, so protecting plants from root pathogens is not its only attribute. Certain species of Gliocladium have similar disease-fighting and stress-reducing characteristics.

Soil bacteria also impacts soil health, with certain bacteria contributing to disease suppression through antibiosis, plant growth promotion, and systemic-induced resistance. Systemic-induced resistance needs a little explaining as it is hard to believe a bacterium on the roots can stimulate a systemic response in the plant that causes the leaves and other above-ground parts of the plant to prepare for and defend against a possible attack or invasion by an insect, bacteria, or fungus. But, this is what happens when certain bacteria are in suppressive soil.



Some of the disease suppression of beneficial microbes is due to competition for nutrients. Some of the good microbes sequester micronutrients such as iron by producing chelates that capture and hold those nutrients. This prevents the critical nutrients from being taken up and used by pathogens. Again the type of organic matter in the soil has a direct impact on these suppressive microbes.

Mycorrhizal-forming fungi (mycorrhiza) assist in preventing root disease in part by utilizing compounds that ooze from roots needed by the pathogen. Some mycorrhiza create a sheath of protection around the roots. The symbiotic relationship these fungi have with roots increases the energy available for the plant’s own production of defensive methods the root creates to ward off plant pathogens.

Enhancing the natural suppressiveness of soils by adding more fiber is a simple procedure. Leaving crop stubble in the field instead of burning it off and adding fibrous compost are “no-brainers.” We have known about suppressive soils and need enhancements for decades. Yet we continue to neglect soil health. Burning off fields is one example of this neglect.

Some people believe they can increase the disease-fighting ability of their soil by purchasing and adding Trichoderma, mycorrhiza, fluorescent pseudomonads, and other suppressive microbes to soil; but unless you have the proper soil nutrient levels and fibrous organic matter, it might not work. This is especially true if your soil has been amended with compost made from a sugar-based organic product like grape, apple, peach and other fruit residue.

As with the microbes in your gut, beneficial soil microbes require a healthy diet to be effective in preventing plant health problems. That healthy diet must include fiber.

Free Press columnist Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at Curtis.Swift@alumni.colostate.edu, 970-778-7866 or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.


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