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Beneficial Soil Organisms
Part 1: Fungi

Paulette Mouchet

Originally published in the Rose Garden newsletter, July, 2003. Updated November, 2006.

Most organic rosarians have heard of mycorrhiza—beneficial fungi that form symbiotic relationships around the roots of roses and other plants and improve plant growth. You might not know that there are hundreds, perhaps thousands, of other types of beneficial fungi found healthy soil and without them, plants do not reach their full potential.

Beneficial fungi are important because

What is biomass and why is it a good thing?

Biomass is a term for the total weight of living organisms in a particular environment. A large fungal biomass is a good thing because it's a way to store valuable macro and micro nutrients in a form (fungi) that is easily converted into use by the plants and is safe from the effects of leaching and soil pH.

One of the most remarkable aspects of storing nutrients as fungal biomass is that these nutrients can be moved into the plant as needed regardless of soil pH. Soils with a large fungal biomass (along with other beneficial soil organisms) and an abundance of organic material can provide almost everything a plant needs without additional supplementation.

What is the soil food web?

From the Soil Biology Primer (published by the Soil and Water Conservations Society in cooperation with the USDA Natural Resources Conservation Service): The soil food web is the community of organisms living all or part of their lives in the soil. A series of conversions of energy and nutrients occurs as one organism consumes another. All food webs are fueled by plants (including lichens, moss, photosynthetic bacteria, and algae) that use the sun's energy to fix carbon dioxide from the air. Most other soil organisms get energy and carbon by consuming the organic compounds found in plants. However, as other organisms decompose complex materials, or consume other organisms, nutrients are made available to the plants.

There are three general groups of fungi.

1) Decomposers—saprophytic fungi—convert dead organic material into fungal biomass, carbon dioxide, and small molecules such as organic acids. This is the largest group of fungi. They are aerobic (require oxygen) and will die when the soil becomes anaerobic (lacks oxygen). Anaerobic conditions often occur when the soil is compacted or over watered.

2) Mutualists—these are the mycorrhizal fungi that colonize plant roots. In exchange for carbon from the plant, mycorrhizal fungi help solubolize phosphorus and bring soil nutrients and water to the plant. Ectomycorrhizae grow on the surface layers of the roots and are commonly associated with trees. Endomycorrhizae (a.k.a. VAM) grow within the root cells and are associated with grasses, row crops, vegetables, and shrubs including roses. Mycorrhizae are aerobic.

3) Pathogens or parasites—these fungi colonize roots and other organisms causing reduced production (parasitic fungi) or death (pathogenic fungi). An example of a pathogenic fungi is verticillium that causes verticillium wilt. Pathogenic fungi can be aerobic or anaerobic.

When the soil food web is properly balanced, pathogenic fungi are kept at bay by the presence of beneficial fungi (decomposers and mutualists). Pathogenic fungi gain the upper hand when the balance of good guys to bad guys is shifted by soil compaction, over watering, or the use of chemical fertilizers and pesticides. Even organic products such as fixed copper, lime sulfur, sulfur powder, and horticultural oils can damage fungal populations and should be applied with caution.

Beneficial fungi along with other beneficial soil organisms are the cornerstones to healthy soil and vigorous, disease-resistant roses that bloom nonstop. Most soils are low in fungal biomass. Adding wood-based compost (ex. redwood compost) and vermicompost are excellent ways to increase the soil's fungal biomass.

Fungi in the Winter

During cooler winter months, most beneficial soil organisms reduce their activity or go to sleep. Not so the fungi (think mushrooms). Beneficial soil fungi remain active during the cool, damp months converting organic matter to biomass and transporting nutrients to dormant plant roots as needed. They are even active when snow blankets the ground!

Read Part 2: Bacteria


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