Soil associated fungi are classed into three broad groups:
Decomposers - saprophytic fungi are able to decompose complex carbon based structures such as lignins, and cellulose found on the forest floor. Their size allows them to bridge the gap between soil and the soil litter to pull nitrogen from the soil surface. Their secondary metabolites are organic acids which contribute to the accumulation of humates in the soil. They are therefore a source of organic material which can persist in the soil for hundreds of years.
Mutualists - mycorrhizal fungi colonize plant roots and in exchange for carbon in the food of sugars, and lipids, which are released as root exudates, provide the roots with micronutrients, phosphorus, nitrogen and perhaps water. It has been hypothesized that fungi helped plants make the evolutionary move from the water to land.
Pathogens - Root-pathogenic fungi, such as Verticillium, Pythium, and Rhizoctonia which are responsible for the major economic losses in agriculture. They are able to penetrate the plant and decompose the living tissue, leading to weakened or dead plants. Where disease symptoms are seen, the pathogenic fungi is usually the dominant organism in the soil. Soils with high biodiversity can suppress soil-borne fungal diseases.
In addition to these roles, the soil fungi are able to aggregate soil particles that have been glued together by bacteria into macroaggregates to create soil structure increasing water retention and infiltration. Hyphae can also trap pathogenic nematodes.
Some plant groups that do not form associations with mycorrhizal fungi and these include the Cruciferae family (eg mustard, canola, broccoli), Chenopodiaceae (eg spinach, beets, saltbush) and Proteaceae (banksia, macadamia). When these plants are included in a rotation, fungi numbers drop. A bare fallow has the same effect. When fungal numbers drop, the amount of organic material available to plant roots also drops because nutrients are also stored in the fungal bodies and released by nematodes and protozoa which eat the fungi. This is likely one of the reasons why crop rotation needs to be done so that at least one crop in the rotation recreates the soil fungal populations.
When fungal populations are high, they acidify the soil which makes nitrogen mainly available as ammonium which can not be used by weeds. When the soil is primarily bacterial dominant, then soil nitrogen is available as nitrates which promote weed growth. This is the reason why weeds are the first to recolonize poor soils.
Fungi to bacterial ratios have been observed for different plant groups. Vegetables such as lettuce, broccoli and carrots like 0.3:1 to 0.8:1 while tomatoes, corn and wheat prefer 0.8:1 to 1:1. Orchard trees on the other hand do well with 10:1 to 50:1 and hardwoods from 10:1 to 100:1.
If you want to increase the fungal populations in your soil, then you can use compost that is high in fungi, and that means the compost has to have a high content of woody materials eg. wood chips. You can pre-innoculate your wood chip pile before adding to your compost pile with fungi taken from forest litter from just under the organic layer. A small amount that fits into a ziplock bag is all that is needed.
Hyphae from mycorrhizal fungi emerging from plant roots. Photo: Paula Flynn, Iowa State University Extension
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