Key Microorganism: Beneficial Fungi
Saprophytic or decomposer fungi, as well as vesicular arbuscular mycorrhizal (VAM) fungi, are beneficial to plant life. We must select for these types of fungi over parasitic fungi by keeping soil and plant surfaces aerobic (above 6 ppm O2). Beneficial fungi manufacture all of the necessary enzymes and organic acids to mine and pull plant-essential nutrients out of the sand, silt, clay, mineral and organic matter fractions of soil, while also binding microaggregates into macroaggregates with their mycelium to create soil structure that allows efficient water and oxygen infiltration.
Penicillium expansum found on a mushroom. CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=53930393
Plants also release foods (exudates) for beneficial fungi. In the case of free-living saprophytic fungi, the nutrients are stored in their biomass (mycelium or hyphae) until their respective micrograzers (fungal-feeding nematodes, springtails, soil mites, etc) consume the hyphae and release the nutrients for root absorption.
VAM, on the other hand, engage in host-specific dynamic intracellular (endomycorrhizae) and extracellular (ectomycorhizae) symbiosis with the plant in which the plant feeds the fungi directly once it has mined the essential nutrients and delivered them to the plant.
Recommended levels
The desired fungal biomass depends on where the crop falls in natural ecosystem succession. The minimum requirement is higher for late successional crops, such as tree crops, and lower for early successional vegetable crops. Fungal biomass should always be above 300 micrograms per gram of soil or compost (ug/g).
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