Thinking About Soil Fungi

I'll bet you have not thought much about the fungi in your soil lately. 70% of the soil biomass (the critters in your soil) is made up of fungi. There are over 25,000 species of soil fungi. It has been estimated that there are 10-20 million per gram of soil and 3-300 m of hyphae per gram of soil. Soil fungi can be categorized in three groups:

• Saprophytes
• Pathogens
• Mutualists

The saprophytes are good guys. They are the primary degraders of organic matter and plant residues which are then further degraded by bacteria decomposers. They recycle C, N, P, and K. Saprophytes are mostly on the soil surface and make up less than 1% of the soil fungi.

The pathogens are bad guys. We know them mostly as the cause of plant diseases. Cropping practices influence their effects. Tillage, crop rotation, plant genetics are all practices to reduce their impact. Natural plant defenses to pathogens include physical barriers on root tips (mucigel), cell walls (lignification), enzymes and proteinase inhibitors produced in the plant, etc.

The mutualistic arbuscular mycorrhizal fungi account for the majority of fungi in your soil. These fungi need a source of carbon for food. They penetrate the plant root and take photosynthetic carbon (sugars) as a food source. In exchange they transfer nutrients along the hyphae into the roots. The mycorrhizae have long hyphae that grow out into the soil up to 8 cm from the roots greatly increasing the size of the root rhizosphere. This benefits the plants and is complementary to the plant's root system. Over 70% of plant species are known to associate with mycorrhizae. Exceptions include most brassica species (mustard, cabbage, canola, and sugar beets, etc.)

Phosphorous is not mobile in the soil and having a good level of mycorrhizae can improve the P nutrition of the crop. Mycorrhizaae is the most efficient mechanism for P uptake, especially under stress conditions. N, iron, copper, zinc, and water uptake is also improved by mycorrhizal fungi. In recent years it has been found that mycorrhizae produce glomalin (a protein) which improves soil structural stability. In addition to greater uptake of P and other nutrients, mycorrhizae improve water use efficiency, increase plant vigor, decrease plant root pathogens, and can decrease the susceptibility to nematodes.

To enhance beneficial fungi in the soil consider the following:

• Reduce tillage to avoid disruption of hyphae networks;
• Reduce fertilizer inputs (especially phosphorous);
• Increase the number of crops in the rotation;
• Plant cover crops to maintain the presence of living roots as hosts;
• Use biocontrol measures for weeds and pests to reduce the impact of fungicides and other pesticides.

Have a good look at your soil, there is a lot more than meets the eye.

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