The Detrimental Effects of Chemicals on Soil Fungi

Fungi and bacteria in the soil are the primary recyclers of nutrients in the soil. Whilst bacteria are much more numerous, fungi provide greater biomass because they are relatively bigger. Fungi may be responsible for greater amounts of nutrient retention and soil organic matter formation than bacteria.

Decomposers. Saprophytes play key roles in SOM production because of their ability to help decompose both plant and animal remains, including animal dung. Animal hair, hooves, claws and feathers become food for particular fungal species, and many moulds thrive on animal droppings. A succession of saprophytes colonise debris on the ground. Sugar fungi break down simple sugars but not the complex sugar chains known as cellulose and hemicelluloses, or the lignins that hold them together. Sugar fungi are eventually replaced by brown rot fungi, which digest cellulose and hemicelluloses and, when they have accomplished their work, leave behind a brown, crumbly residue rich in lignin. The white rot fungi that replace them have the ability to digest lignin, the residue most resistant to decomposition, and leave behind wood strips that look bleached and stringy.

Parasites. Parasitic fungi can seriously damage crop plants. The presence of a host plant is necessary for parasitic fungi to proliferate. Normally, they are specific to certain crops or species, but some can affect several plant species. Continuous planting of the host plant will encourage growth of parasitic fungi, so it is important to promote high biodiversity in farm soils.

Mutualists. Called mycorrhizae (myco=fungi; rhizo=root), these fungi invade plant roots but form mutually beneficial relationships which result in better plant nutrition. Many plants probably cannot survive without the mychorrhizae. They extract sugars from plant roots to obtain energy. In exchange, plants gain a lot more in terms of root protection from soil-borne disease-causing organisms and parasites, and better growth rates. Mychorrhizae also produce glomalin, a type of protein, which is important in soil aggregate formation. Glomalin acts as a glue to bind plant cells, fungi, bacteria and microorganisms with soil particles to form larger particles of organic matter which help in providing good soil porosity, promoting water infiltration, and facilitating drainage. At least 90% of agricultural plants form symbiotic relations with mycorrhizae.

In terms of interaction with cultivated plants, the mycorrhizae would be of greatest interest to farmers. Mycorrhizae promote root development, increase uptake of nutrient elements (especially nitrogen and phosphorus), protect plants against pests, diseases and drought, and improve soil aggregation.

The detrimental effects to farm soil, and consequently farm yields, from the use of chemicals involve the mycorrhizae. Insecticides and systemic fungicides can decimate mychorrhizal populations when applied, while herbicides may remove plants that affect fungi distribution. Methyl bromide, a broad-spectrum biocide, is usually used to kill parasitic nematodes and pathogenic fungi, but it also kills mycorrhizal fungi.

Mycorrhizae also become ineffective in soil conditions where nutrient levels are very high or very low. In very low nutrient-level conditions, their sugar extraction activity from plants has a parasitic effect. Their effectiveness is reduced when there is a good supply of phosphorus. Generally, mycorrhizae are most efficient in soils of relatively low fertility that receive little inorganic fertiliser. They are also quite active in soils with ample organic matter, where crops are rotated but with little or no tillage.