Though not as numerous as the bacteria, soil fungi are very essential in many soil processes. They can form molds, mushrooms, and yeasts. In soils, only molds and mushrooms are considered important. They are very important in the breakdown of starch, cellulose, lignin, gums, sugars, and proteins. Specifically, they break down the soil organic complex, and of the greatest importance, they break down those organic molecules that other soul microorganisms cannot decompose. Fungi are also important in the aggregation and stabilization of soil particles, thus increasing the soil’s ability to provide a suitable environment for plant growth.
Mycorrhizae have become an important study relating to fungi. Mycorrhizae means “fungus root” and are known as the association between soil fungi and the roots of higher plants. This fungal association was first discovered in tree roots, but has since been found to be common throughout most agronomic crops. The fungal relationship between plants and fungus can be of great economic importance. The fungus has the ability to bring more nutrients to the root system of the plant for absorption and utilization. This association is much like nitrogen fixation in legumes where bacteria provide the plant with nitrogen compounds in return for certain plant proteins. This symbiotic association provides the fungus with sugars and other organic compounds that the fungus is able to utilize in its life cycle. In return, the fungi provide enhanced availability of several essential nutrients, including phosphorous, zinc, copper, calcium, magnesium, manganese, and iron.
There are two types of mycorrhizal associations that are of considerable importance, ectomycorrhizae and endomycorrhizae. The endomycorrhizae group, also called vesicular arbuscular (VA) mycorrhizae, are perhaps the most common and most widespread of the mycorrhizae. The roots of most agronomic crops, including corn, wheat, potatoes, beans, alfalfa, sugar cane, cassava, and dryland rice, have these VA mycorrhizal associations. The root cortical cells of host plants are penetrated by the hyphae of VA mycorrhizae. Inside the plant cells the fungi form small structures known as arbuscules. These structures are the sites of nutrient transfer from the fungi to the host plants. The increased nutrient availability from mycorrhizae is thought to be due to the nutrient-absorbing surfaces provided by the fungi. Soil fungi will have as much as 10 times the absorbing surface in association with roots, meaning that roots with mycorrhizal associations can absorb up to 10 times the nutrition. Also, water stress in mycorrhizal-infested plants is less during drought than in uninfested plants.