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Mycorrhizal fungi are soil livestock

Mycorrhizal fungi are soil livestock
They can work for you on your farm to increase efficiency and crop profitability

By Heidi Johnson

If you are strictly a crop farmer, you may not think you are involved in animal husbandry but you are managing livestock in your soil. OK, these are not livestock in any traditional sense since you don't have to get up at the crack of dawn to milk them and you certainly can't sell them for profit but they do require care and management. And they can work for you on your farm to increase your efficiency and crop profitability. 

There are many categories of microorganisms that perform many vital functions in your soil. Arbuscular mycorrhizal fungi are an important example of soil livestock that can provide agronomic services with the proper care and management.

Mycorrhizal fungi are soil livestock

Arbuscular mycorrhizal fungi are soil organisms that colonize the roots of the majority of crop plants. This sounds like a bad thing but they actually are in a symbiotic relationship with the plant, meaning they are beneficial to each other. The AMF attach to the plant root and create masses of root-like structures (hyphae) in the soil that mine the soil for nutrients, greatly increasing the potential nutrient uptake by the plant.  In return for this favor, the plant feeds the fungi. But this is at a minimal cost to the plant in comparison to what the plant is gaining by increasing its nutrient uptake. The AMF also improve soil function by improving soil aggregation which increases water infiltration and retention, improves soil structure and reduces erosion. This increase in soil function indirectly improves crop production.

Arbuscular mycorrhizal fungi have been shown to be very beneficial for corn production. The fungus is particularly good at increasing phosphorous uptake by the plant. This makes much more efficient use of phosphorus in the soil and can reduce the farmer's need for phosphorous fertilizer. High rates of phosphorous fertilizer have been shown to suppress AMF so growers need to be careful to not over apply and cheat themselves out of the benefits of this fungal partner.

If too much phosphorous is readily available in the soil, the plant roots will reject the fungus and not create the association. This means money could be wasted on fertilizer.  his is similar to what happens when nitrogen fertilizers are provided to legume crops, the plant can reject the bacteria that creates root nodules and use the readily available fertilizer.

Starter phosphorus fertilizer is commonly used in corn, even in soils that test high or excessively high in phosphorus, to avoid phosphorus deficiencies that can sometimes be found in young corn seedlings when the soil is cool and the seedlings haven't developed a large root structure. This early season deficiency can lead to dry matter yield losses. Research on corn silage in British Columbia, Canada has shown that this practice did not reduce mycorrhizal colonization and should still be used to protect silage yield.  Even when there are sufficient amounts of AMF in the soil, the fungal attachment to corn roots will not occur immediately, typically starting at the V4 growth stage.

Innoculation of soil with mycorhizzal fungi is generally not needed or recommended because these fungi are generally abundant in most agricultural soils and promotion of these fungi can be easily obtained with proper management. Agronomic practices that impact the fungal abundance are tillage and crop rotation.

Tillage is very damaging to the hyphae of AMF and set back its development so systems that include aggressive tillage will reduce the fungal populations. Low disturbance tillage and no-till systems on the other hand will promote mycorrhizal fungal development and improve soil function and crop production. 

Cover crops and crop rotation are also important for maintaining AMF levels in the soil. Generally cover crops provide a food source for the fungi during times when there isn't a cash crop planted, increasing the AMF population in the soil. However, it should be noted that brassicas (e.g. mustard, radish, and turnip) do not serve as hosts to mycorrhizal fungi and will neither increase nor decrease AMF pop. A whole season fallowing without cover crops can also reduce fungal abundance. Unfortunately, there are no simple ways to measure AMF in soil, but utilization of soil conservation management practices such as reduced tillage and cover crops will increase the AMF population in your soil, which can lead to improvements nutrient uptake and crop growth.

Johnson is the Dane County Extension crops and soils agent.

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