AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

From Agronomists of the Potash & Phosphate Institute 655 Engineering Drive, Suite 110 Norcross, Georgia 30092-2837 Phone (770) 447-0335

Winter 1999, No. 2

    “You scratch my back and I’ll scratch yours.” This likely best describes the symbiotic relationship that exists between plant roots and VAM colonization. In return for a food supply of plant photosynthate the VAM fungi extend the reach of the plant root system, improving the uptake of water and nutrients. They are particularly beneficial in the uptake of immobile nutrients like phosphorus which depend on diffusion into an adjacent root for movement into the plant.

    Mycorrhizae increase plant uptake of available solution forms. While the presence of VAM on a plant root system increases phosphorus uptake through a greater surface area, it still only has access to soil solution phosphorus. That phosphorus which is temporarily unavailable to plants due to fixation on the soil complex must move into soil solution before it can be accessed by either plant roots or mycorrhizae.

    A number of crop management practices influence VAM colonization. The VAM fungi spores germinate in the vicinity of roots in response to the secretion, or leakage, of plant sugars from root cells. The phosphorus nutrient status of a crop influences this root leakage, with plants low in phosphorus having the highest amount of leakage and, in turn, mycorrhizal fungi colonization. High plant phosphorus content reduces the permeability of root cells, minimizing leakage, and the resulting infection of VAM. Starter phosphorus rates used by farmers in the Great Plains generally do not result in excessive plant phosphorus concentrations. As a result, the VAM fungi actually help to increase crop response to fertilizer phosphorus additions.

    Tillage and fallow have a disruptive impact on mycorrhizal populations. The mycorrhizal fungi develop long filaments called hyphae to expand the surface area of the root system. These hyphae are very fine, allowing them to explore areas of the soil complex where roots are excluded. In no-till systems, there is little to damage this network of hyphae from year to year, thus permitting rapid establishment of the “expanded” root system on seeding a crop the next year. Tillage disrupts this network, requiring its reestablishment each year. Where fallow is practiced the VAM hyphae network is not only disrupted but becomes inactive due to the absence of any plant to associate with for an entire growing season.

    Making the most of mycorrhizal fungi requires optimum phosphorus fertility. Given the modest starter phosphorus application rates, typical of the Great Plains, it is unlikely that “luxury consumption” of phosphorus will occur. As a result we can depend on the presence of VAM fungi to help improve fertilizer phosphorus uptake through expansion of the plant root system. By partnering with this microbial agent, you can leverage some additional benefit from annual phosphorus fertilizer applications.