V esicular arbuscular mycorrhizal fungi (V AM) are beneficial root symbionts of most crops, including corn and soybeans. These fungi colonize plant roots, facilitating nutrient ex­ change between the plant and the fungus. The plant supplies carbon for fungal growth; in turn, the VAM enhances uptake of relatively immobile nutrients such as phosphorus (P). The network of external fungal filaments, or hyphae (see photos below), extends outside the root up to several centimeters in the soil, allowing the fungus access to soil P otherwise unavailable to the plant.