In many cases the roots of vascular plants cannot nutritionally support plant growth alone—they simply cannot reach all the nutrients available in the soil. Mycorrhizae expand the root surface area 10-fold to 1,000-fold, increasing the amount of soil that can be mined for nutrients. In addition, because fungal hyphae are much finer than root hairs, they can get into pores in the soil that are inaccessible to roots. In this way, mycorrhizae probe a vast quantity of soil for nutrients and deliver them into root cortical cells.

The primary nutrient that the plant obtains from a mycorrhizal interaction is phosphorus. In exchange, the fungus obtains an energy source: the products of photosynthesis. In fact, up to 20 percent of the photosynthate of terrestrial plants is directed to and consumed by arbuscular mycorrhizal fungi. Such associations are excellent examples of mutualism, an interaction between two species in which both species benefit. Mutualism is a type of symbiosis, in which two different species live in close contact for a significant portion of their life cycles. As is conveyed in Investigating Life: Mycorrhizal Fungi Can Replace Fertilizer in Cassava Cultivation, the relationship between mycorrhizae and plant roots can be exploited by increasing the fungal population. Many species of fungus invade roots. Identification and genetic improvement of these species is now possible, which holds great promise in improving phosphorus nutrition in important crops.