The demand for a particular mineral nutrient depends on plant internal requirements, while the supply of that nutrient primarily depends on its availability and mobility in soils (Russell 1977, Marschner 1995).
Mineral nutrients such as phosphorus have very limited mobility in soils so that depletion zones – where all the available nutrient has been utilised quickly from around roots (Bhat & Nye 1972, Russell 1977, Marschner 1995). Therefore, to obtain more phosphorus, plants must bypass these depletion zones by further root activity elsewhere in the soil. The outcome of this quest for phosphorus (and other relatively immobile soil resources) should largely be determined by the surface area of a plant’s root system.
The most important role of mycorrhizal fungus hyphae is to extend the surface area of roots as is explained in the graphic that follows :
Plant with fine roots and long root hairs
Plant with coarse roots and no root hairs
NOTE : These diagrams have been simplified to assume that phosphorus is uniformly distributed in the soil and is equally available to roots and hyphae. They show the extreme situations of a plant with very fine roots and long hairs (such as many grasses) and a plant with thick roots and no root hairs. There are also many plants with intermediate root systems.