The concept of a root demand coefficient.
If the "root absorbing power" (α) for a nutrient is given by the flux across the root surface divided by a concentration in solution at the root surface (C1), it is shown that any limitation of uptake by transport processes in the soil is determined partly by the magnitude of αr, where r is the root radius, αr is expected to vary widely for each piece of root but the plant mean αr may be more constant. Total uptake rate from culture solutions is 2π αr C1L, where L is the total length of root, and αr thus defines the mean effectiveness of the root system. In soil-grown plants, the variation of αr over the root system becomes important, since the external concentration C1 is a function of αr. The concept is useful if depletion around the roots is small, if αr varies only moderately over the root system, or if αr for each piece of root follows a similar time course, αr cannot be calculated if αr for appreciable parts of a root system is very large, as soil properties then largely control uptake rate.