Phosphate diffusion in soil and uptake by plants

Summary

The changes in solid-phase P³¹ concentration with radial distance from the surface of actively absorbing cylindrical roots have been calculated from data previously obtained for the apparent self-diffusion coefficients (D) of P³² in Seddon soil over a range of P³¹-additions. Values were computed for hourly time intervals to at least 5 days for soils of different initial P³¹-status and for two root radii corresponding to the measured dimensions of the main root and root hairs of wheat plants.

SinceD was not constant but was related to the level of addition, it was not possible to obtain explicit solutions to the appropriate mathematical equations so numerical methods were used. The changes in concentration which occur at the surface of the growing root during active uptake are not known so some assumptions necessary for the formulation of boundary conditions are outlined.

Subsequent integration of the depletion data around the root axis enabled the calculation of the rate of uptake of phosphate per unit length of radial sink of designated radius and initial level of added phosphate. The uptake rate per unit length of a model root, proposed on the basis of microscopic examination of growing wheat roots, was calculated by combining the appropriate data for each component, root and hairs. When the computed data for the model root was compared with measured uptake rates per plant, the length of model root required to achieve the actual uptake was found to be 2–6 metres per plant, which was considered to be a small fraction of the total root system. Thus it was concluded that diffusion is an important mechanism for transfer of phosphate from soils to plant roots.