The influence of soil water content on the uptake of ions by roots. III. Phosphate, potassium, calcium and magnesium uptake and concentration gradients in soil.
Concentration gradients of phosphate, K, Ca and Mg were measured near a plane of onion seedling roots at a range of soil water contents, in a special apparatus utilizing a fine soil fraction. After fourteen days, the concentration of phosphate in solution at the root surface, relative to the initial concentration, fell to about 0.1 in the wettest soil (-0.1 bar at the root surface), but remained near 1.0 in the driest soil (-3.5 bar). The relative solution concentration of K at the root surface fell to 0.01 in the wettest soil and even lower in the driest soil. The relative concentrations of Ca and Mg both rose to above 1.3 in the wettest soil. While Mg rose slightly higher in the driest soil, Ca remained about 1.0. The general shape of the phosphate and K gradients was reproduced by numerical solution of the equation for transport of adsorbed ions to a planar root surface, but the spread of the K gradients was generally underestimated. Estimates of the root absorbing power indicated that the mechanism of phosphate uptake was much more sensitive to decreasing water potential than that of K, and that Mg was more sensitive than Ca. Ion uptake was predicted quite accurately over a very wide range of conditions; the main limitation is lack of information about root absorbing power, although this may be unimportant when there is strong depletion at the root surface.