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

(1) Concentration gradients of phosphate, potassium, calcium and magnesium were measured near a plane of onion seedling roots at a range of soil water content. (2) 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 potassium at the root surface fell to 0.01 in the wettest soil and even lower in the driest soil. The relative concentrations of calcium and magnesium both rose to above 1.3 in the wettest soil. While magnesium rose slightly higher in the driest soil, calcium remained about 1.0. (3) The general shape of the phosphate and potassium gradients was reproduced by numerical solution of the equation for transport of adsorbed ions to a planar root surface, but the spread of the potassium gradients was generally underestimated. Closer agreement between theoretical and measured gradients might be obtained if the desorption behaviour of ions could be estimated more accurately. (4) Estimates of the root absorbing power indicated that the mechanism of phosphate uptake was much more sensitive to decreasing water potential than that of potassium and that magnesium was more sensitive than calcium. (5) Ion uptake was predicted quite accurately over a very wide range of conditions. The main limitation on prediction is lack of information about root absorbing power, although this may be unimportant when there is strong depletion at the root surface.