The uptake of nitrogen from soils in relation to solute diffusion

Abstract

When either a sand or a loam was wetted with solutions of given concentration, the apparent diffusivities of nitrate, KNO3, and ammonium, KNH3, increased with the volume of solution added. Values of KNO3 (0.5-5 x 10-3 cm2/sec) were similar to those reported previously for KCl, and were one order of magnitude higher than the values for KNH4 (0.4-3 x 10-7 cm2/sec). The rate at which wheat roots depleted the sand and loam of nitrate or ammonium was measured by two methods. In one method root mats were used, and in the other single roots were grown through a thin layer of soil. In both methods the flow of water to the roots was minimized. The uptake of nitrogen followed a very different pattern from that reported for stirred solutions. Nitrate uptake increased linearly with soil solution concentration over a wide range, 100-1000 p.p.m. of nitrate nitrogen. Also, although ammonium is generally absorbed more rapidly than nitrate from stirred solutions, nitrate was absorbed far more rapidly than ammonium when equivalent amounts were added to the soil. The observed variations in nitrogen uptake could be largely attributed to changes in the resistance that the soil offered to the diffusion of solutes.