INFLUENCE OF THE SPATIAL DISTRIBUTION OF NITRATE ON THE UPTAKE OF N BY PLANTS: A REVIEW AND A MODEL FOR ROOTING DEPTH

Published literature is re-examined in an attempt to understand the influence of the spatial distribution of soil nitrate on N uptake in order to devise a simple method for estimating the depth to which nitrate must be leached before it becomes unavailable to the crop. The evidence suggests that most crops can continue normal growth with less than 15% of their roots exposed to nitrate. A simple model of nitrate uptake is constructed in which nitrate is assumed to be totally available above a set depth and totally unavailable below it. This effective rooting depth is assumed to coincide with the depth at which uptake per unit length of root declines to half of the maximum rate. Estimates of effective rooting depth have been made from root distribution data for various vegetable crops grown at Wellesbourne and cereal crops at Rothamsted. The results were found to fit a simple regression equation which can be used to calculate effective rooting depths at any stage of growth from the dry weight and population density of the crop and the mean cross-sectional area of its roots. This equation is used in the succeeding papers to estimate the effects of leaching on the N fertilizer needs of crops.