A model of S,P and N uptake by a perennial pasture

An approach is presented for simulating the uptake of S, P and N by a perennial pasture, based upon diffusion theory and plant uptake kinetics. Plant uptake was limited by either (i) diffusion to the root, (ii) root uptake capacity, or (iii) plant demand. For S and P, uptake as limited by diffusion was calculated from the nutrient depletion pattern around the root, which is dependent upon soil diffusivity for the anion, and the length of time since the root grew into the new soil. To avoid time-consuming calculations at various uptake times and distances from the root, the nutrient depletion pattern was simplified into an effective rooting radius, whereby all plant-available nutrient within the radius was made available to the plant over a 30 day period, and none from outside the radius. Mycorrhizal enhancement of P uptake was simulated by increasing the radius by a factor of between 1 and 3 when P was the primary limiting nutrient. Limitations (ii) and (iii) were included for S and P for the rare occasions when nutrient concentrations were high either in a single layer, or in the profile overall. For N, uptake was limited by uptake capacity, adjusted for soil N concentration, root length density and soil moisture content.Because of a lack of data on diffusion coefficients close to the plant root, a calibration factor was applied to the effective rooting radius, which is described in the second paper in the series.