Interpretation of Phosphorus Uptake by Barley From Acid Soils Using a Diffusion Model

Abstract

Phosphorus uptake by barley @er cent of dry tissue) grown on four acid soils each treated with four amounts of lime and P, correlated equally well with EPC (equilibrium P concentration = concentration of P supported in solution at field capacity) and with diffusion supply of P to root cylinder surfaces calculated from the soil indices including EPC, a soil capacity factor maintaining that concentration, and a diffusion coefficient.Calculated rates of P supplied by diffusion to primary root cylinder surfaces were too low to account for P found in plants. If the remainder of the observed P uptake was assigned to root hairs, the ratios of calculated root hair surface area to estimated root cylinder surface areas necessary to account for the difference were in reasonable agreement with ratios for barley reported by other workers. IntroductionPLANT P uptake has been empirically correlated with soil P in solution, with labile P content of soil, and with the P buffering capacity (the ratio between labile P and solution P) of soil (CUMV and Sutton, 1967, Ozanne and Shaw, 1968;Dalal and Hallsworth, 1976;Holford and Mattingly, 1976). In a theoretical analysis of the factors controlling P uptake, Olsen et al. (1962; 1963; 1965; 1968) derived an equation for the diffusion rate of P through soil to the root surfaces. Applying this equation to calcareous soils, they predicted the minimum solution P concentration that would allow plants to take up P at a rate reportedly optimal for good growth. Their equation included the soil diffusion coefficient, the soil buffering capacity to replenish solution P, and the P-concentration difference between soil solution and root surface. Using the same variables, Brewster et al. (1977) simulated Pdelivery to plant roots by diffusion processes.We tested the applicability of this equation to four acid soils from southeastern U. S. These soils are rich in kaolinite, differ widely in their concentrations of A1 and Fe oxides and have a more complex P chemistry than calcareous soils. We measured P uptake by barley and tested the correlations of per cent P in barley values with Equilibrium P concentrations (EPC = Concentration of P supported in solution at field capacity) and the amount of diffusible P calculated from measured soil indices. These were compared with estimates from the plant -uptake data.