Soil Phosphate Diffusion Coefficients: Their Dependence on Phosphorus Concentration and Buffer Power

Abstract Measured values of effective P diffusion coefficients, D e , were compared with D e values calculated on the basis that P only diffuses in the liquid phase. Two soil blocks differing in P content were placed into contact for 2 wk, then quick frozen and cut into slices of 0.2 mm with a refrigerated microtome. The slices were extracted with 4 M HCl to obtain a P concentration‐distance curve. An average diffusion coefficient, D̅ e , was determined from the total amount of P that had diffused from the soil of high P concentration to that of low P concentration. Effective diffusion coefficients for a given soil P concentration, D ec , were obtained by analyzing the concentration‐distance curve. Phosphorus buffer curves were established by adsorption in the lower concentration range and by desorption in the higher concentration range, according to the process occurring during P diffusion in soil. The value of D̄ e increased from 2.2 to 13.9 × 10 −13 m 2 s −1 when the average soil P concentration increased from 380 to 580 mg P kg −1 soil. The D ec values increased with increasing P concentration, although the relationship was not unique. For example, the same D ec value of 5 × 10 −13 m 2 s −1 was obtained at 340 mg P kg −1 when P was being adsorbed and at 600 mg P kg −1 when P was being desorbed during the process of diffusion. This result was related to different P‐buffer behavior of the soil during adsorption and desorption. The P diffusion coefficient in soil is, therefore, dependent not only on the buffer power, which is influenced by concentration, but also on whether P is being desorbed or adsorbed and on the time available for reaction. The value of D e can be calculated if suitable values for the buffer power are used, and D e for a wide P‐concentration range can be calculated if a weighted average buffer power is used.