SOIL PHOSPHATE DIFFUSION-COEFFICIENTS - THEIR DEPENDENCE ON PHOSPHORUS CONCENTRATION AND BUFFER POWER

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, DBAR(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 DBAR(e) increased from 2.2 to 13.9 x 10(-13) m2 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 x 10(-13) m2 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.