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

Indian Inst. of Technology, Kharagpur-721302, W.B., India

J. Kaselowsky, N. Claassen and A. Jungk^*

Inst. für Agrikulturchemie, Georg-August-Universitaet, von Siebold-Str. 6, D-3400 Goettingen, West Germany

* Corresponding author.

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, _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 _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.

Contribution from the Inst. für Agrikulturchemie, Georg-August-Universitaet, Goettingen.

Received for publication October 24, 1989.

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