PHOSPHATE DIFFUSION-COEFFICIENTS IN SOIL AS AFFECTED BY BULK-DENSITY AND WATER-CONTENT

Phosphate diffusion coefficients (D(e)) were determined by the quantity of P that diffused from a soil block with P addition into a soil block without P addition. To compare the results with theoretical concepts and to quantify the influencing factors, D(e) was also calculated using the equation of Nye (1968). This equation takes into account the P diffusion coefficient in water, D1, the volumetric water content, theta, the impedance factor, f, and the buffer power, b, of the soil. The results show that D(e) strongly depends on volumetric water content whereas the effect of bulk density on D(e) values was relatively small. If the weighted average buffer power was used, calculated D(e) values were in good agreement with measured values at higher soil moisture contents. At theta < 0.22 g cm-3 the measured values for D(e) were smaller than the calculated. This effect is attributed to incomplete contact between the two soil blocks. The only small influence of bulk density on D(e) is caused by the fact that bulk density affects both theta and b in a way which compensates each other.