PHOSPHATE ION TRANSFER FROM SOIL TO SOIL SOLUTION - KINETIC-PARAMETERS

In a soil, phosphate nutrition of crops depends on the rate at which phosphate ions in the soil solution are renewed from phosphate ions in the solid phase. Before the present study, however, no method was available to determine the rate of phosphate ion renewal from soil. Using the isotopic exchange method of (PO4)-P-32 ions in soil solution systems maintained at steady state and the stochastic analysis of kinetic data (table II), 3 kinetic parameters were defined: (i) the mean renewal rate of phosphate ions from soil which theoretically can vary to a great extent (table IV), (ii) the mean time phosphate ions remain in the soil solution, (iii) the mean exchange flow between soil and solution. These parameters were determined in soil samples (table V) taken from long-term experimental designs where residual effects of phosphate fertilization were studied. The mean exchange rate and the mean time phosphate ions remain in the soil solution were not greatly modified by crop phosphorus uptake or phosphorus input but the mean exchange flow was greatly modified (table III). Comparison of dry matter yields in the experimental designs with the mean exchange flow shows that the mean flow could be a limiting factor in phosphate nutrition of crops (table III). These results show that attention should be paid to hydric nutrition in order to satisfy phosphorus nutrition.