Phosphate sorption data for twenty-nine soils were found to fit the Freundlich adsorption isotherm better if a measure of native labile P was first added to the sorption data, and this sum plotted against intensity. The exponent from the isotherm thus obtained was closely related to exchangeable A1 (in acid soils) and exchangeable Ca (in neutral and calcareous soils), with a mole ratio of one P to 6 A1 or Ca.

The fate of superphosphate applied at 500 and 1,000 Ib PzO, per acre in a field experiment with twenty-four sites was followed by measuring changes in the L-value over 5 years. The L-value decreased exponentially at a rate that was expressed as the time in years for half the applied phosphate to become non-labile (the half-life). The two fertilizer rates gave similar values in most of the experiments and half-life values were satisfactorily measured for nineteen soils ; the values ranged from I to 56 years.Some relationship between half-life values and a number of soil properties was sought. The only significant correlation was with the hydrogen-ion concentration, from which it is suggested that the decrease in labile phosphate could be the result of the formation of a crystalline basic calcium phosphate at a rate that increases with soil pH.

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Short-and long-term uptake of phosphate by ryegrass grown in pots were both well correlated with combinations of the logarithm of the phosphate concentration in solution (logp]), with a capacity factor (L-value). It is suggested that logIp] measures an intensity/kinetic complex that takes account of intensity, rate, and diffusion factors. The best correlation with a single parameter WBO with phosphate adsorbed by anion-exchange resin, which gives a good measure of the capacity factor and also some measure of the intensity/kinetic complex. On soils of normal phosphate status, the capacity factor tended to be more important, suggesting local exhaustion of labile phosphate. With enriched soils, the intensity/ kinetic complex measured by log [PI was dominant both for short-and long-term uptake.