Towards a single-point method for measuring phosphate sorption by soils

Data from 4 separate studies using widely differing soils were used to assess whether single-point estimates of the relative buffering could be used as estimates of buffering derived from fitted curves. The relative buffering capacity of soils for P was summarized using the 'O and S value'. This is the increase in P sorption between concentrations of 0.25 and 0.35 mg P/L. Phosphate sorption curves were described by the modified Freundlich equation: S = ac^b - q, where S (μg/g) is the measured sorption, c (μg/mL) the solution concentration, and a, b, and q are parameters. The fitted O and S value is given by: a (0.35^b - 0.25^b). If a single pair of values for sorption and concentration is measured, rather than a sorption curve, the a value can be estimated from (S + q)/c^b. An estimate of the O and S value can be obtained from (S + q) (0.35^b- 0.25^b)/c^b. Because the b parameter occurs in both the numerator and the denominator, the single-point estimate of the O and S value was not very sensitive to the value allocated to b provided the value of c was in the range 2-5 mg P/L. For most of the soils of low to moderate buffering, the P extracted by the Colwell reagent was a useful estimate of q. For these soils, the single-point estimates using this value of q were similar to the values found from the fitted curves. One group of exceptions were some soils of low buffering which had received a single application of phosphate at a high rate. For these soils, the Colwell reagent extracted large amounts of phosphate. Another group of exceptions were soils of high buffering. For these the Colwell reagent gave values much lower than the fitted value of q and was of little value in improving the calculation of single-point estimates. The single point estimates for these soils were about 20% too low but nevertheless well correlated with the values from the fitted curves.