Phosphate sorption curves were measured for 103 Western Australian soils sampled from farmers' fields. A measure of phosphate buffering obtained from a fitted curve was compared with indexes obtained from single-point measurements of phosphate sorption. The single-point indexes were adjusted for the phosphate present in the sampled soils by adding the phosphate extracted by the bicarbonate reagent to the measured sorption. Iron and aluminium extracted by an oxalate solution were measured as indirect indexes.The single-point indexes were closely correlated with the values obtained from the sorption curve. Their correlation was much larger than that of indirect measures.
Allen, D. G., Barrow, N. J., & Bolland, M. (2001). Comparing simple methods for measuring phosphate sorption by soils. Australian Journal of Soil Research, 39, 1433-1442. https://doi.org/10.1071/SR00078