The study of extractable and soluble phosphorus on an agricultural watershed in Quebec

The purpose of this study was to determine how much phosphorus could be lost from soils in Quebec. Samples of four soil series and 3 sediment samples from the St. Esprit watershed, Quebec, Canada were treated with KH2PO 4 solutions of 0, 50,100, and 500 mg kg--1 of soil. The relationship between water-extractable phosphorus (soluble phosphorus) and Mehlich III available phosphorus was determined at water: soil ratios 100:1, 200:1, 500:1. Measurements were made on a LACHAT QuickChem AE instrument (based on EPA method 365.3; USEPA, 1983) after 4 hours of shaking. More than 90% of the soluble phosphorus was released after 3 hours of shaking for all the soil samples and the sediment sample. Therefore, the shaking time for release of soluble P was set at 4 hours for all soil groups of the watershed. Mehlich III extractable phosphorus was also determined for each soil and sediment sample. Using a modified form of a well-known equation, it was possible to show that, with appropriate values for the constants, linear relationships exist between the logarithm of soluble phosphorus and the logarithm of Mehlich III extractable phosphorus at different water soil ratios. This was true for all soil groups and the sediment sample. Thus, given the soil type of a particular watershed, and using the linear relationship (isotherm) for that type, it becomes possible to predict the phosphorus yields from agricultural lands with reasonable confidence.