Depletion of soil phosphorus as assessed by several indices of phosphorus supplying power

This study attempts to clarify the advantages and shortcomings of various laboratory tests that are used to characterize the status and dynamics of soil phosphorus (P). We analysed a set of soil samples differing in P status for inorganic P fractions, water‐soluble P (Pw60) and bicarbonate‐extractable P (P_Olsen) both before and after a pot experiment in which soil was depleted. Their P exchange characteristics were determined by Q/I plots, fitted by a modified Langmuir equation, and the supplying power was further assessed by extracting soil samples with various amounts of water. The sensitivity of the P tests in assessing the decrease in inorganic reserves of P was expressed using a response index that was defined as the ratio between the decrease in inorganic P and the decrease in soil P test value. For P_Olsen, the index ranged from 6 to 11 and for Pw60 from 10 to 26. Desorption curves obtained by extracting soil samples with different amounts of water indicate increased buffering power and that large reserves of P were released from soil in weak solutions (P < 0.030 mg l^?1) around soil particles. Although the constants of the Langmuir equation responded reasonably well to the changes in reserves of P in the soil, the results suggest caution is needed in the quantitative interpretation of the constant Q₀ (instantly labile P) of the Langmuir equation.