Comparison of soil extraction procedures for estimating phosphorus release potential of agricultural soils

Excessive phosphorus (P) discharge from agricultural soils to freshwater may contribute to eutrophication processes. The study of the release of P from soil, using laboratory simulations (long-term desorption experiments with different solution:soil ratios) can provide an idea of the amount of P that can be released from soil particles to water (potentially desorbable P). In the studied soils, the P release patterns are clearly related to the dominant P forms and soil properties. In this sense, the fraction of Total P (TP) that is released at 340 days (solution:soil ratio = 10,000) is positively correlated (P<0.001) with the Index of Surface Saturation by P (SI), defined as the ratio between the Isotopically Exchangeable P at 1 day (IEP1) and the maximum P sorption capacity calculated using the Langmuir equation (X(m)-). A figure of the amount of potentially desorbable P from an environmental point of view can be provided by extrapolation of the amount of released molybdate reactive P (MRP) to a reference concentration of MRP in the solution which can be assumed to be 0.01 mg L-1. The ratio between this predicted amount (Q0.01) and TP in soil is also closely related to SI, indicating that the amount of P that can be released from soil to water depends on soil properties and especially on the degree of surface saturation by P. Phosphorus extraction methods using chemical extractions and P sinks can provide a quantification of the potential for P to be released from soils to water. Goethite, that is a near-infinite P sink, and mixed resins [cation and anion exchange resins (CAER)] are effective in extracting soil P, and the extracted amounts using these methods are closely related to the estimated amounts of potentially desorbable P. The extraction using goethite at 2 days is also a useful method for estimating the Total P Available for Plants (TPAP). Thus, this extraction method constitutes an alternative method to traditional agronomic soil tests in order to assess agronomic P availability as well as the P content of soils with environmental significance.