Characterization of Available P by Sequential Extraction

Phosphate availability is a function of chemical equilibrium-controlled solubility and rate- limited processes. Most methods for available P determination attempt to quantify P solubility using different extractants, but few relate this to P supply rates that are relevant to plant uptake. Soil test methods for P do not measure the quantity of P available to a crop, but extract a portion of soil P that is related to plant-available P. This relationship is usually established over years of agronomic experimentation and testing of fertilizer responses through regression equations. These equations relate plant performance to soil test P levels, or indicate fertilizer requirement for optimum crop production. Results obtained with this approach are not always transferable between crops or soil types, and different equations are established by soil testing services for varying crops and soils. The approach does not work when perennial plants or natural ecosystems are examined, because measurable pools are often small, and P cycling is the major determinant of P availability. Since any ‘‘immediately available’’ pool of P is constantly replenished through dissolution or desorption of ‘‘less-available’’ P, and through the mineralization of organic P, ‘‘plant-available’’ P is strongly time-dependent.