ABSTRACT
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.
