Although waterlogged lowlands exploitation with agricultural activities endanger the ecosystem, there are not many information about amounts of P to be applied seeking to avoid environmental impacts. P dynamics in these soils are ever far too complex, because reactions of reduction and hydrolysis of other compounds (oxides and hydroxides) affect P contents in soil solution (Lindsay, 1979). Hence, P dynamics in flooded soils can be well described by approaching forms of P in soils using method of fractionation (Hedley et al., 1982). The P fractions of soils related to available P by extractors, would allow the identification of the most extractable pool of P for each soil, and then, the selection of the most efficient method for a particular area (Barbosa Filho et al., 1987). Obviously, the P amounts obtained by extraction should be correlated to growth and total P taken up by plants.
The objective of this work was to estimate critical levels of P in four waterlogged soils cultivated with rice by extractors Melhich 1 and resin, compare the performance of these extractors as for the evaluation of the P availability, and to find out which organic and inorganic forms of soil phosphorous contribute most to P nutrition of rice in these systems.