PHOSPHORUS FRACTIONS AND THEIR RELATIONSHIP WITH PHOSPHORUS UPTAKE BY FLOODED RICE IN RIO GRANDE DO SUL

Soils used for flooded rice in the State of Rio Grande do Sul, Brazil, evolved from different materials which led to variations in the distribution of P fractions. In order to quantify the phosphorus fractions and relate them with the P uptake by rice plants, a greenhouse experiment was conducted with four soil samples where rice was grown for 35 days, without and with the addition of 39.30 mg kg(-1) P. In the soil samples before cultivation, the available P was determined (Mehlich-1) and the fractionation of P in the samples was determined by the Hedley's method. The P uptake by rice plants was correlated with the labile P fractions (Pi RTA and Pi NaHCO3) and correlated also with the moderately labile fractions (NaOH 0.1 mol L-1 and 0.5 NaOH mol L-1). The P extracted from the soil samples by resin and Mehlich-1 correlated with the P extracted by labile and moderately labile fractions; the association was stronger for resin (Pi RTA). The P added most recently to the soil samples as triple superphosphate increased mainly the fraction of moderately labile P (Pi NaOH 0.1 mol L-1) and, to a lesser extent, the labile fraction (Pi NaHCO3 and Pi RTA).