PHOSPHATE AVAILABILITY IN TROPICAL SOILS AS RELATED TO PHOSPHORUS FRACTIONS AND CHEMICAL-TESTS

Laboratory and greenhouse experiments were conducted to study the relationships among phosphate (P) fractions, P soil tests, and P bioavailability of tropical soils. In both studies, soil samples were equilibrated with two levels of CaCO3 and three of P during two incubation periods. A sequential fractionation procedure was followed to extract inorganic P fractions (Pi) resin-P, Bic-P, NaOH-P I and II, HCl-P, H2SO4-H2O2-P. Dry matter and P uptake by maize (Zea mays L.) were evaluated in the greenhouse experiment. Results indicated that amounts of Pi varied widely according to levels of lime, P, and incubation time. Resin-P and Bic-P, the most abundant Pi, were highly correlated with dry matter weight and plant uptake of P regardless of incubation interval. Highly significant correlations (P < 0.01) were found for Bray P-1 versus resin-P and Bic-P, Olsen-P versus most P fractions, and Mehlich I-P versus resin P, Bic-P, and residual-P, probably because of the ability of most of these chemical extractants in identifying selectively P fractions. Relationships between plant growth parameters and soil tests were also examined.