Fertilización fosfatada y fracciones de fósforo en alfisoles, ultisoles y oxisoles

phosphorus (p) is the main limiting factor in highly weathered tropical soils. therefore, it is important to know the p fertilizer fate in the short and long term. the aim of this work was to evaluate the changes that take place in the inorganic and organic forms of the soil-applied p. the hypothesis of this study is that p fertilizer is first incorporated into the most labile soil fractions and later it undergoes transformations to more resistant fractions. soil samples were collected from the surface horizons alfisols, ultisols and oxisols in misiones (argentina) and a greenhouse experiment was conducted with three p levels and three repetitions. after a period of stabilization, samples were taken periodically for a year and were analyzed for p fractions by the hedley method modified by sattell and morris. the moisture content fluctuated during incubation, causing increases in the pi-nahco3, pi-naoh and po-naohus fractions for the three orders,corresponding to decreases mainly of po-nahco3 and of the residual fractions. the fertilizer added in the first 120 days increased labile and moderately labile inorganic fractions (nahco3-pi and naoh-pi). from incubation day 60, sonicate-pi fractions began to show the p rate effects, reaching the maximum value at day 120 and becoming from that moment a source of the added p. p from fertilizer, which was found in the most labile fractions at the start of the experiment, was converted to more resistant forms through time, reaching the highest values after 360 days of incubation.