LONG-TERM REACTIONS OF PHOSPHATE ROCKS WITH AN OXISOL IN COLOMBIA

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Abstract
The objective of this study was to characterize the reaction products of fertilizer phosphorus (P) and the phosphate rocks (PRs) that remained unreacted in a Colombian Oxisol (Tropeptic Halplustox) under field conditions cropped with Brachiaria decumbens during the 5 yr after application of triple superphosphate (TSP) and six sources of PRs at a rate of 176 kg P ha-1. A strong agronomic response to P fertilizers was still observed at the 21st cutting of the crop, which was harvested after soil sampling. Soil chemical analyses revealed that soil pH, exchangeable Ca, total Ca, total P, Al-P, Fe-P, Ca-P, and Bray-1 P were all increased in the soil samples treated with PRs. The fertilizer P treatments appeared to have no significant effect on soil-exchangeable Al, oxalate-extractable Al and Fe, and P sorption isotherm. Data show that the amounts of the undecomposed PRs in the soil ranged from 2.5% (Fosbayovar rock) to 20.9% (Pesca rock) of the initially applied PRs. Almost 88% of the excess total treatments was found in the clay fraction. The results obtained in the magnetic separation study suggest that the reaction products formed from PRs may be less crystalline than those from TSP, even though the reaction products are probably in the same forms, namely, Al-P and Fe-P. This may explain why the residual available P of PRs was more effective than that of TSP for the plant.