Available Phosphorus Extracted by Ion Exchange Resin and Pi Strip in Brazilian Tropical Soils Treated with Phosphogypsum.

Resin-HCO₃ and Pi tests were used to assess available phosphorus (P) in five tropical soils treated with phosphogypsum (PG) which has been preferred to lime to ameliorate the Al-toxicity in the subsoil. Samples from the top 0–20 cm of representatives Brazilian tropical soils (Oxisol, Quartzipsamment, Ultisol, Vertisol, and Entisol) were collected in Sao Paulo, Bahia, and  Pernambuco States, Brazil. The soil samples were incubated in laboratory at rates up to 75 g kg^-1 of PG containing 0.3% total P and up to 100 mg kg^-1 of P as either triple superphosphate (TSP) or phosphate rock (PR). The results of this incubation study showed that Pi-P content increased with increasing PG rate for the treatments of TSP, PR, and control (no P added). For resin-P, it was underestimated due to the reaction of resin-HCO₃ and CaSO₄ that resulted in the formation of CaCO₃, meaning a sharp contrast to Pi-P results. Consequently, the strength of resin-HCO₃ was weakened and the CaCO₃ formed could also further adsorb/precipitate soluble P and therefore resin-P was underestimated. Thus, resin-P can be underestimated by the presence of significant amounts of gypsum (either natural or applied to the soil). This explanation for the negative effect of PG on resin-P agrees with other authors, who have worked with resin-HCO₃ and Olsen soils tests in incubation and greenhouse experiments, always in alkaline soils. In all of the five soils studied in this incubation experiment, PG has affected negatively P extraction for resin, but does not for Pi strips. It is concluded that as long as a significant amount of PG applied to tropical soils still remains in the soils, it should be cautious to select a proper soil P test.