Agronomic Performance of Phosphate Fertilizers Varing in Solubility to Soybean in Oxisol of Brazilian Cerrado.

The generalized deficiency of phosphorus (P) in most of the Brazilian soils justifies the requirements for constant fertilization in order to increase plant available P levels. The demand for P is high especially under Brazilian Cerrado, where soybean is grown in vast areas with weathered high P fixing soils. Acidulated fertilizers (e.g.single superphosphate, SSP) are the most important P sources in Brazil. However, due to the depletion of high quality phosphate ores and the use of marginal ores, the presence of insoluble compounds/impurities is increasing in the fertilizers. Brazilian government lowered the legislation requirements in order to allow the commercialization of those less soluble P fertilizers. However, there is no field evidence about the agronomic performance of these new P sources compared to traditional highly soluble formulations. Therefore, we aimed to evaluate the effectiveness of acidulated SSP fertilizers varying in water and citrate solubility under field conditions. The trials with soybean were performed for three years in Itiquira, Mato Grosso, Brazil, with a high P fixing soil (clayey Typic Hapludox). The four SSP fertilizers produced from low grade ores presented 50%, 60%, 70% or 85% (standard) as the ratio between water- and citrate soluble P (fi). During the three years of evaluation, the sources with fi 60% and 70% were as efficient as the standard SSP (85%), indicating their suitability for use under field conditions. The lowest fi fertilizer (50%) showed higher soybean yields when compared with the control treatment (no P added), but it was still lower than the yields achieved when a more soluble source was applied. Therefore, fertilizers containing impurities (fi ≥ 60%) can be alternative to the use of highly soluble SSP formulations. These results provide support for revision on legislation requirements and contradict the old premise that water solubility is always associated with agronomic effectiveness.