Agronomic Effectiveness Of Fully Acidulated Phosphate Fertilizers Varying In Water and Citrate Solubility.

Single superphosphate (SSP) is the main phosphate fertilizer in Brazil. Because of the increase in the use of low grade phosphate rocks for fertilizer production, it raised the concerns about the reduction in  the agronomic performance of these sources of phosphorus (P) due their  water-insoluble compounds. Our aim was to evaluate the agronomic effectiveness of fully acidulated phosphate fertilizers varying in water and citrate solubility under contrasting soil conditions. An acid Quartzipsamments collected under Brazilian cerrado was used to grow maize in pot experiment, and the following P fertilizers were tested: standard single superphosphate (SSP-85), sulfuric acidulated phosphate (SAP-70), sulfuric acidulated phosphate (SAP-60) and a low grade phosphate (LGP-50). The number after each acronym is related to its fi value which is defined by fi% = WSP / Available P (WSP: water soluble P; Available P: WSP + citrate soluble P). SSP-85 and SAP-60 are commercial products, while SAP-70 and LSP-50 were manufactured exclusively for this research. Changes in the initial soil condition were obtained by changing soil pH (5.0 and 6.5) and soil available P content (4 and 30 mg P kg^-1), resulting in four conditions to be tested. Four P rates for each P source were applied: 0, 25, 75 and 150 mg P kg^-1. Data of maize response to all P sources were described by a semi-logarithm function. Overall, the values of slopes of all fertilizer response curves were not significantly different, regardless of the fi values.  The best performance for each P source was observed with the combined soil condition of pH 6.5 and 30 mg P kg^-1 of soil available P. The effect of initial soil available P on crop P response was more pronounced than soil pH. When P sources were compared under each soil condition, the agronomic effectiveness was statistically no different among all P fertilizers in 12 of 16 possible evaluations under four soil conditions for maize shoot dry weight and P uptake (two harvests). Therefore, water solubility as % of available P of SSP-or SAP-based fertilizers did not affect the agronomic effectiveness of P fertilizers with fi values ranging from 50 to 85%. These results contradict the assumption that higher water solubility of P fertilizers should always be associated with better agronomic performance. Acidulated commercial P fertilizers containing water-insoluble but citrate-soluble P compounds such as SAP-60 can be as effective as those fertilizers with higher water solubility such as SSP-85.