Influence of cationic impurities in acidulated phosphates on the availability of phosphorus to two corn crops

Based on the problems that arise from the presence of cationic impurities in rock phosphates for fertilizer production, a greenhouse experiment consisting of two consecutive corn crops was conducted in order to evaluate the plant availability of phosphorus (P) in the fraction soluble only in neutral ammonium citrate (NAC) and also in the NAC+H2O fraction of acidulated phosphate fertilizers produced from Brazilian raw materials with different amounts of cationic impurities. The experiment was conducted with samples of a Red-Yellow Latosol (Typic Hapludox) in a completely randomized design with four replications. Four acidulated phosphates obtained by sulfuric acid (H2SO4) solubilization of different Brazilian raw materials were studied. Monocalcium phospate [Ca(H2PO4)2H2O](MCP) was included as a standard source of P as well as samples which were previously leached to remove the water-soluble P, and therefore, contained essentially the NAC-soluble fraction. The fertilizers were thoroughly mixed with the whole soil in the pots (mixed application), or with only 1% of its volume (localized application), at the rates of 50 and 100 mg P kg-1, based on the calculated content of P soluble in NAC+H2O. Corn (Zea mays L.) was the test crop grown in two sequences of 35 days. After each 35-day period, dry matter yield and P accumulated in the plant tops were determined. Results were evaluated by analysis of variance considering the factors, acidulated phosphates, rate of P application, leaching, and methods of application. In a second analysis, the leached phosphates were considered as additional levels of the phosphate factor as well as for MCP. The Tukey test at the 0.05 significance level was utilized for mean separation. Results from this study clearly demonstrated that increasing the amounts of cationic impurities in the raw materials decreased the concentration of water-soluble P and NAC+H2O-soluble P as well as water-soluble P and NAC+H2O-soluble P ratio of the fertilizer obtained. From the results in the first corn cropping, the P in the NAC fraction for the studied Brazilian phosphate was not as available to plants as was the P in the NAC+H2O fraction or in the pure MCP. The NAC+H2O method was not an adequate index for evaluating the P availability of the studied sources. No interaction between P sources, leaching, and method of application was found in the second corn cropping.