INFLUENCE OF CATIONIC IMPURITIES IN ACIDULATED PHOSPHATES ON THE AVAILABILITY OF PHOSPHORUS TO 2 CORN CROPS

Based on the problems that arises from the presence of cationic impuri ties in rock phosphates for fertilizer production, a greenhouse experi ment consisting of two consecutive corn crops was conducted in order t o evaluate the plant availability of phosphorus (P) in the fraction so luble only in neutral ammonium citrate (NAC) and also in the NAC+H2O f raction of acidulated phosphate fertilizers produced from Brazilian ra w materials with different amounts of cationic impurities. The experim ent was conducted with samples of a Red-Yellow Latosol (Typic Hapludox ) in a completely randomized design with four replications. Four acidu lated phosphates obtained by sulfuric acid (H2SO4) solubilization of d ifferent Brazilian raw materials were studied. Monocalcium phosphate [ Ca(H2PO4)(2) . H2O] (MCP) was included as a standard source of P as we ll as samples which were previously leached to remove the water-solubl e P, and therefore, contained essentially the NAC-soluble fraction. Th e fertilizers were thoroughly mixed with the whole soil in the pots (m ixed application), or with only 1% of its volume (localized applicatio n), at the rates of 50 and 100 mg P kg(-1), based on the calculated co ntent of P soluble in NAC+H2O. Corn (Zea mays L.) was the test crop gr own in two sequences of 35 days. After each 35-day period, dry matter yield and P accumulated in the plant tops were determined. Results wer e evaluated by analysis of variance considering the factors, (i) acidu lated phosphates, (ii) rate of P application, (iii) leaching, and(iv) methods of application. In a second analysis, the leached phosphates w ere considered as additional levels of the phosphate factor as well as for MCP. The Tukey test at the 0.05 significance level was utilized f or mean separation. Results from this study clearly demonstrated that increasing the amounts of cationic impurities in the raw materials dec reased the concentration of water-soluble P and NAC+H2O-soluble P as w ell as water-soluble P and NAC+H2O-soluble P ratio of the fertilizer o btained. 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 NA C+H2O method was not an adequate index for evaluating the P availabili ty of the studied sources. No interaction between P sources, leaching, and method of application was found in the second corn cropping.