Plant availability of phosphorus in the neutral ammonium citrate fraction of Brazilian acidulated phosphates

In Brazil, where the rock phosphates are high in impurities, no attempt has been made to evaluate the P supplying efficiency of the neutral ammonium citrate fraction (NAC) of P fertilizers, or to verify if the NAC + H2O extraction solution (AOAC) is satisfactory for estimating the P availability. To attain these objectives, a greenhouse experiment was carried out with samples of a Typic Hapludox soil. Four acidulated phosphates obtained from Brazilian raw materials were studied; monocalcium phosphate p.a. [Ca(H2PO4)(2). H2O] was included as a standard source of P, as well as leached samples containing no water-soluble P. The fertilizers were thoroughly mixed with the whole soil in the pots or with only 1% of its volume, at the rate of 50 mg kg(-1) of P, soluble in NAC + H2O. Corn plants (Zea mays, L.) were grown for 35 days and the amounts of dry matter and P accumulated in plant tops were determined. Increasing the amount of cationic impurities in the raw materials decreased the concentration of water-soluble P, NAC + H2O-soluble P and water-soluble P/NAC + H2O soluble P ratio of the fertilizers obtained. The P in the NAC fraction was not as much available to plants as in the NAC + H2O fraction or in pure MCP. The great variation found in dry matter (5.4 to 17.1 g pot(-1)) and in P uptake (6.3 to 22.2 mg pot(-1)) indicates that the AOAC method is not an adequate index for evaluating the P availability of fertilizers with high amounts of cationic impurities.