In Brazil, where the rock phosphates are high in impurities, no attempthas been made to evaluate the P supplying efficiency of the neutral ammoniumcitrate fraction (NAC) of P fertilizers, or to verify if the NAC +H2O extraction solution (AOAC) is satisfactory for estimatingthe P availability. To attain these objectives, a greenhouse experiment wascarried out with samples of a Typic Hapludox soil. Four acidulatedphosphates obtained from Brazilian raw materials were studied; monocalciumphosphate p.a.[Ca(H2PO4)2·H2O]was included as a standard source of P, as well as leached samplescontaining no water-soluble P. The fertilizers were thoroughly mixed withthe whole soil in the pots or with only 1% of its volume, at the rateof 50 mg kg-1 of P, soluble in NAC + H2O. Cornplants (Zea mays, L.) were grown for 35 days and the amounts of dry matterand P accumulated in plant tops were determined. Increasing the amount ofcationic impurities in the raw materials decreased the concentration ofwater-soluble P, NAC + H2O-soluble P and water-soluble P/NAC+H2O soluble P ratio of the fertilizers obtained. The P in theNAC fraction was not as much available to plants as in the NAC +H2O fraction or in pure MCP. The great variation found in drymatter (5.4 to 17.1 g pot-1) and in P uptake (6.3 to 22.2 mgpot-1) indicates that the AOAC method is not an adequate indexfor evaluating the P availability of fertilizers with high amounts ofcationic impurities.
acidulated phosphates available P cationic impurities corn phosphorus sources water soluble P Zea mays