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

Abstract

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.