DISSOLUTION OF ALUMINUM AND IRON PHOSPHATE BY HUMIC ACIDS

An investigation was conducted to study the effect of humic (HA) and f ulvic acid (FA) on the dissolution of aluminum phosphate (AlPO4) and i ron phosphate (FePO4), to analyze the dissolution products, and assess their availability to plants. The rate of dissolution was determined by shaking 10 mg of Al- or FePO4 with 0 to 800 mg L-1 of HA or FA solu tions at pH 7.0 for 0 to 192 hours. The phosphorus (P) concentration w as measured in the extracts by spectrophotometry, whereas the nature o f P-humic acid complexes was determined by P-31 NMR analysis. Availabi lity of dissolution products was studied by growing corn plants in aer ated hydroponic solutions receiving treatments of 50 mg Al-or FePO4 an d 0 to 800 mg L-1 of HA or FA at pH 5.0. The results indicated that th e amount of P released by HA or FA increased with time. Humic acid was more effective than FA in dissolving the metal phosphates. The P-31 N MR analysis showed that the dissolution products contained free orthop hosphates and minor amounts of P-humic acid complexes. This confirms t he role of HA as a powerful chelator of Al and Fe, liberating in this way the orthophosphate anions. Corn plants grown in hydroponics, with AlPO4 or FePO4 as the source of P, exhibited better growth performance when HA or FA are present.