| Humic substances were isolated from different soils and aquatic environments in the Southern Coastal Plain of Georgia. They were characterized by {dollar}sp{lcub}13{rcub}{dollar}C NMR (nuclear magnetic resonance), IR (infrared) spectroscopy, SEM (scanning electron microscopy), and chemical analysis. The interaction between humic matter and phosphate compounds was studied by reacting humic (HA) and fulvic (FA) with apatite and insoluble metal phosphates at different pH levels. The nature of release P and/or humo-phosphate compounds was analyzed by {dollar}sp{lcub}31{rcub}{dollar}P NMR, whereas availability of the P dissolution products was determined by growing corn plants in hydroponics.; Soil and aquatic humic substances were composed of aliphatic, polysaccharide, aromatic, and carboxyl groups. Humic acids from tropical soils contained more aliphatic compounds than HAs from temperate regions. The latter was more aromatic and higher in COOH groups. Aquatic HA had an analogous composition to soil HA, suggesting that aquatic humic substances were similar in origin as terrestrial humic substances. However, after transfer from soils into streams, the humic matter may have been subjected to further distinct changes.; Humic substances increased the dissolution of apatite and Al- and FePO{dollar}sb4{dollar}. The rate of dissolution increased with increased rates of HA and FA treatments. The reactions were pH dependent. The dissolution of apatite increased by lowering the pH and the dissolution of Al- and FePO{dollar}sb4{dollar} increased by raising the pH. Humic acid was more effective than FA in the dissolution process. {dollar}sp{lcub}31{rcub}{dollar}P NMR spectroscopy indicated that a small amount of P was complexed by HA at pH 7 or higher. Complexation of P was less at pH {dollar}<{dollar} 5, resulting in increased concentration of adsorbed or free P ions. The latter was identified by {dollar}sp{lcub}31{rcub}{dollar}P NMR to be orthophosphate. Corn plants grew better in the presence of apatite (or metal phosphates) and humic matter at pH 5. The results of this investigation confirm the role of humic substances as a metal chelator, dissolving in this way insoluble phosphate compounds. |
