Precipitation of calcium phosphates in the presence of soluble organic matter

Dicalcium phosphate dihydrate (DCPD) and octacalcium phosphate (OCP) are important P solid phases is soil systems. They both have been recognized as precursor phases to the formation of thermodynamically more stable hydroxyapatite (HAP). Their metastability with respect to HAP may be explained by precipitation kinetics and the influence of dissolved organic C (DOC) on crystal growth. Precipitation rates of DCPD and OCP were measured at respective pHs of 5.7 and 6.0, and 25{dollar}spcirc{dollar}C in the absence and presence of organic acids common to soil solutions using a seeded crystal growth method. Initial total soluble (TS) C levels added as humic, fulvic, tannic, and citric acids ranged from 0.26 to 9.0 mM C for DCPD experiments and 20 uM to 2 mM C for OCP experiments. Precipitation of DCPD and OCP was inhibited by adsorption of these organic acids onto crystal seed surfaces blocking sites that act as nuclei for new crystal growth. The efficiency of adsorbed organic acids at inhibiting DCPD and OCP crystal growth is related to their functional group content, size, hydrophobicity, geometry, and orientation on the crystal surface. Of the three Ca phosphate minerals considered, precipitation inhibition increased in the order DCPD {dollar}<{dollar} OCP {dollar}<{dollar} HAP.; Soil incubation and plant available P experiments were conducted to examine the influence of organic acids on P status of soil systems. For the incubation study, a soil was treated with solutions containing 6 to 8 mM Ca{dollar}sb{lcub}rm TS{rcub}{dollar} and PO{dollar}sb{lcub}rm 4TS{rcub}{dollar}, with and without 2 mM C as humic acid. Studies examining bioavailability of P to spring wheat (Triticum aestivum L.) and sorghum-sudan grass (Sorghum sudanese L.) were carried out in controlled environment chambers on soils treated with organic acids. These experiments demonstrated that organic acids were able to increase P solubility and plant availability in soils.; Fulvic acid isolated from a water soluble wheat straw extract was chemically characterized and was most similar to fulvic acids isolated from aquatic systems. It was representative of the hydrophobic, nonhumified fraction of the initial leachate of plant residues and found to inhibit Ca phosphate precipitation. Thus, plant residue breakdown products can enhance P fertilizer efficiency by increasing P bioavailability in soils.