ORGANIC AMENDMENTS AND PHOSPHORUS DYNAMICS .3. PHOSPHORUS SPECIATION

Previous studies have shown that organic amendments decreased P sorpti on and affected distribution of P fractions in soils. The objective of this study was; to determine the effects of different amendments on t he chemical speciation of P in son solution, Soil samples of Jory (Xer ic Haplohlamult) and Tolo (Typic Viltrandept) were Created with 5% man ure or 3 times the CaCO3 or CaSO4 required to neutralize exchangeable Al and incubated for 28 d, After incubation the samples were air-dried and subsequently brought to field capacity with deionized water or wi th a P solution of KH2PO4 (33 nmol P kg soil) and allowed to. react (e quilibrate) for 12 h. Soil solutions were extracted by centrifugation. Total concentrations of metals, inorganic ligands and DOC were measur ed in the soil solution. The computer program GEOCHEM was used to calc ulate the concentrations of the free metals, ligands and their complex es, Adding manure decreased the total soluble Al and Fe in soil soluti on and increased base cations and SO4. Total P in solution increased i n: the samples that received manure or CaCO3 in Jory soil; and manure or CaSO4 in Tolo soil. Phosphorus was oversaturated with respect to st rengite and the percent distribution of phosphate with respect to this solid tended to decrease in the presence of manure, Calculation of th e ion activity product (IAP) for variscite showed that P was in equili brium with an amorphous, analog of variscite. Citrate of the organic a cids measured (malic, maleic, malonic, citrate, succinic, formic, and acetic) was the only organic acid that had a significant effect on P s peciation, Model calculations suggest that citrate forms soluble compl exes with Al, Ca, and Fe, preventing precipitation of strengite and so lid Ca-PO4.