ORGANIC AMENDMENTS AND PHOSPHORUS DYNAMICS: III PHOSPHORUS SPECIATION

Abstract

Previous studies have shown that organic amendments decreased P sorption and affected distribution of P fractions in soils. The objective of this study was to determine the effects of different amendments on the chemical speciation of P in soil solution. Soil samples of Jory (Xeric Haplohumult) and Tolo (Typic Vitrandept) were treated with 5% manure or 3 times the CaCO 3 or CaSO 4 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 with a P solution of KH 2 PO 4 (33 mmol P kg -1 soil) and allowed to react (equilibrate) for 12 h. Soil solutions were extracted by centrifugation. Total concentrations of metals, inorganic ligands and DOC were measured in the soil solution. The computer program GEOCHEM was used to calculate the concentrations of the free metals, ligands and their complexes. Adding manure decreased the total soluble Al and Fe in soil solution and increased base cations and SO 4 . Total P in solution increased in : the samples that received manure or CaCO 3 in Jory soil ; and manure or CaSO 4 in Tolo soil. Phosphorus was oversaturated with respect to strengite and the percent distribution of phosphate with respect to this solid tended to decrease in the presence of manure. Calculation of the ion activity product (IAP) for variscite showed that P was in equilibrium with an amorphous analog of variscite. Citrate of the organic acids measured (malic, maleic, malonic, citrate, succinic, formic, and acetic) was the only organic acid that had a significant effect on P speciation. Model calculations suggest that citrate forms soluble complexes with Al, Ca, and Fe, preventing precipitation of strengite and solid Ca-PO 4 .