Aluminum and iron fractions affecting phosphorus solubility and reactions in selected sandy soils
2001
Zhang, Min | Alva, A. K. | Li, Y. C. | Calvert, D. V.
Phosphate solubility and reactions were investigated in each of three horizon samples from six cultivated soils representing Spodosols, Alfisols and Entisols, with pH ranging from 4.6 to 7.7. Soils were incubated for 30 days near field capacity moisture content without P application or with 44.5 mg P kg soil applied as KH2PO4. Soil solution was extracted by centrifugation, and the concentrations of anions and cations were determined. The ionic activities were calculated using the MINTEQA2 speciation program. Sequential-fractionation data suggested that amorphous forms of Al- and Fe-phosphates and P associated with crystalline Al-and Fe-oxides accounted for a significant portion of total P. The activities of Al and Fe in most cases were adequate to support the stability of wavelite, crandallite, variscite, and strengite and were close to the equilibrium point of amorphous Al- or Fe-phosphate without applying P. Following the application of the equivalent of 100 kg P ha, the soil solution chemistry from two Spodosols and one Alfisol suggested the formation of amorphous varscite- and strengite-like minerals. However, the soil solutions were undersaturated with respect to amorphous Al- and Fe-phosphates in two Entisols and in another Alfisol with higher pH, where Ca-phosphate minerals seemed to control the soil solution phosphate activity. The research results suggest that liming practices under citrus production increase the soil pH of the surface horizon and, in turn, shift the control reaction and solubility of the phosphate minerals and influence the availability of phosphorus in these sandy soils.
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