Phosphate fixation by ando soils different in their clay mineral composition

Abstract

The phosphate fixation capacity at pH 4.5 and an equilibrium concentration of 250 mM phosphate was measured. The soil samples were divided into five groups according to their clay miner. alogical composition. The first group soils contain opaline silica and allophanelike constituents, and some unidentified minerals, the second opaline silici\ and crystalline layer i1icates, the third opaline silica and crystalline layer silicates with additional allophanelike constituents or aluminarich gel-like materials, the fourth allophanelike constituents, allophane and imogolite and the fifth crystalline layer silicates, allophanelike constituents and alumina-rich gel-like materials, plus some halloysite-like minerals, respectively. The first group soils had phosphate fixation capacities of 3,000 to 8,000, the second group soils 1,000 to 3,000, the third group loib 2,000 to 13,000, the fourth group soils 8,000 to 15,000 and the fifth group soil. 5,000 to 12,000 mg P.OI/lOO g oven-dry soil, respectively. The fourth group loils in which allophane and imogolite predominated showed the highest phosphate fixation capacity. The fractions which dissolved from almost all soil samples by treatments with 6% HIO" Na, SIO,-NaHCO,-Na citrate and 2% Na, CO, were estimated to have very high phosphate fixation capacities (8,000 to 19,000 mg PIOI/IOO gdry-matter), and there was not much difference among the loil samples examined. Iron and aluminum combined with humus, allophanelike constituents, alumina-rich gel-like materials and halloysite-like minerals in addition to allophane and imogolite contribute to the phosphate fixation of Ando soils. © 1977 Taylor & Francis Group, LLC.