Studies on the Role of Free Oxides of Iron and Aluminium in Fixation and Transformation of Phosphate in Some Acid Soils of West Bengal

Department of Agricultural Chemistry & Soil Science, University of Kalyani, Kalyani, West Bengal Manuscript received 1 April 1974; accepted 2 May 1975 The effect of free iron and aluminium oxides on the fixation of applied phosphates in one acid lateritic and one alluvial soil of West Bengal has been studied. It has been found that on incuba­tion at 50% moisture holding capacity for 7 days the free oxide removed soils fix only 14.5% (lateri­tic soil) and 20% (alluvial soil) of the soluble phosphate added while the corresponding original soils can fix 58% and 50% respectively of the applied phosphates. The studies reveal that greater phosphate fixation in lateritic soil is due to the presence of larger amounts of free iron oxide (4.5%) and aluminium oxide (2.7%) as against 1.2% and 0.8% respectively in the alluvial soil. The phosphate fractionation studies have further indicated that the phosphates fixed as Fe-P, Al-P and reductant soluble-P in such acid soils depend almost entirely on the occurrence of the above free oxides. With the increasing period of incubation transformation of water soluble phosphates is markedly diminished by the removal of free oxides in both the soils. The free oxide removed alluvial soil has been found to fix relatively more phosphates (4.2 mg/100 gm of soil) in comparison to the corresponding lateritic one (3.51 mg/100 gm soil) on incubation for a maximum period of 60 days