Department of Agricultural Chemistry & Soil Science, Bidhan Chandra Krishi Views. Vidyalaya, Kalyani, West Bengal Manuscript received 21 October 1976, accepted 7 February 1977 The mobilization of native and applied phosphate in relation to ferrous-ferric equilibrium in son has been studied. Native phosphate has no significant and immediate role on the conversion of ferrous Iron to ferric state. But the increase in phosphate activity enhances the degree and rate of oxidation of ferrous ions and as a result most of the applied phosphate are transported to the non-labile pool. The fixation of applied phosphate particularly in acid sons is very little effected by the exchangeable ferric iron. Possibly it is the free ferric oxide which is active in retention of phosphate.