The effect of variations in physical form such as particle size and mixing with other materials, on the rate of dissolution of sulphate and phosphate from solid fertilizers, has been investigated. It is apparent from these results that the degree of subdivision of a solid fertilizer is particularly important in determining its susceptibility to leaching. It has been demonstrated that leaching of sulphate is appreciably restricted when the granule size is 2 mm or greater in diameter. Except when leached immediately after application, the retention of phosphate is determined largely by 'reversion' to the relatively insoluble dicalcium phosphate form. combination of sulphate with phosphate in superphosphate or the addition of various additives to gypsum markedly reduces the rate of sulphate leaching in comparison to pure gypsum. A comparison of pore size distribution suggests that pores within the intermediate range from approximately 500 A to 105 A play a significant part in the dissolution process.
