Reactions of mixtures of granulated superphosphate and ammonium sulphate in calcareous soil

Abstract

Dynamics of P dissolution from fertilizer granules and its transport and transformation in a calcareous soil were studied in order to check the effects of additions of ammonium sulphate (AS) to superphosphate (SSP) granules on these reactions. This was done by checking changes with time of total P, water-extractable P and bicarbonate-extractable P in both fertilizer granules and soil rings around the granules. Fertilizer dissolution induced water movement into granules, which was followed by flow of fertilizer solution into the surrounding soil materials, creating a distinct wetted zone whose area increased with proportion of AS in granules. The increased proportion of AS accelerated the dissolution rate of the fertilizer mixture and the build-up of bicarbonate-extractable P as well as water-extractable P and Ca in the soil rings around the granules. This, in turn, induced a faster fixation rate of P in soil materials that reacted with granules of higher AS content. Thus recovery of water and bicarbonate-extractable P from both granules and soil materials decreased with increasing AS content in fertilizer granules.