Pellets containing 15 mg of P prepared from each of the following salts: KH2PO4 (MKP), NH4H2PO4 (MAP), (NH4)2HPO4 (DAP), Ca(H2PO4)2·H2O (MCP), and CaHPO4·2H2O (DCPD) were placed in cavities in the surface of Bradwell very fine sandy loam (pH = 7.8) and Monroe silt loam (pH = 5.8). The treated soils were stored at 5, 20, and 35C. Extent of dissolution of fertilizer pellets and development of soil-fertilizer reaction zones were measured during the first 14 days of storage. At the end of 2, 7, 14, 28, and 56 days the soil-fertilizer reaction zones were sampled and the concentration of water-soluble P and the percentage of fertilizer P soluble in the reaction zones were determined.
Rate of dissolution of the water-soluble P fertilizers increased markedly as temperature increased. Some pellets were incompletely dissolved at the end of 14 days at 5C, whereas, at 35C dissolution was usually complete in less than 4 days. For each 15C increase in temperature there was about a 33% reduction in concentration of water-soluble P and the proportion of fertilizer P soluble in water.
During the first 14 days after adding the water soluble fertilizers there was a rapid decrease in concentration of water-soluble P and proportion of dissolved fertilizer P while after 14 to 28 days the changes were much samller. Extraction of water-soluble P decreased in the order MKP>MAP = DAP>MCP>DCPD>check. Differences among sources became smaller with increased time of contact with soil. DCPD was more soluble in the acid Monroe soil than in the calcareous Bradwell soil.