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The mobility of P applied as diammonium orthophosphate (DAP), triammonium pyrophosphate (TPP), or ammonium polyphosphate (APP) was studied in columns of Hartsells fine sandy loam. The fertilizer dissolved in soil moisture that moved towards the application site. This water movement was sometimes against a gradient in soil moisture content, but it was along a gradient in the total potential of soil water.The extent of P movement from all three sources was similar, but the distribution patterns were different. The extent of P movement was influenced more by the initial soil moisture content than by the source of P.A higher fraction of the added P was precipitated when the source was TPP or APP than when it was DAP. The ability of the polyphosphates to sequester soil Fe and Al did not prevent the precipitation of these phosphates nor did it make them more mobile than the orthophosphates. It only delayed the precipitation reaction to a degree that depended on the polyphosphate content of the fertilizer material.

Several granule sizes (ranging from 20 to 40 mesh to < 325 mesh) of dicalcium phosphate dihydrate (DCPD) and anhydrous dicalcium phosphate (DCPA) were compared in a greenhouse experiment with corn forage grown on Mountview silt loam, pH 6.6. Several comparable screen‐size separates varied with respect to the size of the component crystals. Plant response increased with decreasing granule size.Geometric surface areas of the fertilizers were estimated from the counted number of granules and the size of the granules. Relative rates of solution, which are roughly proportional to geometric surface area, were also measured.The linear correlation coefficients between (a) availability coefficient indexes and geometric surface area were 0.94 and 0.69 for the DCPA and DCPD fertilizers, respectively, and (b) availability coefficient indexes and relative rates of solution were 0.98 and 0.85 for the DCPA and DCPD fertilizers, respectively.The results of these experiments indicate that the availability coefficients of different granule sizes of DCPD and DCPA are primarily dependent upon geometric surface area of the granules and are not much influenced by variations in size of the component crystals in the granule. Per unit of surface area, the availability coefficient of the DCPD is about 2.3 times that of the DCPA.

The effect of associated nonphosphatic salts on the transformation of reagent‐grade monocalcium phosphate monohydrate (MCP) during dissolution at the granule site was examined in Hartsells fine sandy loam (pH 5.2) and in calcareous Webster silty clay loam (pH 8.3). The fraction of the added phosphorous that remained as a residue at the granule site varied from 92% when MCP was mixed with CaCO3, to only 2% when ammonium sulfate was mixed with MCP. Uptake of phosphorus by plants grown on the soil‐fertilizer systems after removal of the residues indicated that plant response was influenced by both the quantity of phosphorus actually transported into the soil, and the composition of the solution in which the phosphorus was transported. The data further support the concept that during dissolution of the granule, the soil has relatively little influence on the chemistry of the reactions taking place in the granule. Consequently, laboratory phase‐rule studies may be quite useful in estimating: (a) the fraction of phosphorus that will remain as a residue at the placement site, and (b) the composition of the solution leaving the granule. The latter is of utmost importance in studies of soil‐fertilizer phosphorus reactions.