Nature of the Reactions of Monocalcium Phosphate Monohydrate in Soils: I. The Solution That Reacts with the Soil

Abstract

The dissolution of monocalcium phosphate monohydrate (MCP) in water was followed over a 17‐day period; and the resulting solution was compared with solubility isotherms of the system CaO‐P2O5‐H2O at 25°C. Shaking excess MCP with water at room temperature resulted in rapid dissolution during the first few minutes. Within an hour and continuing for more than 24 hours, the solution was in metastable equilibrium with newly formed dicalcium phosphate (DCP) dihydrate and undissolved MCP. The composition of the solution later changed as anhydrous DCP slowly precipitated and the first‐formed DCP dihydrate dissolved. The final solution was in equilibrium with anhydrous DCP and undissolved MCP. Band placement of MCP in soil was found to initiate a series of reactions. The solution phase from the reaction zone was sampled by means of filter papers separating several 5‐mm. layers of Hartsells fine sandy loam. Reduced water vapor pressure at the fertilizer band caused water to move inward from the surrounding soil. This transport of moisture resulted from both liquid and vapor movement. A wetted zone formed in the immediate vicinity of the band. The solution sampled at the fertilizer band approximated very closely the composition of the metastable triple‐point solution (MTPS) in which P is 3.98M, Ca 1.44M, and pH 1.48. This highly concentrated solution moved slowly away from the fertilizer band into the partially dried soil, dissolving Fe, Al, Mn, and other constituents from the soil. In this complex chemical matrix many different phosphate compounds of Fe, Al, Ca, and Mn are expected to precipitate as the solution reacts with more soil and pH rises gradually to that of the surrounding soil. Water continued to move into the fertilizer zone even after MCP had dissolved. This caused dilution of the first‐formed solution and enabled it to move farther into the surrounding soil.

Phosphorus diffusion from monocalcium phosphate co-applied with salts in a calcareous soil

“…2). Similar results have previously been reported (Lindsay and Stephenson 1959;Lindsay et al 1962;. As the dissolution of MCP produces a solution with pH as low as 1.5 , it is to be expected that diffusion of this acidic solution would reduce soil pH.…”

Dissolution of granulated CuZn-superphosphate in soils

“…MCP). These amounts correspond to the congruent dissolution of MCP rather than to incongruent dissolution with precipitation of DCP and/or DCP dihyrdate as has been observed in laboratory studies of MCP and OSP dissolution [14,16]. This interpretation of the chemical data is supported by the XRD patterns of residual pellets which at no stage showed evidence for the formation of any DCP or of significant amounts of DCP dihydrate (Figure 3).…”

“…The chemical reactions occurring within granules of micronutrient-superphosphate applied to soils are generally understood [13]. However some inconsistencies are apparent from the published literature on this topic and require further investigation.Various proportions (18-33%) of the initial P content of superphosphate granules remained in soils in laboratory experiments as the poorly soluble compounds DCP (dicalcium phosphate) and DCP dihydrate [12,14,16,20,21].Residual OSP granules recovered from beneath farmers' crops after periods of 5-20 months in the soil contain about 25% of the initial P [3]. However this P is not as DCP but as MCP (monocalcium phosphate) and the waterinsoluble constituents of OSP, i.e.…”

Nature and transformation of crystalline phosphates produced by interaction of phosphate fertilizers with slightly acid and alkaline soils

“…Lindsay and Stephenson (1959a) showed that shaking excess Ca(H?P04)2'H20 with water at room temperature resulted in the formation of a solution which was in metastable equilibrium with freshly precipitated CaHP04-2H20…”

“…Investigations by Lindsay and Stephenson (1959a) The ratio of water-soluble calcium to water-soluble magnesium in soil was found by Racz and Soper (1967) to be related to the types of crystal line phosphates formed when the soils interact with solutions of phosphate fertilizers. A calcareous soil with a ratio of 0.69 moles of watersoluble calcium to 1 mole of xjatar-soluble magnesium reacted with to form.…”