Immobilization and mobilization reactions of soil phosphorus depend on chemical, physical, and biological properties of soil and these soil properties strongly depend on soil water content and rate of phosphate application. Laboratory study was conducted at 30oC for one week on two calcareous soils from southern part of Iraq, Hartha soil (silty clay) and Zubair soil (loamy sand) to study the effect of superphosphate application (0, 60, and 120 mgPkg-1 soil) and water potential (field capacity, 3/2 field capacity, and ½ field capacity) on solubility and vertical movement of phosphate for four soil depth (0-5), (5-10), (10-20), and (20-40) cm. Results showed that increasing P-rates application increased soluble P and it was decreased with decreasing of soil moisture content as follow: FC> 3/2FC > ½ FC. Formation of calcium phosphate minerals in the studied soils according to saturation index (SI) values showed that Ca-P minerals were MCP,OCP, and HA which their values were above 1 (SI>1), the degree of saturation index was related mainly to the differences between studied soils and soil moisture, rate of application. Calcium phosphate minerals didn’t change with changing of soils depth.
 

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