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Immobilization of Diammonium Phosphate and Monocalcium Phosphate in Calcareous Soils¹

ABSTRACT

Immobilization of monocalcium phosphate (MCP) and diammonium phosphate (DAP) was investigated in an alluvial soil (4.1% CaCO₃) and in a desert soil (39% CaCO₃). Laboratory experiments were performed to study P solubility in the fertilized soils as affected by wetting and drying cycles and by moisture tension. In addition, the relationship between NH₃ volatilization and P solubility was evaluated for DAP, and the agronomic effectiveness of the two fertilizers was tested in a greenhouse experiment. At 4,900 ppm P both fertilizers saturated the alluvial soil during the first 10 cycles with respect to dicalcium phosphate dihydrate (DCPD). The MCP-treated desert soil was DCPD-saturated for the first cycle then suffered a sharp decline in soluble P, but maintained DCPD saturation with the addition of organic matter. DAP, on the other hand, failed to saturate the desert soil with DCPD and a negative relationship was obtained between NH₃ volatilization and P solubility. In soil columns soluble P increased with depth for DAP while for MCP it was dependent on the moisture tension, the lower the tension the higher the solubility. Greenhouse results were in agreement with the laboratory findings. DAP and MCP were equally effective in the alluvial soil but MCP was superior in the highly calcareous desert soil, especially when combined with organic matter application.

¹ Contribution from the Fertilizer Efficiency Project, Soil & Water Science Dep., Univ. of Alexandria, Egypt. The project is supported in part by the Int. Dev. Res. Centre. Presented before Div. S-2, Am. Soc. of Agron., Fort Collins, Colo., August, 1979.

² Professor, former Graduate Students, Univ. of Alexandria, and Senior Soil Specialist, Salinity Lab., Ministry of Agriculture, Alexandria, respectively. The second author is now Assistant Professor, Univ. of Tanta.

Received for publication December 14, 1979. Accepted for publication July 7, 1980.