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Agronomic Effectiveness of Pyrophosphate as an Additive to Monocalcium Phosphate and Diammonium Phosphate in Calcareous Soils¹

ABSTRACT

Recovery of phosphorus (P) from monocalcium phosphate (MCP) and from diammonium phosphate (DAP), in the presence and the absence of sodium pyrophosphate (NaPP), was determined in the greenhouse on two calcareous soils which had been cropped successively with wheat (Triticum aestivum L.), corn (Zea mays L.), and clover (Trifolium alexandrinum L.). Addition of NaPP to MCP at a pyro-P/ortho-P ratio of 1:49 or 5:45 markedly increased wheat grain yield on the desert soil (39% CaCO₃), and responses to 50 ppm P with additive were not statistically different from those to 100 ppm P without additive. Sodium pyrophosphate tended to increase P recovery from MCP by the wheat and by the following corn on the alluvial soil (4.1% CaCO₃), and only by the wheat on the desert soil. By the end of the cropping experiments total P recovery was considerably lower on the desert than on the alluvial soil, especially with DAP. The highest recoveries were obtained with the additive treatments. At the rate of 50 ppm P, total recovery from MCP increased with 1 ppm additive from 48.1 to 55.4% on the desert soil and from 57.5 to 71.6% on the alluvial soil. With DAP, the beneficial effect of the additive was not significant at the 0.05 level. In soil columns, addition of NaPP enhanced mobility of MCP to a greater extent than that of DAP. The results suggest that DAP may not be recommended for soils high in CaCO₃ content.

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. Center, Canada.

² Professor, Univ. of Alexandria; Senior Soil Specialist, Salinity Lab., Ministry of Agriculture; and Assistant Professor, Univ. of Alexandria, respectively.

Received for publication September 22, 1981. Accepted for publication January 5, 1982.