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Laboratory and Greenhouse Studies with Monocalcium, Monoammonium, and Diammonium Phosphates¹

ABSTRACT

Pellets of monocalcium phosphate monohydrate (MCP), monoammonium phosphate (MAP), and diammonium phosphate (DP) containing 15 mg. of P were placed on the surface of moist Hartsells fine sandy loam (pH 5.2) and Webster silty clay loam (pH 8.3). After 3 and 5 weeks the distribution and solubility of the soil-fertilizer P reaction products were measured with soil samples taken at various radial distances from the center of the pellets. Plant response to pellets of fertilizer was measured in greenhouse experiments.

With both soils, distribution of total fertilizer P did not change appreciably in the interval between sampling periods. Fertilizer P moved about 2.5 and 1.5 cm. from the center of the pellets with the Hartsells and Webster soils, respectively. Differences among sources in the same soil were relatively small.

With the Hartsells soil, 6.5 and 11% of the fertilizer P was soluble in 1:15 soil-water extracts with the MCP and DP sources, respectively; with the Webster soil the corresponding figures were 5.2 and 1.0%. The measurements of water-soluble P in the laboratory explained 85% of the variation in plant uptake of P with the different fertilizers in greenhouse experiments. In laboratory and greenhouse experiments DP was superior to MCP in the Hartsells soil, while MCP was much superior to DP in the Webster soil. MAP was intermediate in behavior between the 2 other sources with the Hartsells soil, but was roughly equivalent to MCP with the Webster soil.

¹ Contribution from the Soils and Fertilizer Branch, Division of Agricultural Relations, Tennessee Valley Authority, Wilson Dam, Ala.

² Soil Chemist and Analytical Chemist, respectively. The authors wish to express their appreciation to Dr. J. R. Webb, Iowa State College, Ames, for supplying the sample of Webster silty clay loam used in these experiments.

Received for publication December 5, 1958. Accepted for publication January 7, 1959.