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Reactions of Ammonium Ortho- and Polyphosphate Fertilizers in Soil: II. Hydrolysis and Reactions with Soil¹

ABSTRACT

Reactions of P from triammonium pyrophosphate (TPP), ammonium polyphosphate (APP), and diammonium phosphate (DAP) were investigated in replicated columns of Hartsells fsl soil (a Typic Hapludult) at a moisture content of 19% that were incubated for 1, 2, or 4 weeks at 25°C. The diffusive movement of P was accompanied by two major reactions: (i) hydrolysis of pyro- and polyphosphates to the orthophosphate form; and (ii) precipitation reaction of all phosphatic anions with soil. Hydrolysis of the water-soluble fraction of the condensed forms of P was rapid, with a half life that ranged from 9 to 16 days. Hydrolysis of the water-insoluble fractions was severely retarded by their insolubility and could not be measured in 4 weeks. Because the rate of hydrolysis was faster than that of diffusion, mobility of the pyro- and polphosphates occurred largely in the orthophophate form and accounted for the similarity of P movement among the three sources. The precipitation reactions of pyro- and polyphosphate were virtually complete in the 1st week, were irreversible and localized within well-defined zones, and accounted for nearly one-fourth of the added P. Precipitation of orthophosphates increased considerably with time and higher concentrations of water-soluble orthophosphate, and decreased by pyro- and polyphosphate competing for the same precipitating agents. Furthermore, orthosphate precipitation was dispersed rather than localized and showed signs of reversibility. The total water-insoluble fractions after 4 weeks accounted for 58, 63, and 45% of added P from TPP, APP, and DAP, respectively.

¹ Contribution from Soils and Fertilizer Research and Fundamental Research Branches, National Fertilizer Development Center, TVA, Muscle Shoals, AL 35660.

² Research Chemists, respectively.

Received for publication June 16, 1978. Accepted for publication September 13, 1978.

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