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

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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.

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