Reactions of Ammonium Ortho- and Polyphosphate Fertilizers in Soil: III. Effects of Associated Cations

Diammonium phosphate (DAP), triammonium pyrophosphate monohydrate (TPP), and ammonium polyphosphate (APP) were surface applied on columns of Hartsells fine sandy loam, a Typic Hapludult, and the influence of these three salts on movement, distribution, and precipitation of soil and fertilizer cations was investigated. In general, the NH₄⁺ ions derived from the fertilizer salts moved in association with the phosphate anions in the form of salt diffusion, and also moved by counter diffusion in exchange for some of the exchangeable soil cations. This latter mode of transport resulted in movement of NH₄⁺ a few millimeters beyond the limits reached by phosphates. Exchangeabe Ca in the first 6 mm of soil contacted by the fertilizer solution from DAP was replaced by NH₄⁺, and the displaced Ca²⁺ was precipitated in place by phosphates. Displaced Ca was transported in the fertilizer solution of TPP and APP for 6 to 8 mm before it was precipitated. Reactions of phosphates with displaced Ca²⁺ caused its activity in these soil zones to drop sharply and created a Ca gradient opposite to that of the phosphates. Consequently, Ca moved from soil on the farther side of the phosphate front into a Ca phosphate sink. This movement of Ca was partly in exchange for NH₄⁺ (counter cation diffusion) and partly following hydrolysis from exchange sites. Depletion of exchangeable cations, especially at or just beyond the fertilizer solution front, caused soil pH to drop as much as 1 pH unit below the initial soil value. Within the soil zone affected by phosphates, pH was about 7.5 with DAP and 6.7 to 7.2 with APP and TPP. Differences in pH between soil in the outer zones reached by phosphates and the inner layers ranged from 1.7 to 2.7 pH units. Soil Al was influenced greatly by all three fertilizer solution. With DAP, the reaction products simply reprecipitated in place without undergoing any movement. Increased acid extractability was the only evidence of the presence of fresh precipitates involving Al. In addition to such a reaction, however, movement of Al occurred with TPP and APP, and there was evidence of rather-well-defined zones of precipitation involving Al pyro- or polyphosphates. There was no evidence to implicate Fe in reactions similar to those described for Al.