EFFECTS OF TEMPERATURE AND PHOSPHATE CONCENTRATION ON RATE OF SODIUM PYROPHOSPHATE AND SODIUM TRIPOLYPHOSPHATE HYDROLYSIS IN SOIL

Abstract

Rates of hydrolysis of water-soluble sodium pyrophosphate and sodium tripolyphosphate, applied to soil at 200 ppm P, were extremely rapid (about 2–7%/h) during the 0 to 24- or 0- to 48-h period following phosphate application but decreased with time of incubation. Rates of hydrolysis increased linearly and increased about two- to threefold as temperatures were increased from 5 to 35 C or from 5 to 50 C. Inconsistent results were obtained at temperatures above 50 C. Increases in incubation temperature increased the total amount of added polyphosphate hydrolyzed by the soil in 120 h from about 40 to 70% at 5 C to about 80 to 95% at 35 C and 50 C. Water-soluble polyphosphate hydrolyzed at a greater rate than 0.5 N H2SO4-soluble polyphosphate. However, the effects of temperature on rate of hydrolysis of acid-soluble polyphosphate and water-soluble polyphosphate were similar. Polyphosphate hydrolyzed at a greater rate in the noncalcareous Newdale soil than in the calcareous Lakeland soil. Rate of tripolyphosphate hydrolysis was usually greater than rate of pyrophosphate hydrolysis. Rate of hydrolysis of water-soluble polyphosphate, expressed as %/h, decreased exponentially as concentration of applied polyphosphate increased from 200 to 1,600 ppm. However, rate of orthophosphate production per unit time increased exponentially with increased substrate concentration.