Hydrolysis rates of inorganic polyphosphates in aqueous solution as well as in soils and effects on P availability

Applications of polyphosphate-based fertilizers have been reported to have a positive impact on crop yields as compared to orthophosphate sources. Since plants take up P mainly as orthophosphate, hydrolysis rates of polyphosphates into orthophosphates will determine their fertilizer ability. Laboratory and soil incubation experiments were performed to evaluate hydrolysis rates of pyrophosphate (PP), tripolyphosphate (TP), and trimetaphosphate (TMP) in water as well as in two soils having different P-fixing capacities. P availability was characterized by measuring the orthophosphate (ortho-P) and polyphosphate (poly-P) concentration in soil solution as well as the calcium-acetate-lactate (CAL)-extractable amounts of both forms. In water, PP was completely hydrolyzed within 15 d, whereas TMP was hydrolyzed only to about 30% after 90 d. In the two soils, polyphosphates hydrolyzed during the incubation period increasing ortho-P concentration in soil solution as well as in CAL extract. At the end of the incubation, no significant differences in ortho-P concentration in soil solution and CAL extract were found in the sandy soil, whereas in the silty-loam soil, polyphosphate applications resulted in higher soil-solution ortho-P concentration. Although polyphosphate hydrolysis is mainly affected by the soil-specific enzymatic activity, it seems that polyphosphates and/or hydrolysis products are preferentially adsorbed/precipitated compared to ortho-P in the silty loam, thereby influencing the P availability from polyphosphate sources.

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