Enhancing Phosphorus Efficiency in Calcareous Soils: Role of Polyphosphate Properties in Nutrient Solubility, Maize Nutrient Uptake, and Growth

Polyphosphate, which can improve crop yield and phosphorus use efficiency in calcareous soils, has gained increasing global attention. However, the effects of polyphosphate properties (pH and polymerization degree) on agronomic effects are still unclear. In this study, the nutrient solubility, nutrient release, nutrient uptake, and maize growth in calcareous soil of alkaline diammonium phosphate (DAP), acidic ammonium polyphosphate (APP) with a polymerization degree of 1.8, and alkaline potassium tripolyphosphate (KTPP) with a higher polymerization degree of 2.8 were evaluated. The results showed that KTPP at pH 8.0 could significantly increase the solubility of Ca²⁺ by 24.3 times and 3.1 times than those by DAP and APP, respectively, and significantly increase the solubility of Mg²⁺ by 172.8 times and 1.6 times than those by DAP and APP, respectively. The nutrient release experiment indicated 100% P was released after 15 d for the three P fertilizers, and the cumulative released amounts of soluble Ca and Mg with KTPP were about 2.9 and 3.2 times higher than those of DAP, respectively. The result of the pot experiment demonstrated that KTPP significantly enhanced shoot biomass by 24.1% compared with that by DAP. The P, Ca, and Mg uptake of maize shoots with KTPP significantly improved by 49.7%, 20.9%, and 20.9%, respectively, compared to that of DAP. However, no significant differences in yield or nutrient uptake between the KTPP and APP treatments were observed. This study demonstrated that polyphosphate with a higher polymerization degree had a better effect on increasing the solubility of cations, crop yield, and nutrient uptake, and polyphosphate application provides a synergistic interaction between P and Ca in calcareous soil, which may promote the application of polyphosphate fertilizers to expand to large-scale agriculture.