The availability of ammonium polyphosphate in soil and the impacting factors

Objectives The purpose of this study is to clarify the changes of available P when ammonium polyphosphate is applied into soil and the impacting factors to phosphorus availability, and provide a reference for the reasonable and efficient application of ammonium polyphosphate.

Methods  A cultivation experiment and a pot experiment were carried out using two types of soil in different pH value and maize as test material. The cultivation experiment lasted 331 hours (14 days). At the equal nutrient supply (P₂O₅ 83.8 mg/kg), the cultivation experiment included four treatments: no phosphate application (CK), mono-ammonium phosphate (MAP), ammonium polyphosphate with polymerization degree of 4 (APP-4) and 6 (APP-6). Soil available P was determined at 0, 3, 24, 96, 144, 240 and 331 h after fertilization. At the equal nutrient supply (N 0.400 g/pot, P₂O₅ 0.764 g/pot, K₂O 0.386 g/pot) in the pot experiment, five phosphorous fertilizer treatments were setup: no phosphate application (CK), mono-ammonium phosphate (MAP), superphosphate (SSP), ammonium polyphosphate with polymerization degrees of 4 (APP-4) and 6 (APP-6). The plants were harvested after transplanting 30 days for the determination of dry weight and total phosphorus contents. The soil was sampled for measurement of the pH and Olsen-P content.

Results  Compared with the MAP, APP increased the content of Olsen-P in soil. In acid soil, the phosphate utilization rates in APP-4 and APP-6 treatment were 49.5% and 84.3% higher than that in the SSP. Compared with the SSP, the APP-4 and APP-6 increased the phosphate utilization rate by 307.3% and 316.2% in alkaline soil, the APP increased the total phosphorus content of aboveground part of maize by 7.9%–12.4% and the APP-6 increased the total phosphorus content of underground part of maize by 13.5% in acid soil. In alkaline soil, the total phosphorus content of aboveground and underground part of maize in APP treatment were 175.0%–177.6% and 111.2% higher than those in the SSP treatment. The phosphorus uptake by maize in the APP-4 and APP-6 treatment were increased by 43.3% and 74.0% relative to the SSP treatment in acid soil and increased by 244.6% and 251.7% in alkaline soil. The application of APP increased the dry weight of maize significantly. Compared with the SSP, APP increased the dry weight of aboveground and underground part of maize by 17.2%–51.9% and 13.3%–49.5%.

Conclusions Ammonium polyphosphate can maintain a relatively higher content and longer period of soil available phosphorus compared with mono-ammonium phosphate, so could promote the growth of maize and use efficiency of phosphorous significantly. The degree of polymerization and soil pH affect the availability of ammonium polyphosphate greatly. Polymerization degree of 6 is better than that of 4, and the availability of ammonium polyphosphate performs better in alkaline soil than that in acid soil.