Polymerization degree and rate of polyphosphate fertilizer affected the availability of phosphorus, Fe, Mn and Zn in calcareous soil

Objectives  Polymerization degree and polymerization rate of polyphosphate fertilizer are critical parameters affecting the availability of phosphorus in soil. The paper studied the effects of polymerization degree and rate of polyphosphate fertilizer on phosphorus availability.

Methods  A pot experiment was carried out using cultivated gray desert soil (Calcaric fluvisals) and maize as test material. At the equal nutrient supply (N 2.4 g/pot, P₂O₅ 1.1 g/pot, K₂O 0.7 g/pot), five phosphorous fertilizer treatments were setup, ie: no P application (CK), mono-ammonium phosphate (MAP), polyphosphate with polymerization degree and rate of 3, 90% (APP/90%); 2.7, 90% (APP-2.7-90%) and 3, 40% (APP-3-40%). The soil weight in each pot was 7.0 kg. The soils were sampled every ten days after sowing for measurement of the water soluble-P and Olsen-P contents. The contents of total-P, micronutrients (Fe, Mn and Zn) and phosphorus fractionation (Guppy method) on the 90th day soil samples were determined. The mazes were harvested after sowing 45 and 90 d for the determination of dry weight, the P, Fe, Mn and Zn contents.

Results  Compared with the MAP, all the three polyphosphate treatments significantly increased the soil available P contents. At the polymerization degrees of 3, the water-soluble P and available P contents in APP-3-90% treatment were 15.7% and 7.9% higher than those in the APP-3-40%, and the soil Resin-P and NaHCO₃-P were increased by 38.0% and by 22.8% relative to the APP-3-40% treatment, respectively, while soil HCl-P decreased by 6.2%. At the same polymerization rate of 90%, the soil available P, Resin-P and NaHCO₃-P in the APP-3-90% treatment were 5.0%, 75.1% and 34.2% higher than those in the APP-2.7-90% treatment, respectively, but soil HCl-P content was 12.0% lower. At the same polymerization rate of 90%, maize dry weight and P uptake in the APP-3-90% were 14.3% and 4.5% higher than those in the APP-2.7-90%, respectively. All polyphosphate treatments significantly increased the contents of available Fe, Mn and Zn in soils regardless of polymerization degree or polymerization rate. In the 90% polymerization rate, the soil available Fe, Mn and Zn in the APP-3-90% were 5.7%, 8.4% and 29.9% higher than those in the APP-2.7-90%, respectively. Similarly, in the polymerization degree of 3, the contents of soil available Fe and Zn in APP1 were increased by 3.0% and 29.0% relative to those in the APP-3-40%, the uptakes of Fe and Zn by maize in the APP-3-40% were 5.7% and 19.5% higher than those in the APP-2.7-90%, but no significant differences between APP-3-90% and APP-3-40%.

Conclusions  Compared with regular ortho-phosphate fertilizer, polyphosphate fertilizers significantly improved the availability of P and Fe, Mn, Zn in soils, thereby reducing fixation of P in calcareous soil. Specially, polymerization degree of polyphosphate fertilizer demonstrates more important role in increasing soil P and Fe, Mn and Zn availability than polymerization rate.