Effects of combining zinc and phosphate fertilizers using different methods on the availability of zinc and phosphorus in soil

Objectives We explored Zn and P availability in the soil amended with phosphate fertilizers resulting from Zn's physical and chemical combination. The aim was to choose a suitable Zn and phosphate combination method for high-efficiency utilization of the nutrients.

Methods Zn sulfate heptahydrate of 0.5 and 5 parts by weight were combined with 99.5 and 95 parts by weight of K₂HPO₄ through the physical mixture (P+Zn) and chemical reaction (PZn) methods, respectively. Further, four Zn-containing phosphate fertilizers were prepared, P+Zn0.5, P+Zn5, PZn0.5, and PZn5. The composite structure of Zn and P were observed using X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonance spectroscopy (NMR). The change in the soil available Zn and P content was studied using a soil culture experiment. The alkaline phosphatase activity and pH of the fluvo-aquic soil were measured simultaneously.

Results 1) The combination of Zn and P increased the available Zn content in the soil by 2.90%, 12.17%, 24.64% and 10.86% for P+Zn0.5, P+Zn5, PZn0.5, and PZn5, respectively. PZn0.5 and PZn5 increased soil available Zn content by 21.13% and 7.37% than P+Zn0.5 and P+Zn5. In addition, PZn0.5 reduced Zn fixation rate by 10.49 percentage points than PZn5. 2) Compared with K₂HPO₄ control, the combination of Zn and P (P < 0.05) increased soil available P content. At 60 days of cultivation, the soil available P content of PZn0.5 and PZn5 were (P < 0.05) higher than P+Zn0.5 and P+Zn5 by 5.76% and 5.70%, respectively. P+Zn0.5, PZn0.5, and PZn5 reduced soil P fixation rates by 1.45, 1.19, and 0.25 percentage points than K₂HPO₄ control. 3) Compared with K₂HPO₄ control, P+Zn0.5 and P+Zn5 increased the soil alkaline phosphatase activity at the beginning of cultivation and the effect of PZn0.5 was better than PZn5. 4) PZn0.5 and PZn5 reduced soil pH at the beginning of cultivation. The reduction was more prominent in PZn0.5 than PZn5.

Conclusions The combination of Zn and phosphate fertilizer by the physical mixing and chemical reaction processes could reduce Zn fixation in the soil, and the products obtained from the chemical reaction method performed better. The chemical reaction between Zn and phosphate fertilizer can also reduce P fixation in the soil, similar to soil pH reduction. Adding 0.5% zinc sulfate heptahydrate to phosphate fertilizer through a chemical reaction is recommended.