Effects of Chelating Agent on Dissolution, Fixation and Fertisphere Transformation of Diammonium Phosphate

Promoting the dissolution of phosphate fertilizer and reducing its fixation are important ways to ensure the phosphate supply and to improve the utilization rate of phosphate fertilizer. In this study, the test products of phosphate fertilizer by adding hydroxyethylidene diphosphonic acid (HEDP) and sodium gluconate (SG) into diammonium phosphate were prepared to investigate its dissolution, anti-fixation and fertisphere transformation characteristics, so as to provide a theoretical and technical basis for the efficient utilization of phosphate fertilizer.

HEDP and SG were physically mixed with powdered diammonium phosphate at the addition ratio of 0, 0.5%, 1%, 3% and 5%, respectively, and the extrusion granulation was used to obtain the nine granular diammonium phosphate test products, specifically, common diammonium phosphate (P), HEDP_0.5%+P, HEDP_1%+P, HEDP_3%+P, HEDP_5%+P, SG_0.5%+P, SG_1%+P, SG_3%+P, and SG_5%+P. The dissolution rate and fixation rate of water-soluble phosphorus in different diammonium phosphate particles were analyzed and compared by using water dissolution rate method and CaCl₂ precipitation method, respectively. A soil cultivation was conducted to investigate the transformation characteristics of diammonium phosphate in the fertisphere.

(1) Adding HEDP and SG increased the dissolution rate of diammonium phosphate on average by 27.7% and 20.0%, respectively. When HEDP and SG were added at 5% and 0.5%, the dissolution rates were 5.1%·min^-1 and 4.8%·min^-1, respectively, higher than other additive rate, and also higher than P by 39.2% and 32.6%, respectively. (2) When HEDP and SG were added, the fixation rates of water-soluble phosphorus in diammonium phosphate decreased by 10.3% and 6.6%, respectively. When the additive rate of chelating agent was at 5% and 3%, the fixation rate of water-soluble phosphorus was lower than P by 58.1% and 61.3% for HEDP, 17.0% and 11.8% for SG, respectively. (3) The content of available phosphorus in the fertisphere soil treated with HEDP+P and SG+P significantly increased by 39.5% and 21.2%, respectively, and HEDP+P had a better performance. When the additive rate of HEDP and SG was at 3%, the available phosphorus content in fertisphere soil was 2.9 g·kg^-1 and 2.5 g·kg^-1, respectively, higher than that of P by 53.3% and 31.9%, respectively. (4) The Ca₂-P contents in the fertisphere soil treated with HEDP+P and SG+P were higher than that of P by 38.2% and 43.0%, respectively, and both of HEDP and SG had a better performance when their additive amount at 3%. The Ca₈-P content in the 5 mm fertisphere soil treated with HEDP+P and SG+P was lower than that of P by 33.6% and 14.5%, respectively. When the additive amount of HEDP and SG was at 0.5% and 3%, the Ca₈-P content of fertisphere was 0.9 g·kg^-1 and 1.5 g·kg^-1, respectively, lower than that of P by 53.4% and 25.8%, respectively. (5) The contents of available phosphorus and Ca₂-P in the fertisphere soil were positively correlated with the dissolution rate of phosphorus fertilizer and negatively correlated with the fixation rate of water-soluble phosphorus.

The addition of HEDP and SG could effectively enhance the dissolution of diammonium phosphate and reduce phosphorus fixation. HEDP had a better performance, and especially when its addition amount was at 3%, HEDP could more improve the available phosphorus content and decrease the transformation from Ca₂-P to Ca₈-P in the fertisphere soil.