The application of a diammonium phosphate coating effectively mitigates direct contact between the phosphate fertilizer and the soil, thus minimizing phosphorus fixation. Humic acid holds a pivotal role in augmenting soil quality and activating the soil’s phosphorus reserves. Notably, when combined with humic acid, diammonium phosphate significantly enhances the utilization efficiency of phosphate fertilizer. However, there is a paucity of literature exploring the dynamics of nutrient transport in soil when humic acid is paired with coated phosphate fertilizers. To assess the impact of the combined application of coated diammonium phosphate and humic acid on wheat yield enhancement, we conducted pot experiments along with leaching and ammonia volatilization simulation tests, aiming to elucidate the effects of this combination on nutrient transport. The study explored the effects of three distinct treatments: coated diammonium phosphate (CP), coated diammonium phosphate combined with humic acid (PHA), and coated diammonium phosphate combined with humic acid (CPHA). The investigation focused on analyzing their impacts on wheat yield, ammonia volatilization, soil-available phosphorus, nitrate nitrogen, ammonium nitrogen, soil-available potassium, as well as the mobilization and transport of calcium and magnesium in the soil. (1) Compared to the P treatment, the PHA and CP treatments significantly increased grain yield by 17.2% and 13.5%, respectively. The PHA treatment also increased effective panicle number by 12.9%. Overall, the CP, PHA, and CPHA treatments improved grain yield by 13.5%, 17.2%, and 19.1% compared to the P treatment. (2) The CP and PHA treatments reduced available phosphorus by 95.6% and 49.2%, calcium by 2.0% and 67.0%, and magnesium by 11.6% and 46.1% compared to the P treatment. Ammonium nitrogen decreased by 37.0% and 64.3%, while nitrate nitrogen increased by 14.0% in CP and slightly decreased by 0.8% in PHA. In the leaching solution, PHA and CP treatments reduced available phosphorus by 96.7% and 62.5%, increased calcium by 5.0% and 78.9%, decreased ammonium nitrogen by 2.2% and 43.4%, and decreased nitrate nitrogen by 10.6% and 13.0%. The PHA and CPHA treatments increased available phosphorus in the 0–20 cm soil layer by 1.4 times and 25.8%, respectively. (3) The CP treatment reduced ammonia volatilization by 87.0% compared to the P treatment, while the CPHA treatment further reduced it by 87.5% compared to the PHA treatment. The application of coated diammonium phosphate efficiently delays nutrient release and reduces nutrient leaching in the soil. Additionally, the integration of humic acid significantly improves the availability of phosphorus in the soil, minimizing phosphorus loss. Notably, the combined application of humic acid and coated diammonium phosphate leads to a significant increase in soil phosphorus content, subsequently enhancing soil nutrient availability, conserving fertilizer, and ultimately resulting in an improved wheat yield.
