Comparative Effects of Biochar and Humic Acid on the Soil–Wheat System in Mildly Saline Soils

Soil salinization is a major constraint on global crop production. While organic amendments are used in mildly saline soils, their seasonal effects require further study. This research applied biochar (BC) and humic acid (HA) annually in 2022 and 2023 separately, with an unamended control (CK), to assess impacts on soil quality and wheat yield. BC significantly reduced soil salt content by 28.1% and 17.5% in 2022 and 2023 at a rate of 7.5 Mg·ha−1, while increasing organic matter and total phosphorus. In contrast, HA lowered soil pH by 3.1% in 2022 and enhanced available nitrogen, potassium, and phosphorus. Both BC and HA increased alkaline phosphatase activity by 7.6% and 6.9% in 2023, respectively. Notably, grain yield showed no direct link to soil nutrients but was positively correlated with phosphatase activity in 2023. Consequently, BC did not improve the soil quality index but raised grain yield by 12.9% and 20.7% over two years, primarily via increased 1000-grain weight. In contrast, HA both improved the soil quality index by 16.1% in 2023 and increased grain yield by 17.2%, driven by enhanced aboveground biomass. In conclusion, soil chemical properties and crop productivity were decoupled in these mildly saline–alkaline soils, highlighting the potential for site-specific application of organic amendments.