A Subsoiling–Aluminum Sulfate Spraying System for Whole–Plow–Layer Improvement of Soda Saline–Alkali Land

Soil degradation caused by soil salinization is an important factor affecting global food security. The soda saline–alkali soils in the Songnen Plain suffer from poor soil structure, hard topsoil and alkalized horizons, and high pH and Na+ concentrations, which pose a major threat to sustainable agricultural development. However, the development of demand–adapted agricultural machinery and agronomic technologies integrating physical and chemical amelioration remains a challenge for soda saline–alkali soil improvement under rain–fed dryland conditions. Based on the water solubility of aluminum sulfate, this study integrated novel equipment and technologies for efficient, engineering–oriented amelioration of soda saline–alkali soils. Laboratory incubation experiments with aluminum sulfate showed that an application rate of 0.6% soil dry weight was a suitable addition level for ameliorating soil pH, SAR, and ESP. Verification tests demonstrated that the soil disturbance coefficient, soil loosening degree, and coefficient of variation for liquid application uniformity among rows were 53.75%, 13.50%, and 4.83%, respectively; these operational performance indicators met the requirements of relevant industry standards. Comparisons of soil physicochemical properties under different amelioration treatments revealed that subsoiling combined with ameliorant spraying (SAS) outperformed subsoiling alone (S), surface ameliorant application (A), and the control (CK). Relative to CK, the SAS treatment increased soil water infiltration rate (WIR) by an average of 63.11%, reduced bulk density (BD) by 0.14 g cm−3, decreased pH by 0.86 units, lowered soil salt content (SC) by 0.71 g kg−1, decreased the exchangeable sodium percentage (ESP) by 7.61%, and decreased the sodium adsorption ratio (SAR) by 4.01 (mmolc L−1)1/2. Field trials on alfalfa cultivation demonstrated that SAS improved the alfalfa seedling emergence rate, dry matter yield and overwintering rate, with increases of 20.59%, 26.06%, and 20.29%, respectively, relative to CK. This study provides equipment and technical references for the comprehensive improvement and utilization of soda saline–alkali land.