Effects of Soil Amendment Application on Soil Quality in Saline–Alkali Land: A Meta-Analysis

Soil salinization and alkalization are major forms of land degradation, and soil amendments are commonly used to improve the quality of saline–alkali soils. This study aimed to quantify the effects of different amendments on soil physicochemical properties in saline–alkali soils in China, compare single and mixed applications, and identify environmental and management factors associated with variations in their effects. A systematic literature search and screening process identified 223 eligible datasets from studies conducted in China. Data from amended treatments and corresponding unamended controls were synthesized using meta-analysis. The amendments included organic and inorganic materials, such as humic acid, phosphogypsum, and desulfurization gypsum, applied individually or in mixtures. Compared with unamended controls, amendment application significantly decreased soil pH, salt content (SC), exchangeable sodium percentage (ESP), sodium adsorption ratio (SAR), and bulk density (BD) by 2.8%, 12.4%, 27.2%, 44.4%, and 6.6%, respectively; soil nutrient levels increased significantly: available nitrogen (AN), available phosphorus (AP), available potassium (AK), and soil organic carbon (SOC) increased by 33.9%, 42.3%, 20.7%, and 20.5%, respectively. Mixed applications showed greater improvements in six of the nine evaluated soil properties, whereas single applications produced greater increases in available nitrogen and greater reductions in salt content and SAR. Among amendments applied individually, organic amendments generally showed larger increases in soil nutrient availability and organic carbon, whereas inorganic amendments generally showed larger reductions in ESP and SAR. Additionally, random forest analysis identified precipitation as the most important predictor of salt-content responses under single applications and application amount under mixed applications. Application amount was also the most important predictor of soil organic carbon responses under both application strategies. Overall, farmyard manure is a promising option for increasing nutrient availability and SOC and reducing BD, whereas inorganic–organic treatments merit consideration where excessive exchangeable sodium coincides with poor soil fertility. Amendment selection should therefore be based on specific soil conditions and local environmental characteristics.