From Fertilizer to Water: A Decadal Shift in Soil Quality Drivers in Salt‐Affected Coastal Deltas

ABSTRACT Salinization in coastal deltas threatens food security for nearly 10% of the global population. Accurate assessment of its soil quality, coupled with characterization of long‐term spatiotemporal dynamics and driving mechanisms, provides the scientific basis for developing soil amelioration strategies and promoting agricultural productivity. However, conventional evaluation frameworks often overlook the unique pedological characteristics of coastal saline‐alkaline environments. This study focused on the Yellow River Delta's coastal saline‐alkali farmlands, integrating 8856 multi‐source data points—including soil, groundwater, climate, and management variables—collected during the 2010s and 2020s across 8000 km of field surveys. We developed a scalable, region‐specific framework for coastal delta soil quality assessment, integrating a Principal Component Analysis (PCA)‐based Minimum Data Set (MDS), a fuzzy logic soil quality index (SQI), and an interpretable machine learning model using XGBoost with SHapley values. Results indicate SQI values increased from 0.19–0.80 in the 2010s to 0.27–0.81 in the 2020s. Fluctuations in groundwater depth (40.8%), changes in nitrogen fertilizer input (30.7%), distance to the Yellow River (12.6%), and variations in the Standardized Precipitation Evapotranspiration Index (11.2%) were identified as the primary drivers of SQI improvement. A temporal shift was observed from a fertilizer‐dominated (2010s) to a comprehensive water‐regulated (2020s) regime driven by groundwater, irrigation, and precipitation dynamics, underscoring the growing influence of integrated water management. These findings highlight the critical role of integrated water‐fertilizer management and call for targeted strategies, including enhanced groundwater monitoring and precision irrigation. Our framework offers a scalable approach for tracking and improving soil quality in salt‐affected coastal agroecosystems worldwide.