Large-scale assessment of soil exchangeable magnesium and crop response to Mg fertilization in Fujian Province, a subtropical region of China

Soil exchangeable magnesium (ex-Mg) deficiency is increasingly recognized as a critical limitation for crop production in subtropical regions. This study focused on Fujian Province, a representative subtropical region in China, to identify the key determinants of soil ex-Mg using a large-scale survey of 6833 soil samples. Field experiments were also conducted to evaluate the effects of Mg fertilizer application on eight crops, including cauliflower, sweet potato, peanut, soybean, citrus, tea, potato, and sweet corn. Results showed that the average soil ex-Mg concentration across Fujian was 83.8 mg·kg–1, with 81.5% of samples below the established sufficiency threshold of 120 mg·kg–1. A clear spatial pattern was observed, with higher ex-Mg levels in downstream plains and substantially lower concentrations in upstream mountainous areas. Random forest analysis revealed that soil exchangeable Ca, precipitation, and soil pH were the primary factors influencing soil ex-Mg concentration. Field experiments demonstrated that supplementary Mg fertilization increased crop yields by 5.3%–22.2%, nitrogen uptake efficiency by 5.6%–10.8%, and quality indicators by 8.4%–28.3%. These findings highlight the importance of incorporating Mg into regional nutrient management strategies to mitigate widespread Mg deficiency and enhance sustainable crop production in subtropical regions.