Abstract: To determine the appropriate amount of combined application of potassium and magnesium fertilizer in alleviating soil acidification in tea garden and to clarify the effect paths, the acidified tea garden soil (pH 3.43) was treated with combined application of potassium and magnesium fertilizers. Potassium and magnesium fertilizations were set at four levels, resulting in a total of 16 treatments. The tested soil was cultured at constant temperature and humidity for 5, 10, 20, 30, 45, and 60 days. The results showed that the combined application of potassium and magnesium fertilizer of all levels could improve soil pH. When K⁺ was 27.5 mg·kg^-1 and Mg²⁺ was 240 mg·kg^-1 (the treatment with low potassium and high magnesium), soil pH was significantly increased by 0.61 (P<0.05), achieving the strongest effect. Under this treatment, the concentrations of exchangeable acid and NO₃^--N decreased significantly, while the concentrations of exchangeable K⁺, Mg²⁺, Na⁺ and NH₄⁺-N increased significantly. The structural equation model (SEM) analysis showed that exchangeable Mg²⁺ and Na⁺ had significantly positive and direct effects on pH, exchangeable Al³⁺ had extremely significantly negative and direct effects on pH (P<0.01), and exchangeable Mg²⁺ and NH₄⁺-N had extremely significantly positive and indirect effects on pH. Therefore, low potassium and high magnesium could alleviate soil acidification mainly through increasing exchangeable Mg²⁺, exchangeable Na⁺ and NH₄⁺-N content, and decreasing exchangeable Al³⁺ content.

Key words: potassium phosphate, caustic calcined magnesia, tea garden soil, exchangeable cation, ammonium nitrogen, nitrate nitrogen, pH.