Objective The spatial-temporal variation characteristics of soil pH and its driving factors were clarified to provide a basis for the improvement of soil acidification in southwest cigar area.

Method Based on soil pH data of 26 sample points of the second general soil survey in Sichuan Province in 1984, 391 sample points of soil testing and fertilizer recommendation survey in Sichuan Province in 2009, and 164 sample points of cultivated land quality survey in Sichuan Province in 2019, Multi-statistics and Geostatistics were used to analyze the spatial-temporal variation characteristics of soil pH, and Random Forest model was used to explore the main driving factors of soil pH change.

Result The mean values of pH in Shifang City in 1984, 2009 and 2019 were 6.40, 6.24 and 5.97, respectively, and the average annual value decreased by 0.01 from 1984 to 2009 and by 0.02 from 2009 to 2019. From 1984 to 2019, except for Jiandi Town and Yinghua Town, where the average soil pH value showed an upward trend, the rest of the towns were on the decline, with the largest decrease of 30.62% in Hefeng Town and the smallest decrease of 0.30% in Shigu Town. The overall soil pH in 1984 was dominated by weakly acidic and neutral properties, with alkaline soils accounting for 2.59% of the total area planted with tobacco, and the spatial distribution was characterized by higher soil pH in three areas: Luoshui Town, Hefeng Town and Majing Town. In 2009, only weakly acidic and neutral soils existed in the study area, and soil pH was spatially high in the centre and low in the north and south. In 2019, the area of weakly acidic and neutral soils accounted for 95.25%, and the area of acidic soils (pH < 5.5) accounted for 4.75%, which were mainly distributed in the towns of Majing, Mazu, Hefeng, Nanquan and Shigu. And soil pH in the south was spatially lower than that in the north. Soils planted with tobacco over the past 35 years have generally shown changes in acidification, with about 53.51% of the soil was acidified (ΔpH < -0.3), mainly distributed in the towns of Majing, Hefeng and Mazu, and about 11.63% of the soil was alkaline (ΔpH > 0.3), mainly in the towns of Yinghua and Jiandi; The pH of 44.98% tobacco growing soils ranged from -1.0 to -0.3, 34.86% ranged from -0.3 to 0.3, and tobacco growing soils with ΔpH ≤ -1.0 accounted for 8.54%. The random forest results showed that mean annual precipitation and soil organic matter were the main factors causing soil acidification.

Conclusion Over the past 35 years, the acidification of tobacco growing soils in Shifang City was aggravated. Some measures should be taken to adjust the situation in production, such as creating new special fertilizer and applying soil conditioner.