Spatial and temporal variability in soil pH of Shaanxi Province over the last 40 years

Rapid development of the economy has increased the occurrence of declining cultivated land quality, such as acidification and salinization in China. To provide a scientific basis for adjustment of topsoil pH and to realize the rational use of land resources in Shaanxi Province, this study examined pH of 705 topsoil samples from Shaanxi Province using a soil/water ratio of 2.5/1 in 2017 and incorporated soil pH data from the second national soil survey in 1980s. The spatio-temporal changes and classification characteristics were analyzed via ArcGIS, and the influencing factors were investigated by correlation analysis and analysis of variance methods. General statistical analysis and normality tests were performed in Excel 2016 and SPSS 22.0; and the GS+9.0 software was adopted to obtain the best fitting model. The ordinary Kriging method was used for spatial interpolation analysis and mapping. The study is important for sustainable agriculture development and ecological environment protection in Shaanxi Province. The results showed that pH values of farmland soil in northern Shaanxi, Guanzhong, and southern Shaanxi were 8.25, 7.91, and 6.25, respectively, and the corresponding levels were 5 (alkalescence), 5 (alkalescence), and 3 (weak acidity), respectively. The order of soil pH in the administrative regions was Yan'an > Yulin > Tongchua > Xianyang > Weinan > Xi'an > Baoji > Shangluo > Ankang > Hanzhong. The topsoil pH in Hanzhong City showed moderate variation, whereas the other cities in Shaanxi Province showed weak variation. Compared with the 1980s, the farmland soil in northern Shaanxi and Guanzhong showed an alkalization trend, whereas the surface soil in southern Shaanxi showed an acidification trend. In the administrative regions, the topsoil in Yulin, Ankang, Hanzhong, and Shangluo showed an acidification trend, and the soil in other cities showed an alkalization trend. The optimal fitting semi-variance function model of farmland soil pH in northern and southern Shaanxi was a linear model, and the optimal fitting semi-variance function model of farmland soil pH in Guanzhong was a Gaussian model; both showed strong spatial correlation. Furthermore, the topsoil pH in northern Shaanxi was primarily influenced by structural factors, whereas the topsoil pH in Guanzhong and southern Shaanxi was influenced by structural and random factors. The spatial distribution characteristics of topsoil pH were sporadic in northern Shaanxi, and that in Guanzhong and southern Shaanxi was higher in the east than in the west. Changes in soil pH were affected by natural and human factors, such as topography, soil type, climate, and fertilization. Slope and elevation were significantly (P < 0.05) correlated with topsoil pH in Shaanxi Province; lower slopes and higher altitudes had higher soil pH. To promote sustainable agriculture development and regional food security, soil salinization should be prevented in northern Shaanxi and Guanzhong and acidification should be prevented in southern Shaanxi.