[Objective] Investigation of the possible effects of salinization on soil acidity could provide some evidence for understanding the acidification process of protected cultivation soil.[Methods] Three types of surface soils and soil profiles with different acidification characteristics were collected from the protected cultivation farmland

open cultivation farmland and natural acid land. The soils were used for studying the differences in relations among active acid

potential acid and base saturation

and the effects of salt accumulation on soil acidity by means of chemical analysis and simulation tests of fertilizer salt addition and salt leaching.[Results] There was some difference in acidity between the protected soils and natural acidic soils. The acid in the protected cultivation farmland was human input

which was different from natural acid soils. Soil acidification of the protected soils mainly occurred in the surface layer

and decreased from the top to the bottom. Under the condition of the same exchangeable acid content

the pH value of the soils from the protected cultivation farmland was the lowest

followed by open cultivation farmland

while the pH value of natural acid soils was relatively high. At the same soil pH values

base saturation of natural acidic soils was significantly lower than those of the protected cultivation farmland and the open cultivation farmland

and base saturation of soils from the protected cultivation farmland was higher than that of open cultivation farmland. The proportion of active acid in the exchangeable acid of the protected cultivation soils was higher than that of the natural acid soil. Increasing the accumulation of soil salt can decrease soil pH value. Salt leaching process of the protected cultivation soils reduced soil salinity

and it was also accompanied by a decrease in soil active acid and an increase in soil pH value.[Conclusion] The study suggested that the accumulation of salts in soil can enhance the transformation of potential acid to active acid. High amount of application of chemical fertilizer not only can promote the decrease of pH value of soil directly through the input of acidic material

but also can further reduce the pH value of the soil to a certain extent as a result of salt accumulation.