Soil acidity is a very serious problem that often occurs on reclaimed land after coal mining. The source of soil acidity in post-coal mining land is caused by acid mine water which contains the elements Al, Fe and SO₄ so that it can reduce soil pH. This research was conducted on forest land and post-coal mining reclaimed land at the PT. Arutmin Satui South Kalimantan site. The method used in this research is exploratory descriptive with a survey approach, while the method used to analyze the data is linear regression test analysis. The parameters analyzed in this research are pH, aluminum exchangeability, dissolved iron and dissolved sulfate. The research results show that there is a significant influence of several sources of soil acidity such as aluminum, iron and sulfate on soil pH on forest land and post-coal mining land. The parameter that is thought to be the source of soil acidity on forest land is soluble sulfate, while on post-coal mining reclamation land, it is exchangeable aluminum and soluble sulfate. In comparison between forest land and post-coal mining reclaimed land, there are significant differences in the parameters analyzed, namely pH, exchangeable aluminum and dissolved iron, while dissolved sulfate parameters do not have significant differences in forest land and post-coal mining reclaimed land