The manual presents general information on the chemistry, pedochemistry, and geochemistry of aluminum, considers the main groups of aluminum compounds in the soil solid phase and the soil solution, as well as processes with the participation of mobile aluminum in the soils of humid extratropical regions. Special attention is given to the toxic properties of aluminum compounds, as well as toxic effects and parameters for the estimation of the toxicity of aluminum compounds. Methods for the determination of toxic aluminum compounds in soil solutions and surface waters. The manual can be used for the courses of lectures in Soil Science, Soil Chemistry, and Ecology.