A method for converting organic waste materials into inert humus-like materials by charring the organic waste materials by heating and drying same in the presence of certain water-soluble inorganic acids, removing residual acid and other water-soluble contaminants by washing the crude char product with water, and utilizing the resulting inert material as a landfill. If desired, the humus-like char products can be reacted with an alkali at elevated temperatures, and the resulting alkali salts of humic-acid-like materials can be mixed with the soil to provide an improved and/or fertilized topsoil.