Abstract: A biostimulating substance, useful in agriculture, horticulture and microbial processes, is composed of acetic acid and a fossil humic substance containing solid humins and humic acid. A method for improving the biostimulating properties and stability of dispersions of fossil humic substances containing peat or lignite includes the step of mixing a fossil humic substance with acetic acid in an amount sufficient that the resulting suspension contains at least about 5 to about 25% by weight organic solids. The modified humic substance can be used as a biostimulant for enhancing plant growth and health and for increasing digestion of organic solids by microorganisms in sewage plants and similar facilities.