A composition for promoting the growth of herbaceous plants in a soil and favouring an accumulation of organic matter in said soil comprises a mixture of microorganisms, spores of a mycorrhizal fungus and a source of humic acids, said mixture of microorganisms comprising at least one microorganism belonging to the genus Lactobacillus, at least one microorganism belonging to the genus Bacillus, at least one microorganism belonging to the genus Stenotrophomonas and at least one microorganism belonging to the genus Candida. A method for promoting the growth of herbaceous plants in a soil and favouring an accumulation of organic matter in said soil comprises administering to said plants and/or to said soil a composition； said composition comprises a mixture of microorganisms, spores of a mycorrhizal fungus and a source of humic acids, said mixture of microorganisms comprising at least one microorganism belonging to the genus Lactobacillus, at least one microorganism belonging to the genus Bacillus, at least one microorganism belonging to the genus Stenotrophomonas and at least one microorganism belonging to the genus Candida.
