PROBLEM TO BE SOLVED: To provide an auxiliary material for fermentation of an organic residue fermentation apparatus which can provide an optimum environment for microorganism propagation and to provide high quality culture soil by the fermentation treatment. SOLUTION: An auxiliary material for fermentation of an organic residue fermentation apparatus contains a carbide material 100 parts by weight, an acidic fibrous material 20-200 parts by weight, and a phosphoric acid material 10-100 parts by weight and is so adjusted as to have pH 6-9 and contain phosphoric acid in 2 g or more and potassium in 1 g or more per 1 litter. The culture soil is produced by mixing the product 10-100 parts by weight obtained by fermentation of an alkali material 0.1-50 parts by weight and the above- described auxiliary material an acidic fibrous material 100 parts by weight with an acidic fibrous material 100 parts by weight and is so adjusted as to have pH 5-7 and contain nitrogen, phosphoric acid, and potassium in 0.2-5 g, 0.5-5 g, and 0.2-2 g, respectively, in 1 litter.