PURPOSE: To obtain a sulfur-added seedling-growing culture medium which is prepared by adding sulfur to a mat-form culture medium made of an inorganic substance, pH of the medium is steadily lowered down to a level suitable for growing seedlings of rice plants without using a mineral acid such as sulfuric acid dangerous to handle, thus is safely used without damage to the formed culture medium.

CONSTITUTION: Inorganic and/or organic fibers such as rock wool are formed into a mat as a culture medium after adding sulfur to the formed culture medium, preferably in amount of 10-50g (per 1 unit of the formed culture medium) to give this sulfur-added formed culture medium for growing seedlings. Rice seeds are inoculated into the medium, irrigated, covered with natural soil and moistened at 32°C for 2-3 days or kept at about 25°C for 4-5 days. By these treatments, sulfur-oxidizing bacteria rapidly proliferate to oxidize sulfur into sulfuric acid and the formed culture medium is acidified.