The present invention relates to a Method for preparing a concentrated inoculating composition for use with leguminous plants, preferably soybeans, alfalfa and beans, based on a sterilized and cooled general use culture medium. The method comprises the following steps for treating the culture medium in the order given: a) The general use culture medium is sterilized and cooled b) The Rhizobium Japonicum bacteria is added to the culture medium and allowed to multiply for 46 hours at a temperature preferably of 28°C c) The result of the two previous steps is cooled to a temperature of 10° or 11°C characterized by the addition to the general use culture medium treated in the way explained of powdered maltose saccharide and liquid maltose saccharide as well as fungicide called potassium sorbate which were treated in accordance with the following steps: i. Previous dissolving in 100 liters of water ii. Sterilization at 121°C for 30 minutes iii. Cooling to 20°C.
