Abstract: Organic fertilizers having a superior fertilizer response (or efficiency), and a higher nitrogen content but a lower water-insoluble type nitrogen content are prepared from spent liquors of sulfite pulp plants according to the method of the present invention. Said method comprises setting the solid matter concentration of said spent liquors in the reaction liquid consisting mainly of said spent liquors to 20-45% by weight and reacting therewith ammonia in an amount of 15-30 mol per 1 Kg of said solid matter, and oxygen or oxygen-containing gas at a reaction temperature of 90.degree.-120.degree. C. under a pressure of 4 Kg/cm.sup.2 or higher and lower than 10 Kg/cm.sup.2.