To provide a method for effectively utilizing the residual liquid obtained after separation of clean water of a digestive juice recovered when organic wastes are treated by a methane fermentation process.

The method for manufacturing the fertilizer response accelerator comprising separating NH3 and CO2 from the digestive juice after methane fermentation of the organic wastes, then making the residual liquid acidic after separation of the clean water by using a reverse osmosis membrane and capturing the deposited organic matter. The fertilizer response accelerator obtained by this manufacturing method and the fertilizer containing the fertilizer response accelerator. The method for cultivating plants comprising cultivating the plants by using the fertilizer response accelerator or the fertilizer.