Soil conditioner and method of producing the same

To effectively utilize a high-molecular weight material containing acrylonitrile-styrene or acrylonitrile-styrene-conjugated diene or one of their used materials as a higher additional value substance. The objective method of producing soil conditioner comprises and acid treatment step where a plolymer material including acrylonitrile styrene and/or a conjugated diene as the constitution units is treated with an acid and a salt formation step where the acid groups formed in the polymer material during the acid treatment and the excessively remaining acid used in the acid treatment are converted into a corresponding salt.