A new method is used to determine the nitrate-nitrogen content in organic fertilizers which contain large amounts of ammonia, colored organic matter, and colloidal substances that can not be removed by usual methods.In this method the nitrate is reduced into nitric oxide by ferrous chloride and thus removed from the impurities, and then converted back into pure nitric acid by air and hydrogen peroxide. The nitric acid formed is determined by the usual phenoldisulfonic acid method.