Method for the production of organic fertilizer comprising providing an organic source of ammonia gas, in particular air contaminated with ammonia gas, directing the air contaminated with ammonia gas through water and forming an ammonium hydroxide solution converting the ammonium into nitrate, dosing, under the control of a control system, the ammonium hydroxide solution to a recirculation stream from a second bio-reactor with bacteria suitable for converting the supplied ammonia into nitrite and nitrate. The amount of ammonium water fed by the control system to the acidic nitrate solution from the second bio-reactor depends on one or more measured pH values in the device.