Method for the production of organic fertilizer or organic raw material for, inter alia, the manufacture of yeast, based on ammonium and/or nitrate, said organic fertilizer or organic raw material having a nitrate concentration between 2 and 25% by weight, comprising the steps of: - providing an organic source of ammonia gas, in particular air contaminated with ammonia gas; - dissolving the ammonia gas in water to form an NH 3 solution in water ; - oxidize the ammonium hydroxide in the NH 3 solution in water with air to nitrite by means of bacteria, and then oxidize the nitrite with air to nitrate by means of bacteria to form an acidic nitrate solution , characterized, in that the method comprises the steps: producing an acidic nitrate solution in a first bio-reactor by means of bacteria; discharging the acidic nitrate solution from the first bio-reactor to a second bio-reactor for a further conversion of nitrite to nitrate by means of bacteria; maintaining an O 2 saturation of at least 80% at the prevailing temperature in the first bio-reactor and the second bio-reactor ; recirculating the content from the second bio-reactor to the first bio-reactor as an acidic nitrate solution recirculation stream ; wherein an NH 3 solution in water is dosed into the acidic nitrate solution recirculation stream to form a feed stream having a pH greater than 6, said feed stream being supplied to the first bio-reactor , wherein the amount of NH3 solution in water supplied to the acidic nitrate solution recirculation stream is controlled by a control system dependent on the measured pH value of the acidic nitrate solution recycle stream and/or the measured pH value in the feed stream . disconnecting the first bio-reactor and the second bio-reactor ; and discharging the nitrate solution of the second bio-reactor . - providing an organic source of ammonia gas, in particular air contaminated with ammonia gas; - dissolving the ammonia gas in water to form an NH 3 solution in water ; - oxidize the ammonium hydroxide in the NH 3 solution in water with air to nitrite by means of bacteria, and then oxidize the nitrite with air to nitrate by means of bacteria to form an acidic nitrate solution , producing an acidic nitrate solution in a first bio-reactor by means of bacteria; discharging the acidic nitrate solution from the first bio-reactor to a second bio-reactor for a further conversion of nitrite to nitrate by means of bacteria; maintaining an O 2 saturation of at least 80% at the prevailing temperature in the first bio-reactor and the second bio-reactor ; recirculating the content from the second bio-reactor to the first bio-reactor as an acidic nitrate solution recirculation stream ; wherein an NH 3 solution in water is dosed into the acidic nitrate solution recirculation stream to form a feed stream having a pH greater than 6, said feed stream being supplied to the first bio-reactor , wherein the amount of NH3 solution in water supplied to the acidic nitrate solution recirculation stream is controlled by a control system dependent on the measured pH value of the acidic nitrate solution recycle stream and/or the measured pH value in the feed stream . disconnecting the first bio-reactor and the second bio-reactor ; and discharging the nitrate solution of the second bio-reactor . <EOS>