Natural method of nitrogen fixation : When lightening occurs, atmospheric nitrogen and oxygen combine to form NO. The nitric oxide [NO] combines with oxygen to from NO₂. This nitrogen dioxide combines with rain water to form HNO (Hyponitrous acid) and HNO₂ or HNO₂ and HNO₃. These nitrous acids and nitric acid are washed into the soil along with rain water and cause Acid Rain. This acidic rain water combines with other mineral salts to form nitrities and nitrates. The nitrites and nitrates are absorbed by the plants.