Air is 78 percent nitrogen and 21 percent oxygen. Plants cannot use nitrogen in the air as a direct source of nutrient. All forms of nitrogen must be converted into ammonium (NH4) or nitrate (NO3) in order to be used by plants. These two forms of nitrogen are found naturally in the soil.
Conversion of Ammonium Nitrite
Ammonium molecules are attracted to particles of clay and organic matter. Nitrate moves with the water in soil. It rises in evaporation. It falls in rain. In the soil, it absorbed by plant roots as a necessary nutrient.
To be useful in fertilizer, ammonium has to be converted to nitrite (NO2) and then to nitrate, the form useful to plants. This is called nitrification, a process that occurs naturally in the soil. Nitrite is poisonous to plants, but the conversion takes place rapidly so plants are not harmed.
A catalyst combined with steam and methane gas (CH4) yields anhydrous (without water) ammonium nitrate that is 82 percent nitrogen by weight. This is a gas that will convert to a liquid under pressure. Anhydrous ammonia must be stored in special tanks and is dangerous to handle. It is the basis of all forms of inorganic commercial nitrogen fertilizer. The other forms, while more expensive, are easier to handle, store and apply.
Making Ammonium Nitrate
When anhydrous ammonia is combined with nitric acid, the result is ammonium-nitrate (NH4-NO3), a dry nitrogen fertilizer that is 34 percent nitrogen by weight. The ammonium and the nitrate separate in water. The ammonium binds with soil particles. The nitrate remains in the soil water, where it can be used by plants.
Anhydrous ammonia (NH3) combined with carbon dioxide (CO2) produces urea. Dry pellets of urea contain 46 percent of nitrogen. They are widely available and are easy to store and handle.
Making Urea-Ammonium Nitrate
Urea and ammonium nitrate dissolved in water produces urea-ammonium nitrate, a watery solution that contains 28 percent nitrogen. Urea needs water in order for a plant to convert it into a useful ammonium nitrate so is best applied when the soil is moist, there is rain, or where the plants are irrigated.
The Haber Process
The Haber process, originally discovered by a German, Fritz Haber, uses high voltage electricity to heat the air to 2,000 degrees Fahrenheit. At this temperature nitrogen and oxygen combine to form nitric oxides. Nitric oxides are processed to form nitric acid, which is further processed to make nitrates useful to fertilize plants. The current drawback to this method is the expense of the electricity needed.