Why Is the Nitrogen Content of Fertilizers So Important?
Crop production per acre in the United States is higher than in many areas of the world. In part, this is the result of extensive use of fertilizers, especially those that supply nitrogen in a form that plants can use readily. Both ammonium and nitrate ions are used; even ammonia gas can be pumped into the ground, if enough water is available in the soil to dissolve it.
Ammonia is toxic to animals, so it often comes as a surprise that ammonia gas itself can be used for fertilization. Plants can assimilate ammonia rapidly, but they usually never get the chance to do so because the nitrifying soil bacteria, especially Nitrosomonas and Nitrobacter, rapidly convert the ammonia firstto nitrite and then to nitrate. The final nitrate product is easily converted back to ammonia, but the process requires energy. Ammonia is especially useful as a fertilizer in the early spring and for germinating plants. In the spring, the soil is usually damp enough to dissolve the ammonia so that it can move to the plants. Because light is less available in the early spring, the young plants do not have enough energy to convert the nitrate back to ammo-nia until their chloroplasts develop fully. Fortunately, because of the condition of the soil, the ammonia goes directly to the plants rather than to the soil bacteria.
The genes for the enzymes for nitrogen fixation have been studied extensively. Much research is going on to determine whether these genes can be incorporated into crop plants, which would reduce the amount of nitrogen fertilizer needed for maxi-mal plant growth and crop production.
Two other sources of nitrogen fixation are often overlooked. The first is the chemical synthesis of ammonia from H2 and N2, called the Haber process, after its discoverer, the German chem-ist Fritz Haber. This reaction is very important for the formation of chemical fertilizers, and it is responsible for a great deal of the organic nitrogen currently found in the biosphere. The second source of fixed nitrogen is that produced by lightning.