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.
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