Animals are wholly dependent upon plants to supply them with nitrogen; animals cannot use the simple nitrogen compounds which plants extract from the air.
The atmosphere does not give up this element lightly. Although above every acre of soil there floats a reserve supply of about 150,000 tons of free nitrogen, this is almost totally inaccessible to plants. Minute amounts are brought down as fixed oxides of nitrogen by powerful lightning flashes. Certain soil-inhabiting bacteria, which
are primitive plants (though lacking in the chlorophyll of their , higher relatives), are able to fix nitrogen from the air. Other bacteria (those that form nodules on roots of legumes like clovers, peas, and beans, to name a few) are also able to convert this element into a form which they use for their own growth, supplying what is left to their host plants.
Except for a mmiscule amount of nitrogen fixed electrically by
man, these limited sources (limited in comparison with the vast
unused store floating above) must satisfy the craving of every living
thing for this vital ingredient of existence. Once captured, it might;
not be held for long, since each time nitrogen is converted from one
form to another, it struggles to escape.
THE NITROGEN CYCLE
An intricate pattern is traced by nitrogen as it is captured, used and released by plants and animals. This is commonly called the nitrogen cycle. At each stage some nitrogen returns to the atmosphere directly because not all of it can be used. Thus, while we call this a nitrogen cycle, a complete recycling through all stages without some return to the atmosphere is never achieved.
Atmospheric nitrogen can enter soil in one of two ways. The first is by direct precipitation from air when the nitrogen is fixed as oxides by electrical discharges during thunder storms. The second is by fixation by specialized soil bacteria or by other forms of bacteria that live on the roots of legumes.
When plant roots absorb nitrogen which was previously fixed by one of these two processes, it is converted into protein by the plant. Any unused portion may return to the atmosphere or be blotted up by avid soil bacteria and fungi which are not specialized and thus cannot fix their own supply. Animals feed on plants, but are continually returning matter (and finally their bodies) to the soil to be reused by other plants. No living thing can escape dependence upon the nitrogen cycle