Short of burning organic molecules in fluorine, the greatest amount of energy can be obtained from them by burning them in oxygen. In this combustion process, rapid in fire and slow and controlled in biological metabolism, oxygen with ON = 0 is reduced to and , which have an oxygen ON of -2, while hydrogen and carbon in the fuel molecules are oxidized from effective ON = 0 to ON = +1 and +4, respectively. Denitrifying bacteria (not the same organisms as nitrogen-fixing bacteria) can oxidize their foods with nitrate instead of . If oxygen is scarce in soils, these bacteria can reduce nitrate to , thereby reducing nitrogen from ON = +5 to 0. They only obtain 90% as much energy in this process because nitrate is not quite as good an oxidizing agent as is gas. The other 10% is not lost, however, for every scrap of energy is used in the interlocking network of life on this planet. After the nitrogen-fixing bacteria have reduced to , a third class of bacteria, the nitrifying bacteria, can use the fixed nitrogen of ammonia or amines as foods, thereby oxidizing them back to nitrates with . With nitrates restored again, the net result of the activities of all three kinds of bacteria is the oxidation of the denitrifying bacteria's foods with .

All of these relationships are summarized in the nitrogen cycle, diagramed on the following page. The three-sided loop (a) represents the oxidation-reduction round-robin we have just considered, loop (b) represents the exchange of nitrogen at ON = -3 during growth and decay, and (c) represents the replenishment of the ON = -3 nitrogen by plants. We do not depend on nitrogen reactions for energy sources, nor do any of the higher plants or animals. From a purely human viewpoint, it might seem that loop (b) was sufficient, and that the other steps in the nitrogen cycle were wasted effort. But this is not true. Plants can use either ammonia or nitrate ion as a nitrogen source for protein synthesis, but ammonia has disadvantages. In the form of ammonium ions, as found in the soil, it is a cation very much like and . It is trapped easily between the silicate layers of clay minerals, and does not migrate rapidly toward the roots of the plants it could nourish. The negatively charged nitrate ions travel more freely through the soil. In this respect nitrates are better fertilizers than liquid ammonia. Nitrifying bacteria therefore help by converting ammonia into the more easily circulated nitrate ions.