. Biology; the story of living things. 60 NATURAL HISTORY. fjring' JoactcHee Bacteria also act upon ammonia formed from plant and animal wastes, one kind (Nitrosomonas) producing nitrites, or nitrate salts, and others (Nitrobader) converting the nitrites into the more stable nitrates. Thus all of the compounds of nitrogen are used over and over, first by plants, then as food by animals, eventually returning to the soil, or in part being released as free nitrogen. This process is called the nitrogen cycle. Although free nitrogen is fixed for use by means of electrical discharges dur- ing thunde
. Biology; the story of living things. 60 NATURAL HISTORY. fjring' JoactcHee Bacteria also act upon ammonia formed from plant and animal wastes, one kind (Nitrosomonas) producing nitrites, or nitrate salts, and others (Nitrobader) converting the nitrites into the more stable nitrates. Thus all of the compounds of nitrogen are used over and over, first by plants, then as food by animals, eventually returning to the soil, or in part being released as free nitrogen. This process is called the nitrogen cycle. Although free nitrogen is fixed for use by means of electrical discharges dur- ing thunderstorms, by man-made machines, by ultraviolet light (which is estimated to return 100,000,000 tons a year to the earth's surface), and from other sources, yet these means give an almost negligible amount of usable nitrogen to the soil, compared with what is used in crop produc- tion, especially since so much nitrogen is lost from the soil in various ways. The nitrogen-fixing bacteria supply the deficiency, thus form- ing one of the most important inter-relationships between plants and animals because of their direct relationship to the production of the food of the world. Rotation of Crops Plants that are hosts for the nitrogen-fixing bacteria are raised early in the season, then plowed under and a second crop of a differ- ent kind is planted. The latter grows quickly and luxuriantly be- cause of the nitrates left in the soil by the bacteria which lived with the first crop. For this reason, clover is often grown on land used later for corn, or cowpeas will be followed by a crop of potatoes. On well-managed farms, different crops are planted in succession in a given field in different years so that one crop may replace some of the elements taken from the soil by the previous crop. This is known as rotation of crops.^ ' Crop rotation is not only a process to conserve the fertility of the soil, but also a sanitary meas- ure to prevent infection of the soil. The nitrogen cycle. Wha
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