734 MAN'S PHYSICAL WELFARE the Chilean nitrate beds and the various processes for the chemical fixation of nitrogen, have already been discussed in Unit IX, Section 5. One of the latest developments in the addition of nitrogen to the soil is the introduction of small quantities of ammonia to irrigation water, from cylinders of liquid ammonia. Cheap sources of nitrogen are now available. It remains for the farmers of the United States to learn to use nitrogen-containing fertilizers. Why do plants require such relatively large amounts of nitrogen? The answer is that nitrogen compounds furnish the nitrogen for the proteins which plants manufacture. These proteins are the basic material from which the protoplasm of every living cell is made. Fig. 312. Soybean root showing nodules which harbor nitrogen-fixing bacteria. (Courtesy of the U. S. Department of Agriculture.) Phosphorus, Potassium, and Sulfur Are Often Deficient in Soils. Phosphorus is important in the formation of new cells. Both phosphorus and sulfur are present in many plant proteins. Potassium does not enter into the important plant products, but it seems to act as a sort of helper. Thus the amount of nitrate absorbed by a plant will depend upon the amount of potassium present. The phosphorus, potassium, and sulfur resources of the United States have already been discussed. Elements Needed in Small Amounts Are Very Important, but Fortunately They Are Seldom Deficient in the Soil. Calcium serves to bind the plant cells together, magnesium is a constituent of chlorophyll, while iron is required for its production. The function of such elements as boron, manganese, zinc, copper, molybdenum, etc. are not well established, w^'M Fig. 313. Effect of potash deficiency on ear formation. (Courtesy of the American Potash Institute, Inc.)
FOODS 735 although the necessity for their presence has been well established; for example, it has been shown that the cracking of tomatoes as they ripen can be prevented by the addition of very small amounts of manganese to the soil. It has been found that one part of molybdenum per one hundred million parts of water is essential to optimum tomato growth. Fig. 314. The effect of adding pulverized limestone at the rate of 4000 pounds per acre. (Courtesy of the National Lime Association.) Animals, Like Plants, Require Certain Foods in Definite Amounts for Normal Growth and Health. Food is defined as ^' any material which, when taken into the body, will (a) yield energy, (b) build tissue, or (c) regulate the body processes.'' The human body is composed of only those elements which are contained in foods which we eat. It is impossible to build normal healthy bodies from foods which are deficient in certain elements. The composition of the human body is approximately as follows Oxygen Carbon Hydrogen Nitrogen Calcium Phosphorus Potassium Per Cent 65 18 10 3 1.5 1.0 0.35 Sulfur . . Sodium Chlorine . Magnesium Iron Iodine . Traces of other elements. Per Cent 0.25 0.15 0.15 0.05 0.004 0.00004 The carbon, hydrogen, oxygen, and nitrogen are contained in the carbohydrates, fats, and proteins. Carbohydrates consist of sugars, dextrins, gums, and starches. The function of carbohydrates and fats in the body is to furnish the heat and other forms of energy through their oxidation by the oxygen taken into the body through the lungs, forming water and carbon dioxide as by-products. Excess carbohydrates and fats are stored in the body, chiefly in the form of fat. Excess weight should be avoided because it materially reduces one's length of life by overworking the various organs, especially the heart.