732 MAN'S PHYSICAL WELFARE elements arc returned. The United States has been skimming the cream from its soil and has depended upon large acreage and modern machinery to produce high yields per man. In Europe, where man power is cheaper and land is scarcer, it has been necessary to produce larger crops per acre. Thus Holland uses 367 pounds of fertilizer per acre; Germany, 72; Japan, 56; France, 35; Italy, States, 8. This spendthrift process of robbing the soil 21; and the United can no longer continue for there are few new lands to exploit. Because of that reason the United States must plan to use much larger quantities of fertilizers in the future. The value of fertilizers can be readily determined by experiment. Statistics also show what can be accomplished. Thus Germany and Belgium produce 35 bushels of wheat to about 15 bushels per acre in the United States. The Mineral Content of Plant Products Depends upon the Mineral Composition of the Soil on Which They Were Grown. It is not sufficient to supply plants with an excess of all of their mineral requirements on the assumption that the plants will then utilize the minerals available in accordance with their needs. Plants, like animals, may take in larger amounts of certain food constituents than they need. Hay grown on soil too rich in potassium nitrate will kill the cattle that eat it; toxic hays may contain from 1.2 to 8.0 per cent of potassium nitrate. Often the heavy intake of one mineral lowers the intake of other minerals. Plants may absorb an excess of potassium, nitrogen, calcium, magnesium, and phosphorus if it is available; but sulfur is absorbed only in accordance with the plant's needs, while other elements are toxic to plants if they are present in too large quantities ("too large" generally meaning less than one part per million). The National Resources Committee reports, In 1787, the year the Constitution was framed, the surplus food produced by 19 farmers went to feed one city person. In recent average years 19 people on farms have produced enough food for 56 non-farm people, plus 10 living abroad. Productivity per farm worker increased steadily, and at very nearly the same rate in agriculture as in industry during the 75 years after 1850. Between 1910 and 1930, output per worker increased 39 per cent in manufacturing and 41 per cent in agriculture.* 1 Technological Trends and National Policy, 1937.
FOODS 733 Plants, like animals, require a proper acid-base balance. Soil containing sodium nitrate in the form of Na+ ions and NOs" ions will soon become alkaline due to the excess Na+ ions left in the soil as the NOa" ions are removed to be utilized by the plant, unless this process is balanced by a process in which such positive ions as K+ ions are taken from the soil at nearly the same rate. An important principle in hydroponics is that oppositely charged ions should be so paired in the solution that they will be equally absorbed by the plant.