194 thf: karth as man's abode act on petroleum, and still others carry out many different kinds of chemical changes. Good soil contains from one-half million to ten million bacteria per gram. Some of these bacteria take nitrogen from the air and change it into compounds essential to plant growth, while others carry out other changes that provide necessary plant foods. Plants are thus enabled to grow. As they grow and die, considerable amounts of plant matter such as leaves, roots, and stems accumulate. Bacteria and fungi bring about the decomposition of this organic matter, and the soil is improved with each new crop of plants. mmm MHWMlffff— i-io. ou. ihese farm implements should never ha\e been used because they destroyed a naturally rich grazing area. Mills, New Mexico, May, 1935. (Courtesy of the Farm Security Administration. Photograph by Lange.) The soil must be kept porous to enable roots to penetrate it. Earthworms eat decaying vegetable matter mixed with soil particles, which are then excreted. The total amount of such castings would amount to twenty tons per acre in one year. The worms penetrate the soil to a depth of five or six feet and thus loosen it up as they form their burrows and enrich it with decaying plant materials. It has been estimated that an acre of healthy soil contains 1,500,000 earthworms, which are equivalent to several skilled gardeners and several tons of rich manure each year. Many of the higher animals supplement the work of the worms by burrowing into the soil to form their homes. It takes such a long time to produce good soil that man should conserve it as one of his most valuable natural resources.
LIFE IS DEPENDENT UPON EARTH CONDITIONS 195 Our Farm Lands Are Headed for the Sea. Someone has said that "dictatorship, rather than democracy, has characterized civihzed man's attitude toward nature during many ages in the past." Instead of cooperating with nature, we have tried to impose our imperious will ; but nature, acting slowly but surely, continues to work in accordance with laws that cannot be changed at will. For example, the Europeans came to North America as ruthless conquerors, exploiting its rich natural resources and its native population, taking what they wanted, and destroying the rest. They cut down Fig. 61. Soil erosion on a Brown County farm. (Courtesy of the Farm Security Administration. Photograph by Jung.) and burned forests to provide fields for crops and pastures for animals. They drained swamps and lakes to make available the rich land at their bottoms. he is It is not until recently that man has begun to see that losing some of his most precious resources because he has failed to study the ways of nature and has failed to seek nature as an ally. Man has tried and is still trying to force nature to do his will and has failed because of his ignorance. When the natural processes of erosion are disturbed, soils, which took centuries to deposit, may be removed in only a few years. This process is called accelerated erosion. If our soil continues to be carried away at the present rate, our huge dams and reservoirs will become useless within a hundred years; Mead Lake, impounded by Boulder Dam, will fill up.