Man's physical universe


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.



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.

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