Man's physical universe

xanabras

LIFE IS DEPENDENT UPON EARTH CONDITIONS 193

Land Plant Life Requires Moist Soil.

Plants must have soil into which they may send their roots for

anchorage and from which they may secure the water and soluble

minerals essential to their growth. Soil is therefore the most important

geological product that is essential to life.

Soil Is a Product of Erosion or Volcanic Action.

Most of the soil is produced from rocks by the various processes of

erosion. They are first broken into small pieces by the various disintegrating

agents. Plants could not grow, however, even in finely powdered

Fig. 59. Plowing on the prairies beyond the Mississippi. (Sketched by Theodore

R. Davis.) Six strong oxen were required to pull this plow which is turning

over the thick virgin sod of the Great Plains. The pioneer's plow destroyed the

luxuriant tall grasses of the Great Plains where the buffalo grazed. (Culver

Service, New York.)

rock. Chemical changes must take place in the disintegrated rock

before it is suitable for plant growth. Oxygen and carbon dioxide of

the air, supplemented by other substances, such as nitric and sulfuric

acids, which are produced in

the air by lightning, volcanic activities,

etc., decompose these rocks.

Even then the decomposed rock particles will not support plant

growth. Nature's chemists, the bacteria, must now go to work. Microorganisms

can live and multiply in water. Some of them can live without

oxygen. Some of them liberate sulfur from the compounds, others

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