CHAPTER XXXII 

 THE SOIL 



211. The Soil is the surface of the earth which con- 

 tains the necessary compounds and characteristics for 

 the growth of plants. It varies in depth from a few 

 inches to many feet, according to the method of forma- 

 tion and the amount of erosion that has taken place. It 

 can be increased in depth by the growth of vegetation and 

 by proper cultivation. 



212. Origin. The surface of the earth at one time was 

 barren rock, unfit for plant or animal life. It was 

 much like the surface of cooled lava which flowed from 

 volcanos. There are yet in the Rocky Mountain section 

 large areas of almost barren rock which are not suitable 

 for cultivation. Some other parts of the earth are covered 

 with vast lava flows which cannot be cultivated, and no 

 plants of any kind can grow on them. 



The surface of the rock which ages ago covered the earth 

 was gradually broken up or decomposed by the action of 

 water, air, and the changes of temperature from one 

 season to another. The oxygen of the air oxidized the 

 iron and certain other substances that it could reach, and 

 the water made it more favorable for this oxidation to 

 occur. You know that iron if wet will rust rapidly, that 

 is, it will oxidize. The water dissolved parts of the rock 

 and carried insoluble particles into localities where they 

 were deposited with the soluble parts left by the water 

 when it evaporated. The expansion and contraction of 



