Rocks and Soil 301 



them away. The wind is especially useful in transporting soil 

 from place to place after it has been formed. 



Air. Air contains oxygen which unites with various sub- 

 stances and causes them to break apart by oxidation. When these 

 substances form a part of rocks, the rocks are affected by the 

 air and soon decompose, crumble, and form soil. The reddish soil 

 seen at the base of some mountains is the result of the oxidation, 

 of rock that contained iron ore. Even manufactured iron oxidizes,, 

 or rusts, so rapidly in moist air that it must be protected by paint 

 or other coatings. 



Plants. Fertile soil must contain a certain amount of organic 

 matter. Part of this, the humus, is provided by plants. It results 

 from the decay of plant tissue. Fertile surface soil found in 

 swamps and in wooded areas contains a large percentage of 

 humus. Dead twigs and fallen leaves change to humus. There 

 are certain minute plants in the soil known as soil bacteria that 

 act upon humus and make it a part of the soil. 



Animals. Part of the organic matter in the soil is provided 

 by animals. Some of it results from the decay of dead animal 

 material and some from the excretions of animals spread on the 

 surface of the soil. A large amount, however, conies from ma- 

 terial that has passed through the digestive systems of earth worms 

 and has there been ground fine and enriched with organic matter. 

 Charles Darwin estimated that over 50,000 worms may be found 

 in an acre of land, and that it is possible for ten tons of soil to 

 pass through their bodies in a year and thus be brought to the 

 surface in the form of worm casts. 



Importance of Life in Soil. The importance of organic 

 life in the soil cannot be overestimated. Earthworms and other 

 burrowing creatures break up the soil and so provide avenues for 

 the introduction of the air and water needed by plants. Certain 

 bacteria add greatly to the fertility of soil, for they take from the 

 air nitrogen, a substance needed for plant growth, and change it 

 into a soluble form that plants can use. 



Kinds of Soil. Soils are classified according to the kinds 

 of materials they contain. A soil having a large percentage of 

 clay is called a clay soil. One having a large percentage of sand 



