HOW SOILS ARE FORMED 43 



ing, primarily, from rock decay, which furnishes food and 

 foothold for plant and animal life. 



Soil Builders. — Soil is derived from two main classes 

 of substances: rock and organic matter. Rocks crumble 

 into fine particles. They are gradually being changed into 

 soil. Plants and animals decay to form the organic matter 

 or humus in the soil. The humus present in our fields is 

 chiefly of vegetable origin. 



Pxa. 24. — A high bank of ^v x pre it has nearly covered the 



fence. (Plant Industiy.) 



How Soils are Formed. — Certain forces are constantly 

 at work changing rock into soil and making soil particles 

 finer. These forces are: (1) air; (2) water; (3) temperature; 

 (4) plants; (5) burrowing animals; and (6) minute plant 

 and animal organisms. These forces act both physically and 

 chemically. 



Exercise. — Soil Forming. — Find places near the school 

 or at home where any of the agencies mentioned are at work 

 forming soil. Tell of the places and what you have seen. 

 Can you find places where water has carried soil? How are 

 the little gulleys formed in hillsides? Where is the soil 

 carried to? , 



