12 SCIENTIFIC AGRICULTURE. 



CHAPTER II. 



THE ORIGIN, COMPOSITION, AND CLASSIFICATION OF 

 SOILS. 



12. THE earthy matter in which plants grow is 

 called the soil. It consists of finely divided parti- 

 cles of rock mixed with organic matter. The organic 

 matter comes chiefly from the decay of plants, and 

 forms generally a very small portion of the soil. It 

 can be easily burned off, leaving the mineral portion, 

 which consists of disintegrated or finely crumbled 

 rock (i). The earth is supposed by geologists to har^ 

 once been a mass of melted matter. As it cooled 

 down, a crust was formed upon which condensed 

 moisture falling as rain wore away the solid rock, 

 being aided in its corroding action by heat and other 

 agencies, until the pulverized or finely divided sur- 

 face was in proper condition to produce the plant. 



Soils consist of a small amount of organic 

 matter mixed with rock which has been re- 

 duced to a fine state of division by mechanical 

 and chemical agencies. 



13. Mechanical agencies are such as merely 

 alter the form or appearance of bodies without 

 changing their character, like the grinding or rub- 

 bing of rocks together, whereby they are broken into 

 small pieces. Each little piece has the same prop- 

 erties as the large mass. Chemical agencies are 



