NATURE OF SOIL 303 



are sudden changes of temperature and the action of frost. 

 When masses of rock are heated by the sun they expand ; 

 sudden cooling at night or from change of weather causes 

 so rapid a contraction that they are rent asunder. Water 

 freezing in crevices also constantly breaks masses of rock 

 into smaller pieces. 



Rocks undergo decomposition largely through changes 

 due to the action of chemicals carried in water. The min- 



A plowing contest. 



eral substances in the rocks are dissolved, and new prod- 

 ucts formed. 



Surface and subsoil. The terms soil and subsoil are 

 used to distinguish the top portion from the soil that 

 lies underneath. The line between the dark humus-colored 

 part and the lighter soil below is sometimes spoken of as the 

 division between soil and subsoil. Another distinction made 

 is to call all that lies below the depth of tillage subsoil. No 

 such dividing line can be sharply drawn, however, as much 



organic matter is found below the humus line, and the depth 

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