iv SUBSTANCES OF A LEAF 33 



How the Exhaustion of the Soil is 

 Prevented 



In order to prevent the soil thus becoming 

 temporarily exhausted, the farmer does several 

 things. 



(1) He digs or ploughs the land, thus turning 

 up fresh earth for the roots to feed on. 



(2) He has found that different crops do not 

 require the same food, and he therefore grows one 

 crop one year and another the next year, rotating 

 his crops. 



(3) He allows the land to lie fallow, that is, grows 

 nothing on it at all during a summer season. By 

 doing this he gives time for fresh plant food to 

 become soluble. At the same time a certain amount 

 of food is permanently lost through the drains. 



(4) He manures the land, that is, adds to it the 

 food which he believes to be deficient in quantity, 

 for the health and luxuriant growth of his crops. 



While he is thus trying to make up for the ex- 

 haustion of the soil, he is being assisted by changes 

 taking place in the soil itself. 



The earthworms are bringing up fresh soil to the 

 surface from below, and the wind is blowing fresh 

 soil or dust upon the top. 



The stones and rocks also are gradually breaking 

 down and supplying the soil with fresh mineral food. 

 This natural process goes on all the more freely if 

 the cultivation is deep, thorough, and frequent. 

 D 



