98 SOILS AND FERTILIZERS 



systems of irrigation interfere somewhat with the use 

 of farm machinery, and the initial cost of any sys- 

 tem is considerable, as is the labor cost of working 

 it. These last two objections have prevented the in- 

 stallation of the system in regions which are already 

 fairly productive, and restricted its use largely to 

 areas which but for irrigation would be arid wastes. 

 In spite of cost, however, the increased yield from 

 its use, with fairly fertile as well as barren lands, is 

 so great that it will repay the expenditure many times 

 over. 



SOIL FERTILITY 



The main agricultural question of the humid sections 

 is that of maintaining soil fertility, or of restoring lands 

 depleted of fertility to their former crop-producing 

 power. 



The broad areas of virgin soil in the United States, 

 which yield abundantly with little cultivation, have 

 encouraged unwise and wasteful methods of farm- 

 ing. These methods have resulted in land almost 

 or quite depleted of its fertility, until there are 

 many sections where the soil no longer responds to 

 cultivation with profitable crops. The problem of 

 restoring these lands to productive power and of 

 preventing other land from becoming barren is one 

 that scientists and farmers together have partially 

 solved. 



How Land loses Fertility. (a) As different plants 

 take different elements or different amounts of the 

 same elements from the soil for food (see page 101), 

 a given crop planted in the same soil for years will 



