ii6 THE WHEAT INDUSTRY 



plant is diminished and crop rotation or summer 

 fallowing prolongs the period of high productivity 

 by maintaining a favorable physical condition of 

 the'soil. 



The Use of Fertilizers. - - The soil must possess 

 in available form the essentials for plant growth, 

 viz., (i) sufficient though not excessive moisture, 

 (2) lime, and (3) the indispensable elements of 

 plant food, nitrogen, phosphoric acid, and potash. 

 The last three are usually contained in barnyard 

 manure, which is considered an excellent fertilizer. 

 The old sections of the United States, as well as 

 most European countries, have ample rainfall, 

 but long-continued cropping has reduced the supply 

 of available plant food in the soil. In order to 

 raise paying wheat crops the deficiency must be 

 overcome. This may be done by applying pre- 

 pared mixtures which are high in the particular 

 plant foods needed. Such mixtures are known as 

 commercial fertilizers. The cost of this fertiliza- 

 tion is heavy, but the returns are so materially in- 

 creased that the farmer is much more than repaid 

 for his outlay. 



In the more newly settled regions commercial 

 fertilizers have not proved profitable. The soils are 

 naturally very fertile and cropping has not continued 

 long enough to deplete the available plant food. 

 This condition is generally true of the states west 

 of the Mississippi River. Here to most of the 



