CONDITIONS AFFECTING SOIL FERTILITY 51 



amount, the lands must be irrigated if crops are to be grown. 

 Many otherwise sterile soils are very fertile when water is 

 applied. Irrigation, however, can be carried on only when the 



13 in. 



Trital 1911 45.57 

 Total 1910 25.53 



Ay. 4 yfs. 35.40 



12 in. 



11 In. 



10 in. 



9 in. 



Sin. 



7 in. 



Gin. 



in. 



4 in. 



Apr. 



May 



June 



July 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



FIG. 26. Precipitation at Joliet, Illinois 



The heavy line represents the average for the years 1908, 1909, 1910, and 1911. 

 See page 45 for the precipitation by months. (Data supplied by F. M. Muhlig, 

 United States Weather Observer) 



land is properly located with reference to a permanent source of 

 water supply. Usually some near-by lake or stream is selected, 

 a dam built across it, and the water which falls in its basin 

 stored for the use of the crops. From the dam, canals are made 

 to run through the lands to be irrigated, and smaller ditches, 



