52 



AGRONOMY 



into which the water may be turned as needed, traverse the 

 fields. Crops on irrigated land are usually certain, since the 

 farmer is relieved from any dependence on the natural rainfall. 

 Dry farming. Twenty inches of rainfall, properly distrib- 

 uted, is about the minimum amount that will produce ordinary 

 crops. In a few localities, where the rainfall is less, crops are 

 produced by a system of dry farming in which the scanty 

 moisture is stored up in the soil until there is sufficient for a 



Tin. 



Cin. 



5 in. 



4 in. 



\ 



Sin. 



A 



Z- 



2 in. 



\ 



\/ 



lin. 



Feb. 



Mar. 



Apr, 



May 



June 



July 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



FIG. 27. Diagram showing precipitation at Joliet, Illinois, in 1909 

 Data supplied by F. M. Muhlig, United States Weather Observer 



crop. This method consists in keeping the surface of the soil 

 constantly loose, partly for the purpose of absorbing all the 

 rain that falls, partly to prevent the evaporation of the water 

 already in the soil. A crop may thus be grown every other 

 year, or two crops in three years, the land remaining without 

 a crop, though carefully tilled, during the intervening time. 

 Crops have been grown by this method in regions where the 

 rainfall is but twelve inches. In some cases it is possible to 

 get a crop annually by cultivating the soil during that part 

 of the year when it is not occupied by plants. The selection 

 of drought-resistant plants may also facilitate dry farming. 



