32 AGRICULTURE FOR COMMON SCHOOLS 



of wheat with a rainfall of only twelve inches per year. The 

 fact that crops can be produced from a very few inches of 

 rainfall when it is properly saved has led to what is called 

 "dry" farming. This is carried on mostly in the semi-arid 

 region. In dry farming the object is to stir the ground deeply 

 so that it will take in all the rainfall and hold it for crops. 

 A deep loose surface is kept over the fields to prevent any 

 moisture from evaporating. By so doing good crops are pro- 

 duced where the rainfall amounts to from ten to twenty inches 

 annually. So in the application of water by irrigation it has 

 been found best to apply about the above amount and then 

 give careful cultivation to save the moisture. The cost of 

 irrigating is heavy, so that farmers try not to use more water 

 than necessary. 



Irrigation is used for other purposes than simply to sup- 

 ply water to the plants. It is sometimes used to carry 

 dissolved plant food over the land. This is especially true 

 where sewage water is spread over the fields. The water in 

 rivers, and that from wells, too, usually has considerable 

 plant food dissolved in it and, when spread over the land, 

 adds fertility to it. Commercial fertilizers that are to be 

 applied to a field are sometimes dissolved in the irrigation 

 waters and so distributed. 



Another use of irrigation is to rid the land of "alkali." 

 The soil on large areas in the dry region of the West is filled 

 with salts which are injurious to plants. Such lands are 

 called "alkali" lands. It has been found that if enough 

 water can be applied to such lands to produce percolation 

 these salts will be dissolved and washed away, and the land 

 made fit for crops. However, the water applied must bie 

 pure water free from any injurious salts. 



