FACTORS OF AGRICULTURAL PRODUCTION 147 



more than 7,250,000 acres already under irrigation, and the 

 acreage has increased considerably since. It has been conserva- 

 tively estimated that there will eventually be brought under irri- 

 gation in the western half of the United States an area equal 

 to the whole of New England and New York combined. The 

 greater part of this land is practically valueless without water, 

 but when brought under an effective irrigation system it becomes 

 as valuable and productive as any land in the country, and much 

 more productive than the greater part of that which is now culti- 

 vated. In addition to the advantage of being able to control the 

 moisture, irrigation has the further advantage of replenishing 

 the soil with the sediment brought down from the decomposing 

 rocks of the higher altitudes. Therefore irrigated land is not 

 only highly productive but it tends to retain its productivity for 

 long periods of time. 



Dry farming. But after all the available water of the moun- 

 tain streams has been diverted and utilized, only a small fraction 

 of the acreage of this vast arid region will be under irriga- 

 tion. Large as the irrigated area will be in the aggregate, it 

 will form only a series of oases in the midst of vast wilder- 

 nesses of desert or semidesert lands, incapable, without water, 

 of being brought to a high state of cultivation. Of this land, 

 however, some of it it is impossible to say how much can 

 be brought under tillage by what has come to be known as dry 

 farming. Except in certain high mountain altitudes, the rain- 

 fall gradually diminishes as one moves westward from the Mis- 

 sissippi River, until one nears the Pacific coast. It is difficult 

 to say just where the region of adequate rainfall ends or where 

 that of inadequate rainfall begins, even when judged from the 

 standpoint of older methods of cultivation. But by a scientific 

 study of the problems of moisture retention, and by the intro- 

 duction of new drought-resisting crops, it has been found that 

 this line can be moved much farther westward ; that is, that 



