FACTORS IN CROP PRODUCTION 583 



Natural Resources " published by the American Academy of 

 Political and Social Science, Philadelphia, 1909) : 



"If all the run-off waters of this region could be conserved and employed in 

 irrigation, the total area reclaimed might, perhaps, be brought to nearly 

 60,000,000 acres. . . . Large portions of the water of the arid region cannot 

 be used in irrigation, as no irrigable land exists upon which it can be brought at 

 feasible cost." 



"With present data, the closest statement is probably under 60,000,000 acres 

 and between 40,000,000 and 50,000,000 acres, including the lands now under 

 ditch." (About 13,000,000 acres are now under irrigation.) 



For comparison it may be noted that the state of South Dakota 

 contains 48,000,000 acres, and the estimated total area of arid land 

 that can still be brought under irrigation in the United States is 

 equal to only one state like Illinois. Director Newell estimates 

 that the total land areas that may possibly be brought under irri- 

 gation might support, directly and indirectly, 10 million people, 

 or about 10 per cent of our present population. 



It should be kept in mind that the fertility even of irrigated lands 

 must be maintained if they are to continue productive. With 

 large use of turbid water there is always soil enrichment, but 

 reservoir water adds little or no fertility to the soil, as witness the 

 low yields of irrigated lands in India. In his Handbook of Indian 

 Agriculture, Mukerji makes the following statements: 



"The best crops of wheat are grown on lands newly brought under canal 

 irrigation. Where canal water is used for irrigation for a number of years, the 

 outturn is found to fall off even below the original level. . . . No manure is 

 required for dearh land which is annually renovated with silt." 



In this connection it is of interest to know that the estimated 

 area of reclaimable swamp land in the United States is less than 

 80 million acres, which would provide about two million 4o-acre 

 farms, thus furnishing homes for another 10 million people corre- 

 sponding to the normal increase in our population for five or six 

 years. 



