LAND AND OTHER NATURAL AGENTS 15 



capital corresponds to the return to labor and capital on land at the 

 extensive margin of cultivation.' ' Is this true? Work out your 

 proof illustrating with a chart. 



ADDITIONAL REFERENCES 



King, Farmers of Forty Centuries. 



Carver, Principles of Rural Economics, chap. iii. 



Shaler, Man and the Earth. 



Kropotkin, Fields, Factories, and Workshops. 



Teele, Irrigation in the United States. 



Crookes, The Wheat Problem. 



Hopkins, Soil Fertility and Permanent Agriculture. 



Hill, Highways of Progress. 



McGee, "Subsoil Water of Central United States," Yearbook U.S. Depart- 

 ment of A griculture, 1 9 1 1 . 



Brown, " Importance of Developing Our Natural Resources of Potash," 

 Yearbook of the U.S. Department of Agriculture, 1916. 



Woods, "The Present Status of the Nitrogen Problem," Yearbook of the 

 U.S. Department of Agriculture, 1906. 



Hirshberg, "Making Foods of Chemicals," World's Work, XXVI, 115. 

 (See also Scientific American Supplement, LXXIII, 310.) 



Woodward, "Land Drainage by Means of Pumps," Bulletin 304, U.S. 

 Department of Agriculture. 



TOPICS FOR SPECIAL PAPERS 



How Science Discovers New Agricultural Resources. 



Fitting the Character of Our Production to the Nature of Our Resources. 



The Possibilities of Irrigation (or drainage) in the United States (or some 



particular state or section). 

 Getting the Full Value of Our Rainfall. 

 How Fertility Is Wasted. 

 The Danger of Declining Fertility. 



Why We Need Not Fear Future Barrenness of Our Fields. 

 A Rational Conservation Program for Agricultural Lands. 



