8 4 



PLANT PRODUCTS 



TABLE 14. CROP YIELDS WITH INCREASING NITROGEN SUPPLY, 

 ROTHAMSTED. 



is profitable when prices of produce are high becomes 

 unprofitable when prices are low. Doubtless if every user of 

 artificial fertilizers were to start using artificial fertilizers 

 in double quantities because the rise in agricultural produce 

 justifies such a procedure, then the prices of the agricultural 

 fertilizers would rise so high as to put a stop to their economic 

 use. Exactly where the dividing line between what is 

 practicable and what is not must be determined in every 

 case by the cultivator of the soil himself. 



REFERENCES TO SECTION I 



Leake, " Some Preliminary Notes on the Physical Properties of the 

 Soils of the Ganges Valley, more especially in their Relation to Soil Moisture," 

 Journ. Agric. Science, i., p. 454. 



Keen, " The Evaporation of Water from the Soil," Journ. Agric. Science, 

 vi., p. 456. 



Luxmore, " The Soils of Dorset," pp. 7, n. 



Russell, " Soil Conditions," pp. 75, 87. (Ixmgmans.) 



Hilgard, " Soils," pp. 83, 107. (Macmillan.) 



Hall, " The Soil," pp. 48, 154. (Murray.) 



Fream, " Soils and their Properties." (Bell.) 



Warrington, " Physical Properties of Soils." (Clarendon Press.) 



Tempany, " The Shrinkage of Soils," Journ. Agric. Science, viii., 

 p. 312. 



Balls, " The Movements of Soil Water in an Egyptian Cotton Field," 

 Journ. Agric. Science, v., p. 469. 



Alvvay, " Studies of Soil Moisture in the ' Great Plains ' Region," Journ. 

 Agric. Science, ii., p. 333. 



Leather, " Memoirs of the Department of Agriculture in India," Feb., 

 1908, p. 79 ; July, 1907, p. 49; April, 1906, p. 3 ; and Feb., 1900, p. 125, 

 (The Imperial Department of Agriculture in India.) 



