ASSIMILATION OF NITROGEN BY PLANTS 237 



nitrogen, even when the so-called ''mineral fertilizers" are 

 extensively used. 



The deposits of saltpeter, however, are being gradually 

 exhausted. This seems to threaten mankind with the possibility 

 of nitrogen starvation. Modern industry, however, has come 

 to the aid of agriculture. Several years previous to the World 

 War, a method of obtaining nitric acid from the air by means of a 

 high-potential electric discharge was found. This method is 

 being widely applied, especially in Norway, a country rich in 

 waterfalls, which produce cheap electric power. Hence the 

 product, calcium nitrate, bears the name of "Norwegian salt- 

 peter." During the World War, the German chemist Haber 

 discovered another method of fixing atmospheric nitrogen by 

 combining it with hydrogen, thus forming synthetic ammonium. 

 At the present time, there are in operation in Europe and else- 

 where several factories that produce hundreds of thousands and 

 even millions of tons of synthetic ammonium and nitrate salts. 

 It is very likely therefore that in the near future the requirements 

 of agriculture will be fully satisfied by artificial nitrogenous 

 fertilizers produced from the air. Then the increase of yields 

 will not depend, as heretofore, on the growing of nitrogen-fixing 

 leguminous plants. 



General References 



Chibnall, a. C. Investigations on the nitrogenous metabolism of the 



higher plants, II. Biochem. Jour., 16: 334-362, 1922. 

 EcKERSON, S. H. Protein synthesis by plants, I. Nitrate reduction. 



Botan. Gaz., 77: 377-391, 1924. 

 Harvey, R. B. ''Plant Physiological Chemistry," Part IV. D. Appleton- 



Century Company, Inc., New York. 1930. 

 Jones, L. H., and J. W. Shive. Influence of ammonium sulphate on plant 



growth in nutrient solutions and its effect on H-ion concentration and 



iron availability. Am. Botany, 37: 355-377, 1932. 

 KosTYTCHEv's "Chemical Plant Physiology," Chaps. Ill, VI. Trans, and 



ed. C. J. Lyon. P. Blakiston's Sons & Co., Inc., Philadelphia. 1931. 

 LoEHWiNG, W. F. Root interactions of plants. Botan. Rev., 3: 195-239, 



1937. 

 McKee, H. S. a review of recent work on the nitrogen metabolism of 



plants. New Phytologist, 36 : 33-56, 240-266, 1937. 

 Maskell, E. J., and T. G. Mason. Studies on the transport of nitrog- 

 enous substances in the cotton plant. Ann. Botany, 43: 205-231, 



615-662, 1929; 44: 1-29, 233-267, 657-688, 1930. 



