94 PLANT PHYSIOLOGY 



billion tons of nitrogen in the form of ammonia and nitric acid. 

 This means about five pounds per acre per year. . 



Man in the laboratory has tried by means of the electric spark 

 to duplicate the fixations found in nature. One method has been 

 to bring about the union of nitrogen with oxygen, forming nitrates. 

 Another has been to unite the nitrogen with calcium carbide, 

 (CaC 2 ) forming cyanimide; while a third has been the attempt to 

 unite nitrogen with hydrogen, forming ammonium compounds. 

 Recently there have been discovered catalyzers which will bring 

 about these unions without the expenditure of such enormous 

 amounts of energy as were formerly required. One such compound 

 made in this manner is sodium cyanide (NaCN), which is formed 

 from sodium carbonate, nitrogen, and carbon in the presence of 

 finely divided iron as a catalyst : 



Na 2 C0 3 +N 2 +4C = 2NaCN+3CO. 



These catalytic methods will put the manufacture of nitrogen 

 compounds into the hands of countries where large water-power 

 sources are not available, and will thus free those countries which 

 lack water power and large deposits of nitrogen compounds from 

 the control of their more fortunate neighbors. 



Fixation by Bacteria. — What man has been striving so hard to 

 accomplish and what inorganic nature is doing to a small ex- 

 tent, many of the lower plants are able to do quite successfully. 



Lawes and Gilbert (1891) showed that legumes had the ability, 

 when grown on a soil in which the nitrogen had reached its mini- 

 mum, to increase the amount of total nitrogen present, while, in 

 the case of grain grown upon a similar soil, no increase in nitrogen 

 took place. Wagner (1891) also showed that peas were not able 

 to profit from fertilizers containing nitrogen to the same extent 

 as oats. These results differ from those of Boussingault, who found 

 that all plants were equally dependent upon fertilizer, but when 

 it is remembered that Boussingault used sterilized soil while the 

 others did not, the discrepancy is explained. 



Hellriegel and Willfarth (1888), whose researches in this field 

 are classic, showed that nitrogen was increased only in unster- 

 ilized soil even in the case of legumes. They noticed further that 

 the addition of nitrogen was associated with the formation of 

 small nodules or tubercles on the roots of the legumes, and pro- 

 posed that these growths were the result of a symbiotic relation- 



