64 TEXTBOOK OF PLANT PHYSIOLOGY 



Hence, the struggle for the possession of the most fertile territories 

 of the earth is in its essence a struggle for fixed nitrogen, which 

 most frequently is the limiting factor in plant production. The 

 growing of legumes is a means of utilizing a free source of a very 

 valuable substance, the molecular nitrogen of the atmosphere. 



In spite of the ever-increasing intensification of agriculture, 

 however, which is the result of an incessant growth of the world's 

 population, the beneficial effects of leguminous plants (as well as 

 that of free-living soil bacteria) have been found to be insufficient 

 already and other more rapid ways of enriching soils with nitrogen 

 must be discovered. Until quite recently almost the only method 

 of achieving this was by the application of Chile saltpeter, enormous 

 deposits of which have been exploited in South America for many 

 years. The introduction into the soil of such organic material as 

 manure, animal residue from slaughter houses, city refuse, and the 

 like, is nothing but an incomplete return to the soil of that which 

 was previously taken from it, as part of the nitrogen always is 

 lost in the processes of decay and denitrification, or is carried to 

 the ocean by rivers. 



A desirable feature of potassium nitrate, when compared to 

 stable manure, is its more concentrated form. One ton (1,000 kg.) 

 of stable manure contains, on an average, only about 4 kg. of 

 nitrogen. In order to obtain the same amount of nitrogen 20 kg. 

 of potassium nitrate are sufficient. On the other hand, stable 

 manure contains other nutritive substances necessary for the 

 plant. Moreover, its application considerably improves the 

 physical properties of the soil. Fertilization with stable manure 

 is therefore one of the chief methods of improving soils poor in 

 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 technique, 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 energy. Hence the product — 

 calcium nitrate — bears the name of Norwegian saltpeter. During 

 the World War the German chemist, Haber, discovered another 



