236 PLANT PHYSIOLOGY 



nitrogen and mineral substances, however, is often quite expen- 

 sive. 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. 



Because of the abundance of nitrogen in all parts of leguminous 

 plants, it is good agricultural practice to sow some leguminous 

 plant on soils poor in nitrogen. When the plants have developed 

 sufficiently, they are plowed into the soil to produce green 

 manure. Cereals sown after this utilize the nitrogen that has 

 been fixed and accumulated by the leguminous plants. 



Owing to the ever increasing intensification of agriculture, 

 however, the beneficial effects of leguminous plants, as well as 

 that of free-living soil bacteria, have been found to be insuffi- 

 cient, 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 Chilean 

 nitrate of soda, enormous deposits of which in South America 

 have been used commercially for many years. 



The introduction into the soil of such organic material as 

 manure, animal residues from slaughterhouses, city refuse, and 

 the like, is nothing but a^ incomplete return to the earth of 

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

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

 through leaching to the ocean by rivers. 



A desirable feature of potassium nitrate, when compared with 

 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, or only one-fiftieth 

 of the weight. That is why saltpeter, NaNOa, may be profitably 

 transported great distances, and many countries in Europe and 

 elsewhere are provided with it from Chili. 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 



