52 TEXTBOOK OF PLANT PHYSIOLOGY 



Province) contain 0.50 per cent of organic nitrogen, 0.02 per cent 

 of ammonia nitrogen and 0.003 per cent of nitrate nitrogen; the 

 gray forest soils (Orel Province, Russia) contain, respectively, 

 0.25, 0.001, and 0.0008 per cent of these substances; and the 

 sandy soils (Leningrad Province), 0.09, 0.002, and 0.0009 per cent. 

 These figures show that most of the soil nitrogen is found in the 

 form of organic compounds, the inorganic ones making up but a 

 small fraction of the total quantity present. 



A further study of the question as to which compounds of 

 nitrogen, the organic or the inorganic, are better utilized by plants, 

 is connected with considerable experimental difficulties. Every 

 agriculturist knows from his experience that the application of 

 organic nitrogenous fertilizers (usually manure), markedly in- 

 creases the yield. This might lead one to believe that organic 

 compounds of nitrogen are most easily assimilated by plants. But 

 the very fact that for fertilizing purposes it is best to use manure 

 which is considerably decayed, suggests that the simpler compounds 

 are better assimilated than the more complex. Experiments with 

 soil cultures, however, so far have not been able to give a definite 

 answer to this question. In every soil are found an enormous 

 number of the most varied microorganisms, which decompose every 

 organic substance introduced into a soil. The final products of 

 this decomposition, as will be shown later, are ammonium salts 

 and the nitrates which are always present in soil. In introducing 

 an organic substance into the soil, therefore, it is impossible to say 

 whether it will be utilized by the plant in an unchanged condition 

 or whether first it will be decomposed by bacteria, and the products 

 of its decay then absorbed by plants. 



To solve this question a method of sterile artificial cultures has 

 been devised, in which the roots of plants are placed in a medium 

 free from bacteria. The leaves of the plants may be left exposed 

 to the air. It has been found that, excepting a few parasitic forms, 

 bacteria falling upon the aerial parts of a plant from the atmosphere 

 are unable to enter plant tissues. At the present time there is a 

 fairly large number of such methods of sterile culture (Maze, 

 Shulov, Petrov, Knudson, and others). Essentially, they are but 

 variations of those used in microbiology for the culture of bacteria. 

 The sterile nutritive medium usually is isolated from the atmos- 

 phere by means of a layer of sterilized cotton. The stem of the 

 germinating seed is then passed through this cotton by means of 



