ASSIMILATION OF NITROGEN BY PLANTS 22i 



nozem") contain 0.50 per cent of organic nitrogen, 0.002 per cent 

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

 the gray forest soils, respectively, 0.25, 0.001, and 0.0008 per cent 

 of these substances; and the sandy podsol soils 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, such as manure, markedly 

 increases 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 fertilizer purposes it is best to use 

 manure that is considerably decayed, suggests that the simpler 

 compounds are better assimilated than the more complex. 

 Experiments with soil cultures, however, so far have given no 

 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. When an organic substance is added to 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, as it has been estabUshed 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 are 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 being isolated from the atmosphere by means 

 of a layer of sterihzed cotton. The stem of the germinating 



