ABSORPTION OF NITROGEN FROM SOIL AND AIR 51 



as a source of this element for the nutrition of plants may be 

 settled best by means of artificial cultures. The numerous experi- 

 ments first carried out by Boussingault in the middle of the nine- 

 teenth century, and frequently repeated since, have definitely 

 shown that molecular nitrogen, N2, cannot be utilized by the 

 higher plants. They cannot transform into a fixed state this 

 highly inert gas. The compounds of nitrogen found in the atmos- 

 phere may be assimilated by plants, but their amount in general is 

 so small that it is entirely insufficient to insure their normal 

 development. Practically all plants, therefore, are forced to 

 obtain the required amount of nitrogen from the nitrogenous sub- 

 stances found in the soil. Hence, in growing plants in artificial 

 cultures compounds of nitrogen necessarily must be included in 

 the nutritive mixture. The salts of nitric acid are most frequently 

 used for this purpose. 



16. Absorption of Nitrogen from Soil. — The nitrogenous sub- 

 stances contained in the soil may be divided into three groups : the 

 organic compounds of nitrogen, the salts of ammonium, and the 

 salts of nitric acid. The total amount of soluble nitrogen is quite 

 small constituting, as a rule, not over a fraction of 1 per cent of the 

 total dry weight of the soil. Owing to the difficulty of com- 

 pletely separating them from the nitrogen-free compounds, it is 

 usually considered sufficient to determine by means of analysis the 

 amount of nitrogen produced by the combustion of the organic 

 substances of the soil. Hence, in reporting analytical data, the 

 amount of organic compounds of nitrogen, as a rule, is not men- 

 tioned, but rather the total amount of nitrogen contained in these 

 compounds. Similarly, instead of determining the weight of 

 ammonium and nitric salts, it is usually considered sufficient to 

 report the weight of nitrogen included in their composition. This 

 has given rise to such terms as: "organic nitrogen," "nitrate 

 nitrogen," and "ammonia nitrogen." These figures may be trans- 

 ferred into those showing the total weight of the corresponding 

 substances, if we accept that the humus of an average soil contains 

 approximately 5 per cent of nitrogen, potassium nitrate, about 14 

 per cent and ammonium sulphate about 21 per cent nitrogen. 



The relative quantities of the different forms of nitrogen in 

 various soils may be seen from certain analyses of Russian soils 

 carried out in the laboratory of Professor Kossovitch. Generally, 

 it was found that the black soils ("chernozem" of Voronezh 



