220 PLANT PHYSIOLOGY 



settled best by means of artificial cultures. The numerous 

 experiments, first carried out by Boussingault in the middle of 

 the nineteenth 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 atmosphere, viz., 

 ammonia gas and oxides of nitrogen, may be assimilated by 

 plants, but their amount in general is so small that it is entirely 

 insufficient to insure normal plant development. Practically 

 all plants, therefore, are forced to obtain the required amount 

 of nitrogen from the nitrogenous substances found in the soil. 

 Hence, in the growing of plants in artificial cultures, compounds 

 of nitrogen necessarily must be included in the nutritive mixture. 

 Ammonium salts and nitrates are most frequently used for this 

 purpose. 



The nitrogenous substances contained in the soil may be 

 divided into three groups: the organic compounds of nitrogen, 

 the salts of ammonium, and nitrates. Their total amount 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 completely 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 usually is 

 not mentioned, but rather the total amount of nitrogen contained 

 in these compounds. Similarly, instead of determining the 

 weight of ammonium and nitrates, it is commonly considered 

 sufficient to report the weight of nitrogen included in their 

 composition. This has given rise to such abridged terms as 

 ''organic nitrogen," ''nitrate nitrogen," and "ammonia nitro- 

 gen." These figures may be transferred into those showing the 

 total weight of the corresponding substances, if it is accepted 

 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. 



The relative quantities of the different forms of nitrogen in 

 various soils may be seen from analyses of Russian soils made by 

 Kossovitch. Generally it was found that the black soils ("cher- 



