vil] EARLY THEORIES 199 



probably such as was described above. It has been 

 found to cause increased crop returns on peaty or other 

 soils rich in humus, or where slow-acting nitrogenous 

 manures have been applied. 



The fermentation which goes on in absence of 

 oxygen, is brought about by a large number of bacteria, 

 some of which are only active in the absence of oxygen, 

 others are aerobic, but will continue their work when 

 deprived of free oxygen. Carbohydrates are decomposed 

 with formation of carbonic acid and other gases like 

 hydrogen and marsh gas, butyric and other fatty acids, 

 a residue of humus being always produced at the same 

 time. The protein bodres readily undergo putrefactive 

 change, with the production of tyrosin and various 

 amino-acids, fatty acids, ammonia, phenol, and other 

 bodies containing an aromatic nucleus, gaseous com- 

 pounds of sulphur, etc. In the main, however, the 

 changes of organic material in the soil fall upon the 

 cellulose ; it loses carbonic acid, marsh gas, hydrogen, 

 etc., and becomes humus with a gradually increasing 

 proportion of carbon; the nitrogenous materials resist 

 attack more than the carbohydrates, and hence tend to 

 accumulate, so that an old sample of deep-seated peat 

 is richer in nitrogen than a more recent sample taken 

 from nearer the surface. Finally the humus thus pro- 

 duced, which may be called peat, is essentially an acid 

 product, and even when aerated and supplied with 

 mineral materials will oxidise with extreme slowness. 



The Fixation of Free Nitrogen. 



In the earliest theories regarding the nutrition of the 

 plant which were accepted after chemistry had become 

 an exact science, it was considered that the plant 

 derived its nitrogen from the humus of the soil, as, 

 for example, in de Saussure's statement that " Plants 



