SELMAN A. WAKSMAN 317 



micro-organisms. Since the carbon content of natural organic materials, including 

 carbohydrates and proteins, ranges between 40 and 50 per cent (with the exception 

 of lignins, which contain 63-64 per cent carbon, but which are not readily available 

 as sources of energy), and since the nitrogen content varies, we may conclude that 

 the greater the percentage of nitrogen in an organic substance, the more rapid is the 

 liberation of its nitrogen in the form of ammonia. The lower the nitrogen content 

 and the higher the carbohydrate content of the organic matter, the more delayed will 

 be the process of liberation of nitrogen as ammonia. 



This process of formation of ammonia can be carried out by practically all soil 

 fungi and actinomyces, and by a large number of bacteria. Some of the bacteria, 

 especially many spore-forming anaerobes and aerobes, are very active in the decom- 

 position of native proteins, while other bacteria, especially certain non-spore-forming 

 organisms and cocci, cannot attack native proteins, but act readily upon protein de- 

 rivatives, such as the various amino acids. The non-protein nitrogenous substances, 

 such as urea, uric acid, hippuric acid, xanthine, hypoxanthine, etc., are also decom- 

 posed by various soil bacteria and fungi, and the nitrogen is sooner or later converted 

 to ammonia. 



The ammonia thus liberated may (i) either be assimilated by soil micro-organ- 

 isms, in the presence of undecomposed carbonaceous materials and changed back 

 into proteins; (2) or be used by higher plants as a source of nitrogen; (3) or be ab- 

 sorbed by the colloidal soil substances and held as ammonia; (4) or be acted upon 

 further by other micro-organisms and changed into nitrates. The last process is 

 carried out by two groups of autotrophic bacteria,' one changing the ammonia to 

 nitrous acid and the other changing the nitrous acid to nitric acid. The latter is 

 immediately neutralized by the soil bases and changed to nitrates. 



The nitrates may again (i) either be used by soil micro-organisms, in the pres- 

 ence of available energy material, and changed into microbial proteins; (2) or be ab- 

 sorbed by the roots of higher plants and used for plant nutrition; (3) or be leached 

 out from the soil; (4) or be reduced by various bacteria to nitrites, to ammonia, to 

 atmospheric nitrogen and simple gaseous oxides of nitrogen. The last process is carried 

 out by various micro-organisms, under partial anaerobic conditions in the absence of 

 atmospheric oxygen, when the nitrates are used as sources of oxygen. The reactions 

 may also take place in the process of assimilation of nitrates as sources of nitrogen. 



The nitrogen which is changed into the gaseous form is lost as far as the supply 

 of combined nitrogen in the soil is concerned. However, certain micro-organisms are 

 capable of utilizing the gaseous nitrogen and "fix" it in the soil, thus making it again 

 available for plant growth. This process of fixation of nitrogen is carried out either 

 symbiotically or non-symbiotically. In the first instance, the bacteria {B. radicicola 

 Beij.) can grow in association with leguminous plants, forming nodules on their roots, 

 the plants supplying the bacteria with the necessary energy and the bacteria "fixing" 

 the gaseous nitrogen, changing it into forms available for the leguminous plants. In 

 some instances bacteria form nodules on the leaves of plants. The non-symbiotic ni- 

 trogen-fixing bacteria, namely, the aerobic species of Azotobacter and other bacteria 

 and the anaerobic species of Clostridia {B. amylobacter), do not need any host plants, 



' Described in chap. xxiv. 



