586 PRINCIPLES OF SOIL MICROBIOLOGY 



not be assumed that the addition of available carbohydrates to various 

 soils is always sufficient to induce non-symbiotic fixation of nitrogen. 

 In the presence of available nitrogen in the soil, the addition of carbo- 

 hydrates stimulates the development of various fungi and bacteria which 

 use the added source of energy and transform the available nitrogen 

 into microbial protein. In the presence of available nitrogen, the non- 

 symbiotic nitrogen-fixing bacteria will act upon the carbohydrates 

 like the other heterotrophic bacteria, merely synthesizing proteins. 

 Only in the absence of available nitrogen is there a probability of nitro- 

 gen fixation by non-symbiotic bacteria. But even when fixation of 

 nitrogen takes place the process is usually a slow one in normal soils; 

 in many cases, the actual amount of nitrogen fixed falls within the prob- 

 able error for the determination of total nitrogen. There are undisputed 

 claims in the literature that very porous soils of a moderately high water 

 content can fix small amounts of nitrogen under sterile conditions. 117 

 There is still more definite evidence that appreciable quantities of nitro- 

 gen can be fixed both in the laboratory and in the field by non-symbiotic 

 bacteria, when there is sufficient available energy. 118 Remy found that 

 considerable nitrogen fixation takes place as long as provision is 

 made for the neutralization of the acids formed and a proper source of 

 energy is present. The nitrogen fixed by the bacteria becomes a proper 

 source of nitrogen for higher plants; it becomes available slowly, although 

 not less so than the most active organic fertilizers. It has been sug- 

 gested 119 that the great economy with which the nitrogen fixing bacteria 

 use the organic matter in the soil is due to their symbiotic action with 

 algae. There is no doubt that the nitrogen content of sand or soil may 

 be appreciably increased by the activity of Azotobacter, if sufficient 

 energy is supplied. 119 About 6 mgm. of nitrogen were fixed per 1 gram of 

 plant residue, under laboratory experiments, and up to 9 mgm. in pot 



117 Warmbold, H. Untersuchungen ilber die Biologie stickstoffbindender 

 Bakterien. Landw. Jahrb., 35: 1-123. 1906. Centrbl. Bakt. II, 20: 121-126. 

 1907. 



118 Koch, A., Litzendorff, J., Krull, F., and Alves, A. Die Stickstoffanreich- 

 ung des Bodens durch freilebende Bakterien und ihre Bedeutung fur die Pflan- 

 zenernahrung. Jour. Landw., 55: 355-416. 1907; 57: 269-286. 1909; Remy, 

 Th. Untersuchungen liber die Stickstoffsammlungsvorgange in ihrer Beziehung 

 zum Bodenklima. Centrbl. Bakt. II, 22: 561-651. 1909; Lohnis, F. Centrbl. 

 Bakt., 15: 361. 1905; Schneidewind. Ibid., 21: 437. 1908; H. Fischer. Ibid., 

 22: 654. 1909. 



119 Krainsky, A. Uber die Stickstoffanreicherung des Bodens. Centrbl. 

 Bakt. II, 26: 231-235. 1910. 



