488 



PRINCIPLES OF SOIL MICROBIOLOGY 



However, this still remains to be confirmed. According to Sohngen, 46 

 urea offers an exclusive source of energy to the urea bacteria but not 

 a source of carbon, so that a carbohydrate is also necessary to insure a 

 growth of the organisms. The maximum hydrolysis of urea, however, 

 accompanies a minimum oxidation of organic compounds. B. erythro- 

 genes, for example, hydrolyzes 500 mgm. urea for every 20 mgm. of 

 carbon assimilated, while Urobac. jakschii hydrolyzes 1800 mgm. of 

 urea for 10 mgm. of carbon assimilated. 



^I^^jjfc/awaiamiae 



7 21 42 9 A days 



Fig. 26. Accumulation of ammonia from cyanamide and dried blood, as in- 

 fluenced by the presence of diacyanodiamide (from Cowie). 



Cyanamide readily breaks down in the soil yielding ammonia which 

 is then nitrified practically quantitatively. Cyanamide may first be 

 decomposed in the soil into urea by a purely chemical process, 47 under 

 the influence of catalyzers, or it may polymerize into dicyanodiamide 

 (especially in the presence of catalyzers such as ZnCl 2 ). Dicyanodi- 

 amide does not nitrify and is even toxic to nitrifying bacteria, but is 



46 Sohngen, N. L. Ureumspaltung bei Nichtvorhandensein von Eiweiss. 

 Centrbl. Bakt. II, 23: 91-98. 1909. 



47 Cowie, G. A. Decomposition of cyanamide and dicyanodiamide in the soil. 

 Jour. Agr. Sci., 9: 113-136. 1919; 10: 163-176. 1920. 



