CARBON AND NITROGEN CYCLES IN THE SOIL 191 



in the soil, if the conditions are at all favourable, that they 

 may reappear unless special precautions are taken to prevent 

 infection. Thus, it is commonly stated that treatment of the 

 soil with carbon disulphide merely depresses without killing 

 the organisms. Russell and Hutchinson found, however, that 

 the organisms did not reappear if the soil was kept carefully 

 free from re-infection (241^). 



In pure cultures the organisms cannot tolerate absence 

 of moisture, but die at once. In soil, however, they are more 

 resistant. Absence of air puts an end to their activity. 



There is some evidence that nitrobacter is more sensitive 

 to adverse circumstances than nitrosomonas ; it is also more 

 rapid in action. Otherwise the two sets of organisms show 

 very similar behaviour to external influences, their main ^ 

 difference being the fundamental one that nitrosomonas ^ 

 oxidises ammonia, but not nitrites, while nitrobacter oxidises [ 

 nitrites, but not ammonia. There are also certain morpho- 

 logical differences. Nitrosomonas, or coccus, occurs in several 

 forms, mostly oval in shape, 0*5 to i /a wide and up to 2 ^ long, 

 but whether these are really distinct varieties is not known ; 

 a zooglea stage is also found ; nitrobacter is rod-shaped, i //, 

 long and about 0*3 /i thick ; only one variety has been re- 

 cognised. No other organisms are known with certainty to 

 produce nitrates in the soil, nor can any other compound 

 except ammonia be nitrified {2iga). 



During the course of nitrification some calcium is dissolved, 

 but apparently no phosphate, except in culture solution.^ 



The Evolution of Gaseous Nitrogen. 



It has long been known that losses of nitrogen may occur 

 during the bacterial decomposition of organic matter which 

 cannot be attributed to the volatilisation of ammonia, and 

 which, therefore, are put down to an evolution of gaseous 

 nitrogen : instances are afforded by manure heaps, by rich 



1 Kelley, Journ. Agric. Res., 1918, 12, 682. But see Hopkins and Whiting 

 (III. Bui., 190, 1916), who maintain that phosphate is rendered soluble during 

 nitrification. 



