3^4 ^ MICROBIOLOGY OF SOIL 



cropping. The mechanical composition of the soil influences the pro- 

 portion of aerobic and anaerobic species, while the chemical composi- 

 tion, particularly that of the humus, influences the rate of multipHca'- 

 tion and the character of the chemical transformation accomplished. 

 It is well known, for example, that additions of fresh organic matter 

 intensify the rate of decomposition of the soil humus, and, likewise, 

 ammonia production as has been already demonstrated by Breal. In a 

 more general way it was proved by Lipman and his associates that, 

 with a constant bacterial factor, ammonia production in soils varies with 

 the chemical and mechanical composition of the latter. In some of 

 these experiments loo g. portions of different soils were each mixed with 

 S g. of dried blood, sterilized in the autoclave, cooled arid inoculated 

 with equal quantities of infusion from fresh soil. The foUowing 

 amounts of ammonia nitrogen were produced in six days: 



Soil Ammonia nitrogen found 



A 31.62 mg. 



B 68.29 ™g- 



C 117.06 mg. 



P 107 . 16 mg. 



E ••. • 156.47 mg. 



With all other factors constant, chemical and mechanical differences 

 in the soil used were responsible for striking variations in ammonia 

 production, as indicated by the figures given above. 



The influence of temperature and moisture conditions is fully as 

 important as that of the chemical and mechanical composition of the 

 soil. The following data secured by Lipman may be cited in this 

 connection as showing the effect of moisture: 



One-hundred-gram quantities of air-dried soil were each mixed with 

 3 g. of dried blood and varying amounts of water added. The ammonia 

 formed was distilled off and determined at the end of eight days. 

 The amounts of ammonia nitrogen found were as follows: 



' Water added Ammonia nitrogen found 



c.c 4.13 mg. 



1 c.c 4. 13 mg. 



3 c.c 5.40 mg. 



5 c.c 10.64 mg. 



7 c.c 26.37 mg- 



loc-c ....,,.. 49.57 mg. 



12 c.c 70.71 mg. 



IS c.c 93.90 mg. 



