98 AGRICULTURAL BACTERIOLOGY 



teria went on unrestrained; hence a greater amount of 

 nitrogen became available for the crop. 



The nitrogen that has been built into the tissues of the 

 animal body is broken down into simpler compounds as a 

 result of the metabolic activities of animals. The nitrogen 

 waste from animals is eliminated in the urine in the form 

 of urea, hippuric acid, and uric acid. The amount and 

 relative proportions of these three compounds vary in 

 the different animals. It is estimated that 60 per cent, of 

 the nitrogen of the food is eliminated by the horse in urine, 

 42 per cent, in the case of sheep, and 31 per cent, in cat- 

 tle. In all cases the nitrogen in the three compounds men- 

 tioned is changed into ammonia by means of a group of 

 bacteria that are most often termed the urea- ferment ing 

 organisms. They differ widely in their form and struc- 

 ture, but all have the common property of forming an 

 enzyme, called urease, which changes the urea to ammon- 

 ium carbonate. This group is found in the soil and in 

 the solid excrement of animals. The mixing of the solid 

 and liquid excrement results in the rapid change of the 

 urea and allied compounds to ammonia. The strong odor 

 of ammonia often noticed in horse stalls is due to the fer- 

 mentation of the urea. 



The urea-fermenting bacteria require oxygen for their 

 growth. They are also favored by a strong alkaline reac- 

 tion, and are able to continue growth in the presence of 

 quantities of ammonia that would quickly stop the growth 

 of the common putrefactive bacteria. The urine of herbi- 

 vorous animals is alkaline in reaction, while that of man 

 and the carnivorous animals is acid. The former is a 

 better medium for growth of the urea-fermenting organ- 

 isms. Uric acid is changed directly to ammonium carbon- 

 ate or to urea and then to ammonia. Hippuric acid is 

 likewise fermented with the formation of ammonium car- 



