172 THE LIVING ORGANISMS OF THE SOIL [chap. 



to be described later, in its neutral reaction and in 

 the readiness with which it can be further oxidised. 

 Neutral in its reaction, it yields but little soluble 

 "humic acid" to the attack of an alkali. 



Besides carbohydrates, most aerobic bacteria require 

 some carbon compound of nitrogen, and will begin to 

 break down protein and other nitrogen-containing 

 materials. The products of their attack are succes- 

 sively peptones, bodies like leucin and tyrosin, even- 

 tually ammonia, and probably free nitrogen, but the 

 ultimate production of ammonia is perhaps the most 

 characteristic feature of the aerobic fermentation of 

 protein bodies. Other amides are also resolved into 

 ammonia, of which a characteristic example is afforded 

 by the change of urea into ammonium carbonate. 

 This process [which is one of hydrolysis, not of oxi- 

 dation, being represented in the gross by the equation 

 CO(NH 2 ) 2 +2H 2 = (NH 4 ) 2 C0 3 ] is brought about by 

 more than one organism, universally distributed and 

 abundant in such places as stables and cattle stalls. 

 In warm weather the conversion of the urea of the 

 urine into ammonium carbonate is very rapid, and as 

 the re3ulting product dissociates into gaseous ammonia 

 and carbonic acid, to this cause is due the smell of 

 ammonia which is always to be noticed in such places. 

 These changes to ammonia are the necessary prelim- 

 inaries to the final oxidation process or nitrification, 

 which, as the means by which the higher plants receive 

 their supplies of nitrogen, will be discussed separately. 

 The various oxidation processes in the soil are, like 

 all other bacterial actions, promoted by a certain 

 warmth, the optimum temperature being about 25°-30°, 

 by a sufficiency of moisture, and by the presence of 

 mineral food, like phosphates and potash salts. In 

 any great quantity, however, salts are harmful, 



