196 THE LIVING ORGANISMS OF THE SOIL [chap. 



essentially the means by which they derive their 

 energy. As an intermediate step between the carbo- 

 hydrate and the carbonic acid, a certain amount of 

 humus is produced "mould," or the "mild humus" 

 of the German writers. Examples of this material 

 can be seen in the leaf-mould collected by gardeners 

 from woods, or the fine, brown powder which can be 

 scraped out of the inside of a hollow tree, particularly 

 of a willow ; this mould differs from the peaty humus, 

 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, event- 

 ually 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 CO] 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 resulting 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- 



