VI. MANURING AND GENERAL MANURES 115 



nitrogen is converted into ammonia. Leucine by its fermentation forms 

 valerianic acid, C 4 H 9 .COOH, ammonia, carbon dioxide and hydrogen. 



(c) Putrefactive fermentation is the rapid decomposition of albu- 

 minoid substances, attended by the evolution of evil-smelling gases and 

 produced by the agency of various species of bacteria. Generally, 

 the first step is the conversion of the insoluble or colloidal albuminoids 

 into soluble and diffusible peptones ; these next split up, yielding 

 amino-acids, e.g., leucine. These in turn are decomposed into fatty 

 acids and ammonia and the fatty acids then ferment as described 

 under (a). 



However, the kind of change produced is determined to a great ex- 

 tent by the admission or exclusion of air from the fermenting substances. 

 Indeed, the presence or absence of air from the decomposing mass de- 

 termines the species of bacteria which can flourish in it. Bacteria are 

 sometimes classed into two great groups : 



Aerobic bacteria, which require the presence of oxygen, and 



Anaerobic bacteria, which only perform their functions in the ab- 

 sence of oxygen. 



The distinction is not altogether satisfactory, as under conditions 

 of air exclusion, certain aerobic organisms can carry on their work if 

 nitrates be present. In the case of the aerobic bacteria the compounds 

 formed by their vital processes are usually of a simple character, e.g., 

 water, carbon dioxide and ammonia. Such fermentation is sometimes 

 called mouldering or decay and is not attended by the evolution of 

 foul-smelling gases. 



The anaerobic organisms, on the other hand, tend to produce un- 

 oxidised products, some of which are highly complex, and the gases 

 evolved consist of methane, hydrogen, sulphuretted hydrogen, etc. 

 Many of these products are possessed of disagreeable smells and the 

 changes leading to their production are considered as true putrefaction. 

 These putrefactive changes can be brought about by a great number of 

 different organisms and the albuminoid substances are the chief raw 

 material for their activity. Many of the bad-smelling gases evolved 

 have not been identified. Among other products, amines, e.g., trimethy- 

 lamine, N(CH 3 ) 3 , volatile acids, e.g., butyric acid, C 3 H 7 .COOH, and 

 caproic acid, C 5 H n .COOH, and mercaptans, e.g., C 2 H 5 .SH, have been 

 detected. 



In a manure heap, both aerobic and anaerobic organisms perform 

 their functions, the former chiefly at first, and until the air in the 

 interstices of the manure has been exhausted, producing mainly carbon 

 dioxide, water and ammonia. Then the anaerobic bacteria begin to 

 operate and evil-smelling gases come off. The evolution of heat, 

 which is often considerable during the life of the aerobic organisms, 

 diminishes. 



(d) Ammoniacal or urea fermentation. This has already been 

 briefly referred to (vide p. 63). The main reaction is there given as 



CO(NH 2 ) 2 + 2H 2 = (NH 4 ) 2 C0 3 , 



thus giving ammonium carbonate ; in addition a reaction expressed by 

 the following equation : 



