VII.] CHANGES IN THE COMPOSITION OF DUNG 185 



bonate is protected from evaporation, no loss of nitrogen 

 will result, but the more surface it exposes to the air 

 and the higher the temperature, the greater will be the 

 amount of ammonia passing off in a gaseous condition. 

 Thus thin films of urine on the floors, walls, or even on 

 the surface of loose straw, easily lose nitrogen by the 

 fermentation of the urea and subsequent volatilisation 

 of the ammonia; the smell of a stable arises in this way 

 and is clear evidence of the escape of ammonia. As 

 will be brought out more clearly later, this volatilisation 

 of ammonia causes most of the loss of nitrogen that 

 takes place in making dung. 



The ammonium carbonate is itself subject to change 

 and even to loss by other actions than evaporation : 

 there are always present in the manure heap various 

 bacteria which can oxidise ammonia into free nitrogen 

 gas and water ; in consequence dung which is allowed 

 to lie about loosely grows poorer in nitrogen from this 

 cause as well as through volatilisation of ammonia. 

 Though the action has been recognised as taking place 

 in practice, little is known of the specific bacteria which 

 set free gaseous nitrogen in this way, a process which 

 is often called " denitrification," though the term is 

 better restricted to the change whereby nitrates are 

 reduced to nitrogen gas. Nor have the conditions 

 favourable to this change been closely investigated ; it 

 is, however, certain that rapid oxidation such as is 

 brought about by a loose condition of the manure or by 

 turning it, will be accompanied by some destruction of 

 ammonia. It is also favoured by the presence of soluble 

 carbohydrates — i.e.^ easily oxidisable material — and it is 

 materially reduced, if not suspended, as soon as these 

 substances have been used up. 



Another group of bacteria which are extremely 

 abundant in fresh faeces are the so-called putrefactive 



