316 SOILS AND MANURES 



free hydrogen. The peculiar organised structure of the 

 straw vanishes, and what remains is humus (p. 109). 

 The humic acid combines with the alkaline carbonates of 

 ammonia and potash, liberated .by the fermentation, 

 forming soluble compounds which impart the dark brown 

 colour to the liquid that drains from manure heaps. 

 This change is characteristic of the conversion from 

 the fresh or long to the short condition. It does not 

 necessarily involve any loss of fertilising ingredients, 

 but on the contrary, by reducing the quantity of dry 

 matter it tends to increase the proportion of fertilizing- 

 constituents in the part that remains. 



Fermentation of the Nitrogenous Matter. The effects 

 of the fermentation of the nitrogenous matter are not so 

 readily apparent as those above described, but they are 

 considerably more important. The nitrogenous matter 

 of the urine consists of comparatively simple compounds 

 and is probably the first to undergo those changes by. 

 which ammonium carbonate is produced. The formation 

 of this substance from urea takes place in two stages 

 which may be represented by the following equations : 



(NH 2 ) 2 CO + H 2 = NH 4 .O.CO.NH 2 



Urea. Water. Ammonium 



carbamate. 



NH 4 .O.CO.NH 2 + H 2 = (NH 4 ) 2 C0 3 



Ammonium Water. Ammonium 



carbamate. carbonate. 



It will be seen that both reactions consist in the addi- 

 tion of the elements of water; they proceed simul- 

 taneously and may be represented by a single equation. 

 The change can be brought about by a number of different 

 organisms and probably takes place either in the presence 

 or absence of oxygen without evolution of nitrogen. 



The nitrogenous compounds of the dung are, for the 



