18 THE FERTILITY OF THE SOIL [CH. 



(2) Nitrates are formed from ammonia alone, 

 and not from any other substance so far as is known. 



(3) Nitrate production is the quickest of all 

 the chain of processes. 



(4) The quickest acting organisms seem to be 

 the most sensitive. 



We now turn to the formation of ammonia. This 

 has not proved so attractive a subject for investiga- 

 tion and we do not yet know much about it. Unlike 

 nitrification it is not a specific property of any one 

 organism, but is effected by many different kinds : it 

 is not even confined to bacteria, but goes on in a 

 vacuum or in presence of antiseptics. But it will not 

 go on after the soil has been heated to 150, whence 

 we may conclude that one of those accelerating agents 

 technically known as enzymes is also at work. It is 

 difficult to say precisely how much of the decom- 

 position is due to enzymes free in the soil and how 

 much to micro-organisms, but it seems certain that 

 the latter are by far the most potent agents. No one 

 has yet gone further back in the chain to discover 

 the compound antecedent to ammonia. The initial 

 compound is the nitrogenous part of the plant re- 

 sidues or of the substances added as manures, and 

 is generally of a protein nature : enzymes, earth- 

 worms, fungi and bacteria may all take part in its 

 decomposition ; we may suppose that its conversion 



