28 THE SPIRIT OF THE SOIL 



nitrate (N0 3 ), which will be found in the heap in 

 some such soluble form as Calcium Nitrate 

 [Ofc(N0 8 )J. 



Apart from the value of its mechanical structure, 

 and apart from the Phosphates and Potash that it 

 contains, the essential value of farmyard manure to 

 the land is that it possesses sufficient carbon com- 

 pounds in assimilable form to act as a food-supply 

 for the bacteria, and that it contains a large supply 

 of the nitrogenous food material essential for plant 

 nourishment. 



This is not the whole story, however. The bac- 

 teria of which I have so far spoken are the fairy god- 

 mothers of the romance the fairy super-godmother 

 has not yet appeared on the stage but the romance 

 itself would be incomplete if there were no hard- 

 featured, cross-grained witch. Experience has long 

 taught that the loosely piled heap will burn away 

 to waste if it is too loosely stored owing to the exces- 

 sive amount of oxygen supplied to the first group of 

 bacteria. In agriculture, as in politics, the advice of 

 Talleyrand especially as regards bacteria surtout pas 

 trop de zele is eminently sound, but however carefully 

 he goes to work, the farmer may find that the cross- 

 grained witch has come uninvited to the feast in the 

 shape of a denitrifying bacterium that seizes on the 

 precious nitrates designed as plant food, and waste- 

 fully turns them into the nitrogen that is useless for 

 plants, and escapes inert into the open air. 



It is the super-godmother, however, that comes to 

 the rescue. A group of bacteria exists; the name 

 of one of the group is Azotobacter, and it has the 



