26 THE SPIRIT OF THE SOIL 



from which other plants and animals can build up 

 their tissues. 



Such are some of the broad generalizations that 

 constitute the foundations on which the plant 

 bacteriologist is called upon to build, and already 

 from them he has been able to formulate other 

 narrower generalizations that are of vital importance 

 to the art of agriculture. We shall see more of them 

 in later chapters, but it may be helpful at this stage 

 barely to outline the chief points on which, as we 

 know it at present, the question of soil fertility from 

 the bacterial standpoint mainly depends. One may 

 accept it as broadly true that we know the con- 

 ditions of soil fertility if we are able to trace the 

 processes by which a load of farmyard manure 

 becomes available as plant food. 



When the manure is thrown freshly on to the heap 

 it consists essentially of a heterogeneous mass of 

 material, loaded up with bacteria, and containing a 

 great variety of Carbon compounds, with traces of 

 Phosphates, Potash, and so forth. It is with the 

 Carbon compounds that we are concerned. These 

 fall into two great classes that in which the Carbon 

 is combined with various quantities of Hydrogen 

 and Oxygen, and that in which Nitrogen is also one 

 of the constituents (the proteins). As they are 

 found in fresh manure neither class is available as a 

 plant food. Common farmyard experience tells us 

 that the manure-heap heats and shrinks, and it needs 

 no great effort of imagination to realize that a process 

 is at work comparable with what occurs when a bon- 

 fire is being burnt. And this is actually what is 



