68 THE SPIRIT OF THE SOIL 



constitution of humus, and it may perhaps be most 

 convenient to consider it first rather from the 

 biological and physical standpoints, and then from 

 the point of view of its chemical constitution and 

 relationships. 



The commonplace of natural history that the growth 

 of living beings is chiefly conditioned by the extent 

 and character of the food-supply is abundantly borne 

 out in the case of soil bacteria. Apart from the 

 organisms connected with the fixation of Nitrogen in 

 leguminous plants, which are only partially dependent 

 on decaying organic matter for their food-supply, the 

 growth, development, and activity of the soil bacteria 

 depend on the abundance and character of the humus 

 in the soil. It is this that provides them with the 

 energy they require in the form of food, which 

 improves the moisture and temperature conditions 

 of the medium in which they live, tending to keep 

 the latter more or less stable, and preventing violent 

 changes in temperature that would prove injurious 

 to their growth. It is only for very broad generaliza- 

 tions such as these that one can regard humus as a 

 simple substance of uniform composition. 



Biologically it falls into two grand classes the 

 mild humus or mull met with in arable soils and 

 woodland, and the raw humus found in heaths, 

 meadows, and swamps. 



Mild humus is either neutral or alkaline in reaction, 

 and is formed in conditions that allow the free entry 

 of air, and facilitate the development of the so-called 

 aerobic bacteria that is, of the bacteria that are 

 able to develop in the presence of oxygen. Raw 



