HUMUS 65 



in the following chapter, as it is chiefly they and 

 their activities that are the subject of the present 

 volume. Before doing so it is necessary to form a 

 clear idea of the non-living medium in which they 

 live and of its properties. 



The soil from the particular standpoint which I 

 am considering may be regarded as a mixture of two 

 constituents (i) mineral debris, and (2) decaying 

 organic matter or humus. 



The mineral debris in the soil plays no very inter- 

 esting part in connection with the life of the soil 

 bacteria. It is in most cases only soluble with 

 difficulty, and, with the exception of lime, influences 

 bacterial life rather from a negative standpoint. 

 Thus, a clay soil will retain an excess of moisture 

 and interfere with bacterial growth ; a sandy soil, on 

 the other hand, by allowing excessive drainage, will 

 unduly parch the ground, and eventually check 

 bacterial activities. Conditions well known to the 

 practical agriculturist will bring about an acid state 

 of the soil, in which the bacteria will find growth 

 impossible. These and other conditions will be 

 considered in the chapter in which the practical 

 application of the bacterized peat or humogen is 

 discussed, but may be ignored here, as they may 

 fairly be regarded as factors superimposed on normal 

 conditions. 



The centre of bacterial activity is the humus or 

 decaying organic matter in the soil. Bacteria differ 

 essentially from plants in that they have no mechan- 

 ism enabling them to derive their energy from the 

 rays of the sun, but, like animals, must secure it from 



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