16 EXPLOITATION OF PLANTS 
organic matter serving simply as a trough for mineral 
plant nutrients. We now think of it as a porous mass 
made up of a skeleton of sand coated with a colloidal 
mixture of clay and organic matter, which serves as a 
nutrient medium for myriads of bacteria whose necessary 
supplies of air and water are contained in its pores. 
To quote Dr. E. J. Russell, “‘ Into the pores of this 
mass we have no means of penetrating ; no microscope 
has been devised that enables us to look into it and see 
what is going on. . . . We shall find the study of the 
soil very unsatisfying and uninspiring if we become too 
much absorbed in its utilitarian aspects and forget to 
stop and reflect on the infinite wonder of its honey- 
combed structure and its dark recesses, inhabited by a 
teeming population so near to us and yet so hopelessly 
beyond our ken that we can only form the dimmest 
picture of what the inhabitants are like and how they 
live.” 
But what is the work they do?’ Knowledge as to 
this has been slowly accumulating during the last few 
years, and we are learning that the all-important work 
of the decay of animal and vegetable matter in the soil 
is carried out through the agency of these bacteria. 
This process is effected in several stages, and distinct 
groups of bacteria are responsible for each stage. One 
thing is certain, they are absolutely dependent for their 
life and activities on the organic matter of the soil. 
No soil can be truly fertile unless it contains organic 
matter—humus material—as food for the bacteria which 
bring about decomposition. 
Hitherto attention has been directed chiefly towards 
the organisms concerned in the nitrogen cycle in the 
