12 EXPLOITATION OF PLANTS 
mineral nutrients have been derived from the soil and 
must be replaced when deficient. 
The prevailing idea, until recently, regarding the 
maintenance of soil fertility, was that each crop requires 
certain mineral elements from the soil, and these must 
be supplied by means of chemical fertilisers when they 
are lacking or are present in insufficient quantities. ~ 
Recent research, however, has shown that this 
“ mineral ” theory, which assumes that the fertility of 
a soil or its crop-producing power is dependent only, 
or even mainly, upon the mineral constituents which 
it may contain, is far from complete. 
The soil is not merely a reservoir for the mineral 
nutrients of plants, but is the seat of complex physical, 
chemical and biological actions which directly and 
indirectly influence soil fertility. These actions are 
intimately associated with the organic matter of the 
soil and its bacterial inhabitants. Mineralogy and 
inorganic chemistry, though helpful, are no longer 
capable of solving soil problems. Biochemistry and 
bacteriology, with their modern conceptions of colloids, 
absorption phenomena, enzymes, oxidising, reducing 
and catalytic actions, etc., are now rapidly extending 
our knowledge of the soil as a medium for plant growth. . 
Organic matter in the form of manure has been used 
from earliest times for promoting plant growth, but 
nothing definite was known as to its specific effect upon 
plants. It was believed that it acted in some mysterious 
way. Certain “ spirits ’’ were supposed to leave the 
decaying manure, enter the plant and produce more 
vigorous growth, leaving behind a leached substance of 
a worthless nature. The trend of early ideas respecting 
