CHAPTER VIII 
BACTERIA AND SOIL MINERALS 
MINERALS NECESSARY FOR PLANTS 
Although the different minerals in the soil are needed by plants 
in smaller quantities than the nitrogenous foods, still they are quite 
as necessary, and vegetation cannot be supported without them. 
They come primarily from the rocks that form the earth’s crust. 
In these rocks there is practically an unlimited supply of the neces- 
sary minerals, but they must be rendered available as plant food. 
Most of these rocks contain their minerals in an insoluble condition 
and, in order to be absorbed by vegetation, they must be dissolved 
in the soil waters. Although this subject has not been studied so 
thoroughly as the transformation of nitrogen, still it is known that 
chiefly through the agencies of the soil microédrganisms the min- 
erals are brought into solution. 
LIME AND MAGNESIA 
These two minerals may be considered together since they are 
closely allied and their relations are the same. ‘The importance 
of lime to soil has long been recognized and our previous study has 
shown one of its most important uses. We have learned how 
necessary is the activity of the soil bacteria in the transformation of 
plant foods, and how, as a rule, bacteria cannot grow in the pres- 
ence of the slightest acid reaction. But general processes of de- 
composition are constantly giving rise to acids, so that the soils 
tend to become more and more acid. As the acidity increases the 
bacterial action declines and fertility correspondingly diminishes. 
The addition of lime to such soils is necessary, therefore, to reduce 
the acidity. Lime also improves the physical condition of the 
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