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 microorganisms the minerals 
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 presence 
of the slightest acid reaction. But general processes of decomposi- 
tion 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. Other needs there may be for lime, but the primary one is 
to keep the bacterial activities in the soil at a high state of activity. 
in 
