Moreover, if it should happen that the soil-water is
stronger than the water within the root-hair, the water would tend
to pass from the plant into the soil. This is the condition that
prevails in many alkali lands of the West, and is the cause of the
death of plants growing on such lands.
It is clear that under these circumstances not only water enters the
root-hairs, but many of the substances found in solution in the
soil-water enter the plant also. Among these are the mineral
substances which are indispensable for the proper life and growth of
plants. These plant nutrients are so indispensable that if any one
of them is absent, it is absolutely impossible for the plant to
continue its life functions. The indispensable plant-foods gathered
from the soil by the root-hairs, in addition to water, are:
potassium, calcium, magnesium, iron, nitrogen, and phosphorus,--all
in their proper combinations. How the plant uses these substances is
yet poorly understood, but we are fairly certain that each one has
some particular function in the life of the plant. For instance,
nitrogen and phosphorus are probably necessary in the formation of
the protein or the flesh-forming portions of the plant, while potash
is especially valuable in the formation of starch.
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