A comparatively large quantity of the plant is, therefore,
drawn directly from the soil. Among the ash ingredients are many
which are taken up by the plant simply because they are present in
the soil; others, on the other hand, as has been shown by numerous
classical investigations, are indispensable to plant growth. If any
one of these indispensable ash ingredients be absent, it is
impossible for a plant to mature on such a soil. In fact, it is
pretty well established that, providing the physical conditions and
the water supply are satisfactory, the fertility of a soil depends
largely upon the amount of available ash ingredients, or plant-food.
A clear distinction must be made between the_ total _and _available
_plant-food. The essential plant-foods often occur in insoluble
combinations, valueless to plants; only the plant-foods that are
soluble in the soil-water or in the juices of plant roots are of
value to plants. It is true that practically all soils contain all
the indispensable plant-foods; it is also true, however, that in
most soils they are present, as available plant-foods, in
comparatively small quantities.
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