silicate of potash. It is only by the slow
disintegration of potash rocks that the potash they contain is set free
for plant uses. When it is applied as an artificial manure, on the other
hand, it is in a soluble form. In most soils the amount soluble in water
probably lies between .001 and .009 per cent; that soluble in dilute
acid solutions from .1 to .5 per cent; and that insoluble from .2 to 3.5
per cent of the soil. It is highly probable that a certain quantity of
potash in the soil may exist in combination with humic and ulmic acids,
forming insoluble potassium humates and ulmates.

_Potash in Plants._

Of all the ash ingredients of plants, potash is the most abundant, as it
forms on an average about 50 per cent of the total ash of plants--about
90 per cent of the alkalies. The ash of plants, indeed, was for long
the chief source of potash. Certain plants remove very large quantities
from the soil. Of these roots, potatoes, the vine, the tobacco-plant,
and hops may be mentioned as examples. It is present in large quantities
in the grain of cereals, although, as we have already pointed out, not
to the same proportional extent as phosphoric acid. It is found in the
plant's extremities, such as twigs and new leaves, in greatest
abundance.[129]

_Potash in the Animal Tissue._

It is also found in all parts of the animal body. Especially rich in
potash salts are the blood corpuscles, which contain about ten times the
amount contained in the serum. It is found in especial abundance in the
fleece of sheep, which may contain more potash than that in the whole
body of the sheep. Animal urine also contains potash in considerable
quantities.

_Sources of Loss of Potash._

The capacity of the soil to retain soluble potash compounds, while not
equal to its capacity for retaining phosphoric acid, is yet very much in
excess of its capacity for retaining nitrates. The result is, that
potash is only found in comparatively minute traces in drainage
water.[130] Taking the same example as we already cited in illustration
of the loss of phosphoric acid, we find that the amount carried away in
the course of a year in the waters of the Elbe from Bohemia is
97,000,000 lb. (43,300 tons).

_Potash removed in Crops._

The amount of potash removed by the different crops from the soil will
be considered in a subsequent chapter. We need only say here that the
class of crops which remove the largest quantity are the root crops