ll not grow in a
soil in which that element is deficient, although wheat or barley, which
require but little lime, may yield excellent crops. Again, if the soil
be deficient in phosphoric acid, those plants only will grow luxuriantly
which require but a small quantity of that element, and hence it follows
that on such a soil plants cultivated for the sake of their stems,
roots, or leaves, in which the quantity of phosphoric acid is small, may
yield a good return; while others, cultivated for the sake of their
seed, in which the great proportion of that constituent of the ash is
accumulated, may yield a very small crop. It is obvious also that even
where a soil contains a proper quantity of all its ingredients, the
repeated cultivation of a plant which removes a large quantity of any
individual element, may, in the course of time, so far reduce the amount
of that substance as to render the soil incapable of any longer
producing that plant, although, if it be replaced by another which
requires but little of the element thus removed, it may again produce an
abundant crop. On this principle also, attempts have been made to
explain the rotation of crops, which has been supposed to depend on the
cultivation in successive years of plants which abstract from the soil
preponderating quantities of different mineral matters. But though this
has unquestionably a certain influence, we shall afterwards see reason
to doubt whether it affords a sufficient explanation of all the observed
phenomena.

It may be observed, on examining the table of the percentage and
position of the ash, that some plants are especially rich in alkalies,
while in others lime or silica preponderate, and it would therefore be
the object of the farmer to employ, in succession, crops containing
these elements in different proportions. In carrying out this view,
attempts have been made to classify different plants under the heads of
silica plants, lime plants, and potash plants; and the following table,
extracted from Liebig's _Agricultural Chemistry_, in which the
constituents of the ash are grouped under the three heads of salts of
potash and soda, lime and magnesia, and silica, gives such a
classification as far as it is at present possible:--

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| | Salts of | Salts of | Silica.
| | Potash and | Lime and |
|