tion of crops is now generally attributed to the
different quantities of valuable matters which different plants remove
from the soil, and more especially to their mineral constituents. It has
been already observed that great differences exist in the composition of
the ash of different plants in the section on that subject; and it was
stated that a distinction has been made between lime, potash, and silica
plants, according as one or other of these elements preponderate in
their ashes. The remarkable difference in the proportion of these
elements has been supposed to afford an explanation of rotation. It is
supposed that if a plant requiring a large quantity of any one element,
potash, for example, be grown during a succession of years on the same
soil, it will sooner or later exhaust all, or nearly all, the potash
that soil contains in an _available_ form, and it will consequently
cease to produce a luxuriant crop. But if this plant be replaced by
another which requires only a small quantity of potash and a large
quantity of lime, it will flourish, because it finds what is necessary
to its growth. In the meantime, the changes which are proceeding in the
soil, are liberating new quantities of the inorganic matters from those
forms of combination in which they are not immediately available, and
when after a time the plant which requires potash is again sown on the
soil, it finds a sufficient quantity to serve its purpose. We have
already, in treating of the ashes of plants, pointed out the extent of
the differences which exist; but these will be made more obvious by the
annexed table, giving the quantity of the different mineral matters
contained in the produce of an imperial acre of the different crops.

TABLE shewing the quantities of Mineral Matters and Nitrogen in average
Crops of the principal varieties of Farm Produce.

+---------------+--------------+---------+----------+---------+-------+-------+
| | Produce per | Total | Total | | | |
| | Imperial | Weight | Mineral | Potash. | Soda. | Lime. |
| | Acre. | in lbs. | Matters. | | | |
+---------------+--------------+---------+----------+---------+-------+-------+
|Wheat--Grain | 28 bushels | 1,680 | 34.12 | 10.11 | 1.20 | 1.04 |
| | at 60 lbs. | | | | | |
| Straw | 1 ton 3 cwt. | 2,576 | 114