quent upon the
removal of both grain and straw from soils which contain but a limited
supply of that substance in an available condition is obvious. It is
clear that under such circumstances the frequent repetition of a cereal
crop may so far diminish the amount of available silica as to render its
cultivation impossible, although the other substances may be present in
sufficient quantity to produce a plentiful crop of any plant which does
not require that element. Beans and peas, turnips and hay, on the other
hand, require a very large quantity of alkalies, and especially of
potash.

Looking more minutely, however, into this matter, certain points attract
attention which appear to be at variance with commonly received
opinions. With the exception of silica, for example, the cereals do not
withdraw from the soil so large a quantity of mineral matters as some of
the so-called fallow crops, and if their straw be returned to the soil
they are by far the least exhaustive of all cultivated plants; and we
thus recognise the justice of that practical rule, which lays it down as
an essential point of good husbandry that the straw ought, as far as
possible, to be consumed on the farm on which it is produced. As regards
the general constituents of the ash, it is also to be remarked that
though differences in their proportions exist, they are by no means so
marked as might be expected; thus there are no plants for which a large
quantity of potash, nitrogen, and phosphoric acid is not required; and
it is not very easy to see how the substitution of the one for the other
should be of much importance in this respect. Indeed, the more minutely
the subject is examined, the more do we become convinced of the
insufficiency of that view which attributes the necessity for a rotation
of crops to differences in chemical composition alone. There can be no
doubt that the nature of the plant and the particular mode in which it
gathers its nutriment, have a most important influence. Certain plants
are almost entirely dependent on the soil for their organic
constituents, while others derive a large proportion of them from the
air, and a plant of the latter class will flourish in a soil in which
one of the former is incapable of growing. In other cases, the structure
and distribution of the roots is the cause of the difference. Some
plants have roots distributed near the surface and exhaust the
superficial layer of the soil, others penetrate into