he same plot of ground, one
crop following the other for a long series of years, and never refresh
the soil with manure, it must be evident that we shall, some day or
other, find the crop fail through the exhaustion of the soil of its
available sulphur, phosphates, lime, or potash. But if this soil were
allowed to lie fallow for some time, it would again produce a crop of
Cabbage, owing to the liberation of mineral matters which, when the
crops were failing, were not released fast enough, but which, during the
rest allowed to the soil, accumulated sufficiently to sustain a crop.
Obviously this mode of procedure is unprofitable and tends of necessity
to exhaustion, although it must be confessed that utter exhaustion of
any soil is a thing at present almost unknown. But, instead of following
a practice which impoverishes, let us enrich the soil with manure, and
change the crops on the same plot, so that when one crop has largely
taxed it for one class of minerals, a different crop is grown which will
tax it for another class of minerals. Take for a moment's consideration
one of the necessary constituents of a fertile soil, common salt
(chloride of sodium). In the ash of a Cabbage there is about six per
cent. of this mineral, in the Turnip about ten per cent., in the Potato
two to three per cent., in the Beet eighteen to twenty per cent. On the
other hand the Beet contains very little sulphur, but both Turnip and
Beet agree in being strongly charged with potash and soda. It follows
that if we crop a piece of ground with Cabbage, and wish to avoid the
failure that may occur if we continue to crop with Cabbage, we may
expect to do well by giving the ground a dressing of common salt and
potash salts, and then crop it with Beet.

The whole subject is not exhausted by this mode of viewing it, for all
the facts are not yet fully understood by the ablest of our chemists and
physiologists, and crops differ in their methods of seeking nourishment.
We might find two distinct plants nearly agreeing in chemical
constitution, and yet one might fail where the other would succeed.
Suppose, for instance, we have grown Cabbage and other surface-rooting
crops until the soil begins to fail, even then we might obtain from it a
good crop of Parsnips or Carrots, for the simple reason that these send
their roots down to a stratum that the Cabbage never reached; and it is
most instructive to bear in mind that although the Parsnip will grow on
poo