nces it has
proved successful.

The process of chalking, which has been carried out on a large scale in
some parts of England, and which consists in bringing up the chalk from
pits, penetrating through the overlying tenacious clay, and mixing it
with the soil, operates, to some extent, in a similar manner, though no
doubt the lime also exercises a strictly chemical action. It is probable
that the mixing of soils might be advantageously extended, and it merits
more minute study than it has yet obtained. Its use is obviously limited
by the expense, because, of course, where good effects are to be
obtained, it is necessary to remove large quantities of soil, in some
instances as much as 50 or 100 tons per acre, but the expense might be
much diminished if it were carried out methodically, and on a
considerable scale. The admixture of highly fertile soils with others of
inferior quality is also worthy of attention; indeed, it is understood
that this has been done, to some extent, with the rich trap soils of
some parts of Scotland, but the extent of the benefit derived from it
has not been made public.

FOOTNOTES:

[Footnote J: Mr. Dudgeon, Spylaw. Transactions of the Highland Society,
vol. cxxix., p. 505.]

CHAPTER VII.

THE GENERAL PRINCIPLES OF MANURING.

In their natural condition all soils not absolutely barren are capable
of supporting a certain amount of vegetation, and they continue to do so
for an unlimited period, because the whole of the substances extracted
from them are again restored, either directly by the decay of the
plants, or indirectly by the droppings of the wild animals which have
browzed upon them. Under these circumstances, a soil yields what may be
called its normal produce, which varies within comparatively narrow
limits, according to the nature of the season, temperature, and other
climatic conditions. But the case is completely altered if the crop, in
place of being allowed to decay on the soil, is removed from it, for,
though the air will continue to afford an undiminished supply of those
elements of the food of plants which may be derived from it, the fixed
substances, which can only be obtained from the soil, decrease in
quantity, and are at length entirely exhausted. In this way a gradual
diminution of the fertility of the soil takes place, until, after the
lapse of a period, longer or shorter, according to its natural
resources, it will become entirely incapable of maintaining