A mode of improvement often resorted to is the paring and burning of
poor land. The efficacy of burned clay, also, even in superseding
manure on good lands, has been highly extolled by some practical men.
1. The effect of paring and burning is easily understood. The matted
sods consist of a mixture of much vegetable with a comparatively small
quantity of earthy matter. When these are burned the ash of the plants
only is left, intimately mixed with the calcined earth. To strew this
mixture over the soil is much the same as to dress it with peat or
wood ashes, the beneficial effect of which upon vegetation is almost
universally recognised. And the beneficial influence of the ash itself
is chiefly due to the ready supply of inorganic food it yields to the
seed, and to the effect which the potash and soda it contains exercise
either in preparing organic food in the soil, or in assisting its
digestion and assimilation in the interior of the plant.
Another part of this process is, that the roots of the weeds and poorer
grasses are materially injured by the paring, and that the subsequent
dressing of ashes is unfavourable to their further growth.
2. Much greater uncertainty hangs over the alleged virtues of burned
clay. That benefits are supposed to have been derived from its use
there can be no doubt, though in many cases the better tillage of the
land generally prescribed along with the use of burned clay, may have
had some share in producing the good results actually experienced
during its use.
By the burning, in kilns or otherwise, any organic matter the clay may
contain will be consumed, and the texture of the clay itself will be
mechanically altered. It will crumble down like a burned brick into a
hard friable powder, and will never again cohere into a paste as before
the burning. It will, therefore, render clay soils more open, and may
thus, when mixed in large quantity, produce a permanent amelioration
in the mechanical texture of many stiff wheat soils. It cannot itself
undergo any chemical change that is likely so to alter its constitution
as to make it a more useful chemical constituent of the soil than
before. Any saline matter we may suppose to be set free could be far
more cheaply added in the form of a top-dressing to the soil.
Bricks, however, are generally more porous than the clay from which
they are formed; burned clay is so also. And all porous substances suck
in and _condense_ much air and many vapours in large quantities into
their pores. In consequence of this property, porous substances, like
charcoal and burned clay, are supposed, when mixed with the soil, to be
continually yielding air to decaying vegetable matter on the one hand,
and as continually re-absorbing it from the atmosphere on the other,
and by this means to be of singular service in supplying the wants of
plants in the earlier seasons of their growth. The vapours of nitric
acid and of ammonia, which float in the air, they are also supposed to
imbibe, and by the beneficial action of the substances believed to be
thus conveyed by burned clay into the soil, the fertilizing virtues
ascribed to it are attempted to be explained.
It must be confessed, however, that on this point considerable
obscurity still rests. It is in some measure doubtful what the true
action of charcoal and of burned clay is, both in _kind_ and in
_quantity_. It is the part of science, therefore, to decline offering
more than a mere conjecture till the facts to be explained are more
fully and satisfactorily demonstrated.