However much lime or other fertilizers may be applied to the soil, still
great benefit is derived from the use of plaster, (sulphate of lime.)
After all, plaster is the main dependence of the potato-grower, a help
on which he may rely with the utmost confidence. Astonishing results are
obtained from its use, when applied in a proper manner. The writer has
seen a field, all of the same soil, all prepared alike, and all planted
with the same variety at the same time, on one half of which, that had
no plaster, the yield was but sixty bushels per acre, and many rotten;
the other part, to which plaster was applied in the manner hereafter
explained, yielded three hundred and sixty bushels per acre, and not an
unsound one among them.
The action of plaster is often puzzling. From the fact that where land
has been strongly limed, a small quantity of plaster applied shows such
decided benefit, there would seem plausibility in Liebig's theory that
its effects must be traceable not to the lime, but to the sulphuric
acid. The ammonia in rain-water in the form of carbonate (a volatile
salt) is decomposed by plaster, the sulphuric acid having greater
affinity for it, thus forming two new compounds, sulphate of ammonia and
carbonate of lime. But as arable soil has the same property of absorbing
ammonia from the air and rain-water, and fixing it in the same or even a
higher degree than lime, there is only the sulphuric acid left to look
to for an explanation of the favorable action of plaster on the growth
of plants.
It is found that plaster in contact with soil undergoes decomposition,
part of the lime separating from the sulphuric acid, and magnesia and
potash taking its place, quite contrary to the ordinary affinities.
These facts show that the action of plaster is very complex, and that it
promotes the distribution of both magnesia and potash in the ground,
exercising a chemical action upon the soil which extends to any depth of
it; and that, in consequence of the chemical and mechanical
modifications of the earth, particles of certain nutritive elements
become accessible and available to plants that were not so before.
It is said plaster is of most benefit in wet seasons; such is not
always the case. It is certainly beneficial to clover, wet or dry; so of
potatoes.
A few years since, when the drought was so intense in this section as to
render the general potato crop almost a total failure, the writer
produced a plentiful crop by the use of plaster alone. On examination at
the dryest time, the bottoms of the hills were found to be literally
dust, yet in this dust the tubers were swelling finely: the leaves and
vines were of a deep rich green, and remained so until frost, while
other fields in sight, planted with the same variety, but not treated
with plaster, were brown, dead, and not worth digging. That gypsum
attracts moisture may be proved by plastering a hill of corn and leaving
a hill by it unplastered; the dew will be found deposited in greater
abundance on the plastered hill. But, according to Liebig, certain
products of the chemical action of plaster enter into and are
incorporated with the structure of the plant, closing its breathing
pores to such an extent that the plant is enabled to withstand a drought
which would prove fatal to it unassisted.
Certain it is that plaster renders plants less palatable to insects,
and, so far as the writer's experiments extend, it is fatal to many of
the fungi family. To obtain the best results, the vines of potatoes
should be dusted with plaster as soon as they are fairly through the
soil, again immediately after the last plowing and hoeing, and, for
reasons hereafter given, at intervals throughout the whole growing
season. The first application may be light, the second heavier, and
thereafter it should be bountifully applied, say two hundred pounds per
acre at one sowing.