The general nature and mode of operation of such mineral substances as
are capable of acting as manures, will be in some measure understood
from what has already been so fully stated in regard to the necessity
of inorganic food to living plants, and to the kinds of such food
which they specially require. A slight notice, therefore, of the more
important of these manures now in use will here be sufficient.
1. _Nitrates of Potash and Soda._—Saltpetre and nitrate of soda have
been deservedly commended for their beneficial action, especially
upon _young_ vegetation. They are distinguished by imparting to the
leaves a beautiful dark green colour, and are applied with advantage
to grass and young corn, at the rate of 1 cwt. to ½ cwt. per acre. The
nitric acid they contain yields nitrogen to the plant, while potash and
soda are also put within reach of its roots, and no doubt serve many
beneficial purposes.
_Sulphate of Soda_, or Glauber’s salt, has lately been recommended in
this country for clovers, grasses, and green crops. Mixed with nitrate
of soda it produces remarkable crops of potatoes.[19]
[19] See the author’s “_Suggestions for Experiments in Practical
Agriculture_,” Nos. 1 & 2.
_Sulphate of Magnesia_, or Epsom salts, might also be beneficially
applied in agriculture, probably to clovers and corn crops. As it can
be had in pure crystals at 10s. a cwt., and in an impure state at a
much less price, from the alum works, it might readily be submitted to
trial.
_Sulphate of Lime_, or Gypsum, is in Germany applied to grass lands
with great success, over large tracts of country. In the United States
it is used for every kind of crop. It is especially adapted to clovers
and legumes.
These three substances all afford sulphur to the growing plant,
while the lime, soda, and magnesia are themselves in part directly
appropriated by it, and in part employed in preparing other kinds of
food, and in conveying them into the ascending sap.
Though there can be no question that these and similar substances are
really useful to vegetation, yet the intelligent reader will not be
surprised to find, or to hear, that this or that mineral substance has
not succeeded in benefitting the land in this or that district. If he
has already bricks enough at hand, you must carry the builder mortar,
or he will be unable to go on with his work: so, if the soil contain
gypsum or sulphate of magnesia in sufficient natural abundance, it is
at once a needless and a foolish waste to attempt to improve the land
by adding more; it is still more foolish to conclude that these same
saline compounds are unlikely to reward the patient experimenter in
other localities.
_Common Salt_ has undoubtedly, in very many districts, a fertilizing
influence upon the soil. The theoretical agriculturist knows that a
small quantity of it is absolutely necessary to the healthy growth
of all our cultivated crops, and he will therefore, early try by a
preliminary experiment upon one of his fields, whether or not they
require the addition of this species of vegetable food. It is in inland
and sheltered situations, and on high lands often washed by the rains,
that the effect of common salt is likely to be most appreciable. The
spray of the sea, borne to great distances by the winds, is in many
districts, where prevailing sea winds are known, sufficient to supply
an ample annual dressing of common salt to the land.
_Kelp._—Among mineral substances kelp ought not properly to be
included, since it is the ash left by the burning of sea-weed. It,
however, partakes of the nature of mineral substances, and may,
therefore, be properly considered in this place. It contains potash,
soda, silica, sulphur, chlorine, and several other of the inorganic
constituents of plants required by them for food. It is nearly the same
also—with the exception of the organic matter which is burned away—with
the sea-weed which produces such remarkably beneficial effects upon the
soil. In the Western Isles a method is practised of half burning or
charring sea-weed, by which it is prevented from melting together, and
is readily obtained in the form of a fine black powder. The use of this
variety ought to combine the beneficial action of the ordinary saline
constituents of kelp, with the remarkable properties observed in animal
and vegetable charcoals.
_Wood-ash_, among other compounds, contains a portion of common
_pearl-ash_ in an impure form, with sulphate also, and _silicate_ of
potash. These are all valuable in feeding and in preparing the food of
plants, and hence the extensive use of wood-ash as a manure in every
country where it can readily be procured.
_Dutch ashes_ are the ashes of peat burned for the purpose of being
applied to the land. They vary in constitution with the kind of peat
from which they have been prepared. They often contain traces of potash
and soda, and generally a quantity of gypsum and carbonate of lime, a
trace of phosphate of lime, and much siliceous matter. In almost every
country where peat abounds, the value of peat ashes as a manure has
been more or less generally recognised.