Calcium. Lime exists in a soil sometimes as carbonate, that is,
air-slacked lime, but most generally as silicate, that is chemically
united to silicic acid, or sand. In the human body, in union with
phosphoric acid, it forms a very large part of the bones. It ex-
ists in combination with different acids, in the root, stalk, blade,
and grain of all plants.

Besides the necessity of its presence to form the structure of
plants, it performs many other important functions in a soil. It
promotes the decomposition of vegetable matter, and thus causes
it to yield the different mineral substances which it contains, as
means for the production of another growth. It improves the
mechanical texture of the land; which is but one of its smallest
uses. It powerfully aids the disintegration of the minerals (grains
of sand) on a soil which contain many of the necessary elements
of plants, and which, without the fluxing or fusing power of lime,
would not yield them. This is one of its greatest and most im-
portant uses, and we cannot yet say, whether or not, lime, when
applied to a soil, does not act more beneficially in this way than
in any other.

Magnesia.—Magnesia is the oxyd or rust of a metal called
magnesium, which never exists naturally in a pure state. It is one
of the necessary constituents of soil, being found in the bones of
animals, and in the roots, stalks, leaves, and grain of plants. It
has, in many respects, the same action as lime, and to a certain
extent can be substituted for it. The quantity of it varies in differ-
ent soils. In some I have found as much as two per cent., equal
to about six hundred bushels to the acre, to the depth of twelve
inches, and in others but a mere trace. The facts to show what
the best quantity is in a soil, are very meagre. As far as my
knowledge extends, I believe that where it exists in a less quantity
than two-tenths of one per cent., in a soil, magnesian lime is the
best application for that soil. There were no experiments on the
subject, and no means taken, any where, so far as I know, before
those in Maryland, to lay down any foundation for experiment that
will be worth any thing when made. My own knowledge, as to
the action of it, is derived from gentlemen who have used both
magnesian, oyster shell and pure stone lime on their lands, and
analyses of the soils which I subsequently made, have led me to
the above opinion, and are the only data yet given to the agricul-
tural community on this subject. [See article—Lime, Manures.]

Potash.—Potash, the oxyd or rust of a metal called potas-
sium, is another of the necessary constituents of soil. In union
with silicic acid, it gives strength and firmness to the stalk,
and to the husks or envelope of the grain.

The proportions in which I have found it to exist in soils are
very variable, from a mere trace in some, to as much as (1.20) one

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