ammonia. The precipitates obtained by the addition of ammonia
to salts of alumina or iron, are true salts in which the ammonia is
contained as a base. Minerals, containing alumina or oxyds of
iron, also possess, in an eminent degree, the remarkable property
of attracting ammonia from the atmosphere and retaining it
* * * * Soils, therefore, containing the oxyds of iron and
burned clay, must absorb ammonia, an action which is favored by
their porous condition; they further prevent, by their chemical pro-
perties, the escape of the ammonia once absorbed. Such soils act-
indeed, precisely as a mineral acid would do, if extensively spread
over their surface.

"The ammonia absorbed by the clay of ferruginous oxyds, is
separated by every shower of rain, and conveyed in solution to the
soil."

Our red and yellow clays need not burning, the iron in them
being already in a state of peroxyd, heat would not benefit them.
It is only in white or dark clays, where the iron is in the state of
protoxyd, that heat would be of any use if applied.

The peroxyd of iron in a soil, also absorbs and retains moist-
ure, and whatever else of the food of plants which exists in the
air.

Third.—It gives firmness and compactness to the soil, and
better fits it for those plants which flourish best in compact soils.

Fourth.—It gives color to soils, which makes them absorb heat
better, and in this way it gives crops a quick, early growth.

Alumina.—Alumina, or pure clay, is another of the constitu-
ents of all soils. It, like iron, is the oxyd or rust of a metal called
aluminum which never exists naturally in its pure state. "The
different kinds of clay of which pipes, porcelain and earthenware are
made, consist of hydrate of alumina, in a greater or less degree of
purity." It has not the same important action as iron in the human
economy, but its uses in the soil are very nearly the same, giving
it tenacity and firmness, absorbing, like it, moisture and food from
the atmosphere, forming "true salts" with ammonia, and having,
like it, the properties of an acid or an alkali; of an acid, by uniting
with alkaline bases such as potassa, lime and baryta; and of an
alkali, by forming salts, with acids. Our red and yellow clays
are by no means pure alumina, but silicates of alumina and the
peroxyd of iron, united with lime, magnesia, and sometimes with
potash, and very rarely with soda—that is, pure sand chemically
united to these substances.

Lime.—The great use of this substance as a manure, and the
frequent benefits that result from its application, make almost su-
perfluous the bringing forward of any proofs of its necessity in a
soil. Lime, like alumina, is the oxyd or rust of a metal, never
existing naturally in its pure state. The metal of lime is called

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