Elements of agricultural chemistry and geology

by Jas. F. W. Johnston

We have seen that all the elements which are necessary to the production of the woody fibre, and of the other organic parts of the plant, may be derived either from the air, from the carbonic acid and watery vapour taken in by the leaves, or from the soil, through the medium of the roots. In the air, however, only rare particles of inorganic or earthy matter are known to float, and these in a solid form, so as to be unable to enter by the leaves; the earthy matter which constitutes the ash, therefore, must be all derived from the soil.

The earthy part of the soil, therefore, serves a double use. It is not merely, as some have supposed, a substratum in which the plant may so fix and root itself, as to be able to maintain its upright position against the force of winds and tempests; but it is a storehouse of food also, from which the roots of the plant may select such earthy substances as are necessary to, or are fitted to promote, its growth.

The ash of plants consists of a mixture of several, sometimes of as many as eleven, different earthy substances. These substances are the following:—

1. Potash.—The common pearl-ash of the shops is a compound of potash with carbonic acid; it is a carbonate of potash. By dissolving the pearl-ash in water, and boiling it with quicklime, the carbonic acid is separated, and potash alone, or caustic potash, as it is often called, is obtained.

2. Soda.—The common soda of the shops is a carbonate of soda, and by boiling it with quicklime, the carbonic acid is separated, as in the case of pearl-ash.

3. Lime.—This is familiar to every one as the lime-shells, or unslaked lime of the limekilns. The unburned limestone is a carbonate of lime; the carbonic acid in this case being separated by the roasting in the kiln.

4. Magnesia.—This is the calcined magnesia of the shops. The uncalcined is a carbonate of magnesia, from which heat drives off the carbonic acid.

5. Silica.—This is the name given by chemists to the substance of flint, quartz, and of siliceous sands and sandstones.

6. Alumina is the pure earth of alum, obtained by dissolving alum in water, and adding liquid ammonia (hartshorn) to the solution. It forms about two-fifths of the weight of porcelain and pipe-clays, and of some other very stiff kinds of clay.

7. Oxide of Iron.—The most familiar form of this substance is the rust that forms on metallic iron in damp places. It is a compound of iron with oxygen, hence the name oxide.