Section Ii.—of the Inorganic Part of Soils.

The inorganic part of soils,—that which remains behind, when every thing combustible is burned away by heating it to redness in the open air,—consists of two portions, one of which is soluble in water, the other insoluble. The soluble consists of saline substances, the insoluble of earthy substances.

1. The saline or soluble portion.—In this country the surface soil of our fields, in general, contains very little soluble matter. If a quantity of soil be dried in an oven, a pound weight of it taken, and a pint and a half of pure boiling rain-water poured over it, the whole well stirred and allowed to settle,—the clear liquid, when poured off and boiled to dryness, may leave from 2 to 20 grains of saline matter. This saline matter will consist of common salt, gypsum, sulphate of soda (Glauber’s salts), sulphate of magnesia (Epsom salts), with traces of the chlorides of calcium, magnesium, and potassium, and of the nitrates of potash, soda, and lime. It is from these soluble substances that the plants derive the greater portion of the saline ingredients contained in the ash they leave when burned.

See pages 51 and 52, where these substances are described.

Nor must the quantity thus obtained from a soil be considered too small to yield the whole supply which a crop requires. A single grain of saline matter in every pound of a soil a foot deep, is equal to 500 lbs. in an acre, which is more than is carried off from the soil in 10 rotations (40 years), where only the wheat and barley are sent to market, and the straw and green crops are regularly returned to the land in the manure.

A further portion, it will be recollected, is carried off in the cattle that are sent to market,—this is here neglected.

In some countries, indeed in some districts of our own country, the quantity of saline matter in the soil is so great, as in hot seasons to form a distinct incrustation on the surface. This may often be seen in the neighbourhood of Durham; and is more especially to be looked for in districts where the subsoil is sandy and porous, and more or less full of water. In hot weather the evaporation on the surface causes the water to ascend from the porous subsoil: and as this water always brings with it a quantity of saline matter,—which it leaves behind when it rises in vapour,—it is evident that the longer the dry weather and the consequent evaporation from the surface continue, the thicker the incrustations will be, or the greater the accumulation of saline matter on the surface. Hence, where such a moist and porous subsoil exists in countries rarely visited by rain, as in the plains of Peru, of Egypt, or of India, the country is whitened over in the dry season with an unbroken covering of the different saline substances above mentioned.

When rain falls, the saline matter is dissolved, and descends again to the subsoil,—in dry weather it reascends. Thus the surface soil of any field will contain a larger proportion of soluble inorganic matter in the middle of a hot season than in one of even ordinary rain; and hence the fine dry weather which, in early summer, hastens the growth of corn, and later in the season favours its ripening, does so, among its other modes of action, by bringing up to the roots from beneath a more ready supply of those saline compounds which the crop requires for its healthful growth.

2. The earthy or insoluble portion.—The earthy or insoluble portion of soils rarely constitutes less than 95 lbs. in a hundred of their whole weight. It consists chiefly of silica in the form of sand, of alumina in the form of clay, and of lime in the form of carbonate of lime. It is rarely free, however, from one or two per cent. of oxide of iron; and where the soil is of a red colour, this oxide is present in a still larger quantity. A trace of magnesia also may be almost always detected, and a minute quantity of phosphate of lime. The principal ingredients, however, of the earthy part of all soils are sand, clay, and lime; and soils are named or classified according to the quantities of each of these three they may happen to contain.

If an ounce of soil be boiled in a pint of water till it is perfectly softened and diffused through it, and, after shaking, the heavy parts be allowed to settle for a few minutes, the sand will subside, while the clay—which is in finer particles, and is less heavy—will still remain floating. If the water and clay be now poured into another vessel, and be allowed to stand till the water has become clear, the sandy part of the soil will be on the bottom of the one vessel, the clayey part on that of the other, and they may be dried and weighed separately.

If 100 grains of dry soil leave no more than 10 of clay, it is called a sandy soil; if from 10 to 40, a sandy loam; if from 40 to 70, a loamy soil; if from 70 to 85, a clay loam; from 85 to 95, a strong clay soil; and when no sand is separated at all by this process, it is a pure agricultural clay.

The strong clay soils are such as are used for making tiles and bricks; the pure agricultural clay is such as is commonly employed for the manufacture of pipes (pipe-clay).

Soils consist of these three substances mixed together. The pure clay is a chemical compound of silica and alumina, in the proportion of about 60 of the former to 40 of the latter. Pure clay soils rarely occur—it being well known to all practical men, that the strong clays (tile clays) which contain from 5 to 15 per cent. of sand, are brought into arable cultivation with the greatest possible difficulty. It will rarely happen, therefore, that arable land will contain more than 30 to 35 of alumina.

If a soil contain more than 5 per cent. of carbonate of lime, it is called a marl; if more than 20 per cent., it is a calcareous soil. Peaty soils, of course, are those in which the vegetable matter predominates very much.

To estimate the lime, a quantity of the soil should be burned in the air, and a weighed portion, 100 or 200 grains, diffused through half a pint of cold water mixed with half a wine glassful of spirit of salt (muriatic acid), and allowed to stand for a couple of hours, with occasional stirring. The water is then poured off, the soil dried, heated to redness as before, and weighed: the loss is nearly all lime.

Unless the soil happen to contain a large quantity of magnesia, which is rarely the case.

The quantity of vegetable or other organic matter is determined by drying the soil well upon paper in an oven, and then burning a weighed quantity in the air: the loss is nearly all organic matter. In stiff clays this loss will comprise a portion of water, which is not wholly driven off from such soils by drying upon paper in the way described.