ON THE

COMPOSITION OF SOILS,

AND THE AGENCY OF THE EARTHS IN VEGETATION.

ALTHOUGH the Earth is in fact a variable compound, as it respects vegetation, we need not pursue it farther than the following simple division; viz. Calx, or the calcareous principle; Silex, or the silicious; Clay, or the argillaceous; Magnesia, or magnesian; and Carbon, the carbonaceous, or, as it is commonly called, Mould.

The first four substances are what Miller properly calls the containing part or body, bed or couch, and the fifth substance, or mould, (which is the result of decayed animal and vegetable matter) the part contained.

It is clearly proved that neither of the four substances, calx, clay, magnesia, or silex, in a pure state, whether separate or combined, will support a plant; and the vegetative power of every part of the earth is determined by the quantity of mould, or animal and vegetable matter it contains.

Earth is proved to be an essential part of vege

tables; but the quantity discovered in them is so small, and of such a nature, as to be contained in and conveyed by water.

Too great an accumulation or concentration of `vegetable and animal matter, or mould, renders it as a soil unfit for the propagation and sustenance of most vegetables: we therefore find it is in the course of nature divided and diluted, by the intervention and admixture of the other primitive substances, and in this state or combination it forms what is called loam.

Every part of the surface of the globe, that supports vegetables, consists of an admixture or covering of loam, of greater or less depth, and the depth and proportion of the admixture, the degree of exposure to the sun and air, the nature of the substrata, and the quantity of water supplied, determine the produce of the general substance or soil.

It is a very general opinion that carbonic acid gas, or fixed air, constitutes the principal Food of Plants, but this is not demonstrated. Carbonic acid gas, which is composed of carbon held in solution by a large portion of oxygene, is no doubt constituted of the two grand principles of vegetation; but it does not appear to me probable or necessary that it should, in a combined or gaseous state, be applicable as Food; but being decomposed by calcareous earths, the acid neutralized, or the superabundant oxygene withdrawn, by forming some other union, and the carbon united with water, it is then converted into Food.

It may perhaps be necessary that carbon should be reduced to the same state, as when capable of uniting with oxygene to form carbonic acid gas, before it can be held in solution by water, and consumed and appropriated by plants.

Every observation proves that a superabundance of oxygene is detrimental to plants; it is also certain, that the formation and emission of carbonic acid gas, either by the soil or the plant, reduces its prolificacy.

The most fertilizing manures are found to be such as are produced from the decomposition of animal and vegetable matter, by such processes as oppose or prevent the formation of carbonic acid gas.

Without an excess of water in a continued state, carbon stops short of that degree of oxygenation which is essential to form it into an acid, and is then said to be converted into carbonic oxyde, and this appears to me to be the state in which it is most convertible to the Food of Plants; hence we find, that stagnant water is detrimental to Plants. 1

Unless the soil be previously charged with a sufficient quantity of alkali or acid, to neutralize each other, or be so subdivided, by the intervention of the siliceous and calcareous earths, as to oppose concentration, the addition of either makes a soil sterile, and detrimental to vegetation.

Fermented liquors, such as beer, containing a great quantity of fixed air, or carbonic acid gas, when applied to the roots of plants in a common

soil, retard and obstruct their growth; and stronger acids, such as the acetous or vinegar, brought into immediate contact with the roots, destroy vegetable life.

It is observed that Plants, when growing in the shade, give out carbonic acid gas, but when exposed to the rays of the sun, they give out oxygene only; by this we are not to conclude, that carbonic acid gas has been taken up ready formed, by the leaves or roots as food, and again emitted in that state, but that a carbonaceous solution in water is taken up as food, and that the sun enables the Plant to digest this food, which consists of water holding in solution earth and carbon, and apply it to its various uses; and thus, by facilitating the escape of oxygene, preventing the formation of carbonic acid gas, and the consequent loss of its carbon, its grand material; and when the sun is excluded, the decomposition or digestion is incomplete the food is then expelled undigested, as carbonic acid, and the Plant becomes weak, unhealthy, and diseased.

Carbon cannot be produced by art in a state of purity, without the application of excessive heat approaching to fire; and there appears good reason to suppose, that it cannot be prepared to admit of the required solution in water, or be reduced to a state fit for the Food of Plants, without the aid of the sun's rays, digestion in the bowels of animals, the heat consequent to fermentation or fire; for we find, that animal or vegetable substances buried

in an organized state, and decomposed under the earth, furnish very little wholesome Food for Plants, whilst excluded from the sun, fire, &c.; but being at any period afterwards exposed to the action of the sun and air, or brought in contact with such heat, with calcareous earths, or with absorbent and caustic substances formed by fire, they are reduced to a state to fertilize the earth, or to become the Food of Plants.

By the urine and excrement of animals, by the application of fire to vegetable and animal matter, and by exposing it to the sun, and an excess of water, alkaline salts are produced, and the natural decomposition of animals and vegetables, by fermentation, is found to produce acids, carbonic acid gas, carburetted hydrogene gas, ammoniacal gas, &c.; and hence it is supposed by some, that these form the grand principle of food and substance of vegetables, but it is proved, that neither alkaline or other salts, nor acids, will of themselves sustain or produce an immediate increase in Plants.

Rank and gross vegetables are sometimes found to grow where the different gases are emitted in large quantities, and appear to devour the solutions of decomposing substances in the most impure state; this is clearly demonstrative in the cabbage tribe when grown for the table, and particularly in sea kale; for when this is grown in a soil richly manured, it retains so strong and rank a flavour, as to be scarcely eatable; but when it is grown in pure loam, or in a soil in which the