However varying in the proportions, yet every soil is composed of silica, alumina, lime, magnesia, oxide of iron, salts, and animal and vegetable remains. The most important consideration is, what proportions those are which constitute a fertile soil ?

The beau ideal of a fertile soil is one which contains such a proportion of decomposing matter and of moisture as to keep the crop growing upon it always supplied with food in a state fit for introsusception, yet not so superabundantly as to render the plants too luxuriant, if the object in view is the production of seed; but for the production of those plants whose foliage is the part in request, as spinach, or of edible bulbous roots, as onions, which have a small expanse of leaves, so as to be almost entirely dependent upon the soil for nourishment, there can scarcely be an excess of decomposed matter presented to their roots.

Spinach, on rich soils, will yield successive cuttings, the same as asparagus; the latter especially demands abundant applications of nourishment to its roots, since, like the onion, it has little foliage and slightly fibrous roots, at the same time that, like the spinach, it has to afford repeated cuttings; and thus, requiring a repeated development of parts, it needs abundant food in its immediate neighbourhood. A soil with a just proportion of decomposing matter will be capable of absorbing moisture, during the droughts of summer, from the atmosphere; for the most fertile soils are always the most absorbent. Yet it must not be too retentive of moisture, which is the case in such soils as contain too much alumina; neither must it too easily part with moisture, a fault which is a characteristic of those soils which contain an excess of silica.

A subsoil of gravel, mixed with clay, is the best, if not abounding in oxide of iron; for clay alone retains the moisture, on the arable surface, in too great an excess; and sand or chalk, on the contrary, carries it away too rapidly. It is, however, evident, that to insure these desiderata in any soil, at all seasons, is impossible; and it is manifest that a soil that would do so in one climate would fail in another, if the mean annual temperature of them should differ, as well as the amount in inches o rain which fall during the same period. Thus, in the western parts of England, more than twice as much rain occurs as in the most eastern counties, or in the proportion of forty-two to nineteen; a soil in the east of England, for any given crop, therefore, may be richer and more tenacious than the soil required for it on the western coast.

Alumina, or clay, imparts tenacity to a soil when applied; silica, or sand, diminishes that power; whilst chal k and lime have an intermediate effect. They render heavy soils more friable, light soils more retentive. These simple facts are important; two neighbouring gardens, by an interchange of soils, being often rendered fertile, which before were in the extremes of tenacity and porosity.

From these statements it is evident that no universal standard or recipe can be given for the formation of a fertile soil; but a soil, the constituents of which approach in their proportions to those of the following, cannot be unproductive in any climate. It is a rich alluvial soil, which Mr. Sinclair, in his invaluable Hortus Gramineus Woburnensis, gives as being the most fertile for the grasses: -

"Fine sand, 115; aluminous stones, 70; carbonate of lime, 23; decomposing animal and vegetable matter, 34 ; silica, 100; alumina, 28; oxide of iron, 13; sulphate of lime, 2; soluble vegetable and saline matter, 7; loss, 8; total 400".

I have already stated what forms a fertile soil; it may be added, that, to constitute it eminently such, its earthy particles must be in a minute state of division; the more so the more fertile it will be.

In the above analysis 185 parts only were separable by sitting through a fine sieve; 215 parts were impalpable; whereas poorer soils will often have 300 parts coarse matter to every 100 of finely pulverized constituents.

In affording warmth to plants the earth is of considerable importance, and the power of accumulating and retaining heat varies as much in soils as the proportions of their constituents. Sir Humphrey Davy found that a rich black mould, containing one-fourth of vegetable matter, had its temperature increased in an hour, from 65° to 88° by exposure to the sunshine, whilst a chalk soil was heated only to 69° under similar circumstances. But the first, when removed into the shade, cooled in half an hour 15° ; whereas the latter lost only 4°. This explains why the crops on light-colored tenacious soils are in general so much more backward in spring, but are retained longer in verdure, during autumn, than those on black light soils. The latter attain a genial warmth the more readily, but part from it with equal speed. Different plants affect different soils. Every gardener most have observed that there is scarcely a kitchen garden but has some particular crop which it sustains in luxuriance, far superior to any other garden in its neighborhood, or to any other crop that can be grown on it.

A garden I once cultivated would not produce, without the preparation of an artificial soil, the common garden-cress (Lepidium sativum), whilst the raspberry was remarkably luxuriant; and we have seen that the composition of a soil has a main influence in these peculiarities. It is certain that a soil is often considered unproductive, and the unproductiveness attributed to some deficiency in its staple, when, in truth, the defect arises from erroneous management.

I have before stated an instance of tap-rooted plants being produced, of superior size and form, by means of applying the manure deep below the surface. In another instance, some pars-neps being of necessity sown in a poor soil, having turned in some manure by trenching full twelve inches deep, I would not allow any to be applied to the surface, but, at the time of thinning I set half the bed out at an average of twelve inches' distance between each plant, the other half at nine inches. When taken up for storing, the whole were alike perfectly fusiform; but those grown at twelve inches apart were the finest, as four and a half is to three. If manure had been applied to the surface, the fibrous roots, I calculated, would be multiplied at the expense of the caudex, to its much greater detriment than by making the few usually produced by this root extend in length, thus enlarging the circuit of their pasturage.

Again, a more silicious, darker colored soil should be employed for the growth of an early crop, of any given plant, than is required by the main crop; because such soil will more readily get rid of the superfluous moisture, and earlier acquire a genial warmth, two great desiderata for vegetation in spring. On the contrary, in autumn, for a late crop of peas, for instance, the soil should be more aluminous, that such moisture may be retained.

The quantity of soluble matter obtainable from a soil, at any one time, is very small, seldom exceeding a one-thousandth part of its weight; and even pure vegetable mould, the debris of entirely putrefied plants, was found by Saussure to yield only one-eleventh of soluble matter. This mould was too rich for horticultural purposes, peas and beans grown in it being too luxuriant; and they were more productive in . a soil containing only one-twentieth of organic constituents dissolvable by wa-ter. Small in amount, however, as is the soluble constituents of the most fertile soils, they are necessary for the vigorous vegetation of plants; for when a soil is deprived of those constituents by frequent washings with boiling water, it is much less fertile than before. Liebig and others have most illogically concluded, from the smallness of the soluble extract contained in a soil, that it is of small importance, forgetting that as fast as it is taken by the roots of the crop, it is generated again by the decomposition of the animal and vegetable remains.

This is one reason why fallowing is beneficial; easily decomposing matters have been exhausted by successive crops; and by a year's rest, and exposure to the putrefactive agency of the air, the more stubborn and more slowly decomposing exuviae; have time to resolve into and accumulate soluble compounds in the soil. - Princ, of Gard.