There are three circumstances to be regarded in regulating the order in which crops should follow each other: - 1. Each crop should be as dissimilar as possible from its predecessor. 2. The exuviae of the preceding crop should not be offensive to its successor. 3. A fusiform-rooted crop should succeed a fibrous-rooted crop, or vice versa.

I. Dissimilarity in the following crop is desirable, because, so far as the saline constituents of the soil are concerned, every tribe of plants in some measure takes from it distinct food. Sir H. Davy truly observed upon this point, that, "though the general composition of plants is very analogous, yet the specific differences in the products of many of them, and other well ascertained facts, prove that they must derive different materials from the soil; and though the vegetables having the smallest systems of leaves, will proportionately most exhaust the soil of common nutritive matter, yet particular vegetables, when their produce is carried off, will require peculiar principles to be supplied to the land in which they grow. Strawberries and potatoes at first produce luxuriantly in virgin mould recently turned up from pasture, but in a few years they degenerate and require a fresh soil; and the organization of these plants is such as to be constantly producing the migration of their layers. Thus the strawberry by its long shoots is continually endeavouring to occupy a new soil; and the fibrous radicles of the potato produce bulbs at a considerable distance from the parent plant.

The most remarkable instance of the powers of the plant to exhaust the soil of certain principles necessary to its growth, is found in certain fungi. Mushrooms are said never to rise in two successive seasons on the same spot; and the production of the phenomena called fairy-rings, has been ascribed by Dr. Wollaston, to the power of the peculiar fungus which forms it to exhaust the soil of the nutriment necessary for the growth of the species. The consequence is that the ring annually extends, for no seeds will grow where their parents grew before them, and the interior part of the circle has been exhausted by preceding crops; but where the fungus has died, nourishment is supplied for grass which usually rises within the circle, coarse and of a dark green colour".

Again, exhausting crops should never be grown successively; and the following observations of one of the best of modern gardeners, the late Mr. G. Sinclair, afford much light npon this point: -

"If we take the weight of nutritive matter which a plant affords from a given space of ground, the result will be found to agree with the daily experience in the garden and the farm; and the following figures represent the proportion in which they stand to each other with respect to the weight of nutritive matter they contain, with their having exhausted the land: -

2. It is important that the exuviae of a preceding crop should not be offensive to its successor.

Thus, brassicas will not grow healthily upon soil where the immediately previous crop was of the same tribe ; but if the ground be pared and burnt, they will grow luxuriantly; and the same occurs to ground exhausted by strawberries: if it be burned and manured, strawberries will grow as vigorously as upon fresh ground, but they will not do so if manure only is applied.

It has also been observed that the roots of plants placed in water give out their characteristic flavours to the liquid; but on this, as evidence that they emit excrements, no great reliance can be placed, for some of the roots, during removal from the soil, must be wounded. The fact that the roots of plants do give out peculiar and varying mat-ters to the soil which sustains them, aids to explain why one rotation of crops is superior to another.

3. As fusiform-rooted crops should precede or follow a fibrous-rooted crop, because the one draws its chief supply of food from a greater depth than another, and, consequently, exhausts a different portion of pasturage; founded upon these considerations, and sanctioned by practice, the following rotations are recommended: -

Onions.

Lettuce.

Cabbage.

Carrots.

Manure.

Turnips.

Celery.

Peas.

Potatoes.

Manure.

Mr. Kelly, of Airthrey Castle, Scotland, says, that "on poor ground the rotation he finds best is celery; second season, cauliflowers and red beet; third, onions ; fourth, German green, or peas. By digging deep, and manuring abundantly, for celery, the ground is brought into such fine tilth, that the whole rotation is often gone through without any further addition, and without failing in any of the crops. Another good rotation is strawberries, celery, cauliflowers." - Gard. Chron.

Mr. Errington, gardener at Oulton Park, Cheshire, recommends the following as good successions: -

"Brassicas after raspberries or strawberries; peas after brassicas; celery after peas; celery after asparagus; beans and brocoli after celery; carrots or parsnips or beet after brocoli." - Gard. Mag.

The writer of the Kitchen Garden Calender in the Gardener's Chronicle for 1S44, (p. 72,) says, "the chief rule is never to have two crops of the same class directly following each other." He adds, that 'celery is a good preparation for carrots, turnips, parsnips, onions, and earlv cauliflowers, or for peas, with potatoes and winter greens or brocoli between the rows. Autumn-sown onions, followed by spinach, lettuce, etc., and early cauliflowers by autumn onions. Spring-sown onions are well succeeded by cabbages in beds, and scarlet runners between; and if the cabbages remain through the summer and next winter, the ground will be for celery, potatoes, and peas in the spring." In gardens of limited extent it is not always practicable to observe a systematic rotation of crops, even though it were as important to successful culture as some writers declare. For all practical purposes deep tillage will suffice, and there can be little doubt that if the land be deeply dug or ploughed after each crop, and the exhaustion supplied by manure, that the same description of vegetable growth may be successfully produced for successive seasons - indeed the only inmate of the garden which we have seen tire the land, as it is termed, is the pea.

Some market gardeners, whether from habit, or an idea that particular localities answer better for certain vegetables, invariably use them for such, and year after year the same crop may be seen growing thereon.