Among the essentials necessary to maintain high cultivation, a proper system of rotative cropping occupies a prominent place. Physiologists differ in opinion with regard to the principles upon which rotation is founded, but they agree in recommending its utility. There are two theories in vogue, which may be termed the excretory and the exhausting. The former is based on the supposition that, during growth, plants throw off by their roots certain speculiar substances injurious to themselves, but which promote growth in those of a different species. It is well ascertained that plants possess this power, but it is so limited as not to be considered sufficient evidence to account for the beneficial results of rotation. Those who advocate the exhausting theory prove it by the fact that, although all plants are composed of the same primary elements, yet, different species require different proportions of them, each having its own peculiar characteristic formation; so that if the soil is deficient in these particular substances required by a plant, it cannot prosper, notwithstanding that a plant of suitable formation may grow luxuriantly upon it.

This points to the possibility of successfully cultivating the same crop on the same ground, by constantly supplying the ingredients extracted by the crop, but we have not as yet attained that perfect knowledge of the exact specific relations between the soil and the plants that grow upon it, to enable us to put the system in practice. Passing over, for the present, many other advantages attending rotation, it may be remarked that there is a general ignorance of the subject, and a wide field for experiment in ascertaining the kinds of plants best suited to succeed each other, as our present systems are founded more upon convenience than science.

It is very evident, however, that crops cultivated for their seeds and fruit, as peas, corn, beans, tomatoes, egg-plant, etc., should be followed by those grown for their leaves and roots, as cabbages, spinach, beets or carrots; the seeds of plants contain a larger portion of mineral matters than the leaves, consequently there is a greater consumption of the inorganic substances of the soil.

Persevere in turning over the soil, and presenting new surfaces to be acted upon by the atmosphere. Even when the ground is frozen six or eight inches deep, it has been found beneficial to turn it over in cakes with crowbars and pickaxes. The most adhesive clayey soils can thus be rendered as friable, and admit of as early working as those of a loamy nature; their latent powers of action are drawn out and rendered available for the purposes of vegetation.

Preparations for cropping should now be commenced by making hotbeds for raising plants and forwarding early crops. Asparagus is a useful early vegetable, and its natural season can readily be anticipated by forcing in frames. Hotbeds are usually made with fresh manure, and much of their efficiency depends upon the preparation of the material. It should first be thrown loosely into a rounded heap; in a few days decomposition will be active and heat generated. It should now be carefully turned over and separated, mixing it thoroughly, and thrown together as before. This turning must be repeated as often as is necessary to prevent its heating to excess, which is indicated by its dry, charred-like appearance in the centre; when in this state it should receive a thorough soaking of water. When violent fermentation declines, it is ready for use. To produce an equal and gradual heat, care must be exercised in building, that it may be of equal firmness; pressure retards decomposition, the fresher the material, therefore, the firmer must it be put together. The making of hotbeds is apparently a simple process, yet few take the trouble to do it properly. Leaves of oak, beech, or chestnut-trees make an excellent hotbed; they decay slowly, and give heat for many months.

A very slight warmth is sufficient to force asparagus, a bed of manure two feet deep will answer; cover with four or five inches of soil, and lay the roots close together, and spread two or three inches of dry soil over their tops. All the future care is simply to give a little air in mild weather, and cover up closely on frosty nights with straw mats, or other suitable protection. Air is the best non-conductor; endeavor to secure a stratum of it between the covering and the roof. For this reason, a loose covering of straw or hay, is more effectual in protection than half a dozen mats that lie compactly together.

A crop of radishes may be secured by sowing the seed when the asparagus is planted. Dwarf beans are also frequently raised in frames; they require to be kept dry and warm. Rhubarb and seakale are also suitable for this method of forcing - but they can be much accelerated by simply covering the plants with old boxes or barrels out of doors, and inclosing all with leaves or manure.

Tomatoes, egg-plant, lettuce, capsicums, etc., are most convenient when sown in boxes; they can then be removed, when found desirable, to a different temperature, or for transplanting - the sooner they are removed (technically, pricked out), the more robust and better rooted will they become; this applies to all plants that are transplanted when young.