Many gardeners do things because they have seen others do them, but they often cannot tell us why or wherefore.

Here, then, we have a few facts, and although they may not establish the opinions herein expressed, they at all events clearly demonstrate the existence of certain functions in the roots of plants, by which refuse or faecal matter is discharged, and also that these rejections are highly injurious to the plant emitting them, if re-absorbed. We are, therefore, very naturally led to inquire, how shall these deleterious influences be neutralised or destroyed? We believe it has been satisfactorily proved, that only to those of their own species, are the rejections of plants injurious. To all others they are perfectly innocuous, and very probably nutrimental. This establishes the propriety and necessity of attending to the alternation and rotation of crops in the field and kitchen garden, and is a discovery of no small importance to every cultivator of the soil. But how is it in the flower garden? so widely different from growing vegetables. The majority of the plants are of perennial duration; therefore attention to this object is of still greater importance, and more difficult of attainment; for no plant should be suffered to remain more than two or three years in the same spot; if it does, degeneration is inevitable.

It is especially to bulbs and tubers these remarks will apply, for to them an occasional removal from the ground altogether, is of very decided benefit; nor is this benefit derived solely from suspended vegetation, but likewise is due to a change of soil. Therefore, this latter fact should be taken into consideration and acted upon, if we desire the treatment to be as perfect as it ought to be.

When grown in pots, plants are far more materially affected by their own rejections, because their roots are confined, and cannot extend themselves into uncontaminated soil; yet much may be done in these cases to remove the difficulty, or to counteract its effects* By some writers it has been declared, but not proved, that from the tips or extremities of the rootlets ugly, is this excrementitious matter voided. This may be true or not; but assuming it to be true, the importance of an annual repotting is too evident to need insisting upon. To all gardeners it is a well known fact, that at the sides of the pot these rootlets are always found in greater or less number. In repotting, the outside surface of the soil is generally removed, and with it is taken away the injurious matter, in the place of which we substitute fresh and sweet earth. So with a great many plants, we find near the surface of the ground their fibrous roots. With these the operation of top-dressing is practiced, and with the very best effects, only taking care to remove the top soil before making the addition of fresh compost.

But these are not the only means of neutralising or removing these unwholesome excretions. There are others of quite as much importance to the horticulturist or gardener, to wit - a thorough exposure to the air, and a free permeation, of water. The advantages derived from the latter of these processes is too palpable to need further elucidation, and it will readily be perceived, that in efficiently draining pots, we are doing something more than is commonly supposed necessary. When the water is allowed to stagnate about the roots, the plants becomes saturated, and in consequence, their rejections are more abundant. A retention of this noxious feculence is thus inevitable, which is not only pernicious, but very frequently destructive.

It has been abundantly proved, that by freely exposing the soil to the varied influences of the atmosphere, the excretions of plants contained therein are thereby either evaporated or decomposed. The great advantage then of spreading out soils to the air, and the influence of heat are light, with frequent turning previous to their being used for potting, will be instantly perceived. From these sources, they derive the greater portion of their electrical properties, without which vegetation would become extinct. Nor is it saying toe much to refer the utility of digging and ridging to the same cause, as well as the pulverization of the earth and the admixture of nutritive substances. With some gardeners, it is a habit to throw away the soil after having been once used for potting plants, but the practice is not to be commended; the rather is it a wasteful and injudicious proceeding. Much better would it be to expose the soil for two or three years to the influences of the atmosphere, and to turn them frequently during that time. Then, by adding a little well rotted manure, the same soil would become again available, and equally as good as it was before.

Where the right kind of soil is scarce, and must be purchased, this is a consideration of some moment, and deserving of no little attention. If soils are not renovated and restored by these atmospherical influences, every portion of the cultivated earth would long since have become a dreary waste, and all our most valuable vegetable productions would now be unknown.

It was once the common practice to purify the soil by burning, and it has been recommended to destroy all extraneous matters by a strong heat. These processes are .doubtless useful, but, except for the better pulverization of strong and stiif earths, they cannot be recommended for general adoption. The atmospherical exposure and frequent turning are greatly to be preferred for their efficiency, and are entirely adequate to the complete regeneration of the soil.

Enough has now been said to show the great importance»of giving attention to this subject in a practical point of view, and we trust that what has been advanced will have the effect of inducing cultivators to investigate the matter with a great deal more minuteness than has yet been bestowed upon it. W. W. Talk, M. D.

Flushing, Sept 11,1852.