This section is from "The Horticulturist, And Journal Of Rural Art And Rural Taste", by P. Barry, A. J. Downing, J. Jay Smith, Peter B. Mead, F. W. Woodward, Henry T. Williams. Also available from Amazon: Horticulturist and Journal of Rural Art and Rural Taste.
From various observations I have made on the use of charcoal in growing Roses, I am disposed to consider it of advantage, whether mixed with the soil in which the plants are grown, or used as a top-dressing. Dr. Gray, in his "Lessons in Botany," says that" carbon itself is a solid, and not at all dissolved by water; as such, therefore, it cannot be absorbed into the plant, however minute the particles." This is a plain statement, and may be regarded as expressing the opinion of scientific men on the subject. It is also believed, that although plants do not receive any of their carbon from charcoal, they are benefited by the use of it, inasmuch as it is an absorbent of the carbonic acid gas which is in the atmosphere, and thereby presenting it in a fit condition to be taken up by the rootlets in greater quantity than when it is not used.
There is another benefit, however, which it confers, - that of promoting a healthy condition of the roots, and by that means securing them in some measure against the attacks of parasitical fungi, an enemy which ought to be guarded against with the utmost care. The most of the readers of the Horticulturist who have had experience in growing Roses in pots can not but, have observed, while planting them out in the spring, or in the fall when lifting them, the roots of some of thorn covered with a white mould, or thread-like process, which is the ordinary way in which fungus manifests itself. And it is most destructive, living as it does upon the roots and tender spongioles, thereby incapacitating them from performing their proper functions, aud ending in the disease and death of its victims, provided restorative means are not resorted to.
In addition to mixing a little in the soil with which to grow roses, I have for several years used it as a mulching for those growing in pots in the greenhouse, and have invariably been satisfied with the effects produced. From actual experiment, I have found that by using it in this way, the temperature of the mould in the pots is raised one degree above what it is in pots where it is not so applied. The gain of even one degree of heat at the roots of the plants, when it is an object to have them early in bloom, is so far advantageous, and more than equivalent to the labor and expense bestowed.
If mildew is caused, as Loudon somewhere remarks, by the temperature of the soil being greatly below that of the atmosphere in which the plants grow, any means which serve to increase the heat of the former, as in the case of charcoal, must be of benefit in protecting them from the attacks of that enemy. It is readily conceded that other causes may, and do, operate in producing the same effects, such as an undue supply of moisture at the roots of plants, while the surrounding atmosphere is comparatively dry, or the reverse. It is more than probable, however, that any material difference of temperature between the roots and the branches of a growing plant predisposes it to become mildewed.
In regard to the opinion entertained by some, that carbon acts beneficially in counteracting the injurious effects caused by the matter excreted from the roots of plants, little need be said, as there does not seem to be anything like positive proof that such exudations act injuriously in any case whatever; such, at least, is the conclusion arrived at by Prof. Gasparrini, after a series of carefully conducted experiments. At the same time, it is of undoubted benefit in decomposing the " decayed suckers and pilorhizas, and the numerous fibres which perish from natural and accidental causes," and thereby enabling the constituent elements of these to enter into other combinations, in which circumstance they may become food for the plants.
It may be remarked, in conclusion, that the result of my experience is, that when not using charcoal in growing Roses, they have been more or less subject to mildew, and the roots of the plants more apt to be injured by fungus, whereas, with the free use of that material they are not at all liable to be attacked. And, besides, when treated in this way the plants are remarkable for their freshness and beauty; the flowers are so much improved that they seem as though they had been "Dipped in color's native well".
We have used charcoal freely, especially in compost for pot plants, and fully endorse all that A. V. says. Charcoal, it is admitted, is not properly a manure; it is rather an absorbent or vehicle of manures, whether liquid or gaseous. We are not quite satisfied as to its specific mode of operation, but we know, that while it improves the soil mechanically, it is highly beneficial to all classes of plants. A. V.'s article should be carefully read. - Ed].