The Gardener's Magazine 7: 252-255 (1841)
Theory of M. Edward Lucas's Experiments on the Effect of Charcoal on Vegetation.
By Dr. A. Buchner, Sen.
Munich, Feb. 29. 1840
(Translated from the "Garten Zeitung" for the "Gardener's Magazine," by M. L.)

The numerous experiments and observations mentioned in this treatise appear to me to be very important contributions, not only to vegetable physiology and dietetics, but also to the founding of a vegetable therapeutic system, which, if I am not mistaken, has not hitherto been taken into consideration. I take leave, therefore, to make a small addition in a theoretical point of view, in order to introduce a clear scientific notion of the effects of charcoal on vegetable life. These effects are founded, undoubtedly, on several causes, of which the following appear to me the most important.

1. Absorption of Light, and Generation of Heat. It is well known that bodies receive the light of the sun the more perfectly, the darker, duller, and looser they are, and that the consequent developement of heat is in proportion to this absorption of light; hence, a black light soil is, under the same circumstances and relations, much more favourable to vegetation than a light-coloured, grey, heavy earth. Heavy clayey soil, with a deficiency of humus, is less suitable to vegetation, inasmuch as it soon loses its porosity through rain and snow, and assumes a smooth surface, by which it is prevented from absorbing air and light and generating heat. Hence agriculturists justly name these clayey soils, which are deficient in humus, cold soils. As charcoal dust is one of the darkest, dullest, and most porous of bodies, it must, on account of its peculiar capacity of receiving the sun's light and changing it into heat, be particularly favourable to vegetable life. M. Lucas, in his experiments next summer, will, no doubt, not omit making comparative thermometrical experiments.

2. Absorption of Atmospherical Air. Among all porous bodies that have the capacity of absorbing gases and vapours, charcoal has been proved by numerous experiments to hold the first rank. If, therefore, clayey soil, deficient in humus, is in general less suitable to the growth of plants than rich loose garden mould, the reason lies, not only in the latter receiving more light and creating warmth, but also in its more readily condensing, by its greater porosity, the constituent parts of the atmospheric air, and consequently supplying oxygen, nitrogen, and carbonic acid gas for the nourishment of the spongioles. We come here to a very important point, the nourishment of plants, which I cannot slightly pass over in elucidating the theory of the effects of charcoal in this respect. Modern vegetable physiologists are, for the most part, of opinion, that plants can receive no solid nourishment from the earth; that is, that every thing that they can assimilate (or digest) must be in a liquid and gaseous or vapoury state. If we, therefore, meet with siliceous earth, chalk, magnesia, oxide of iron, in short, such substances in plants as could only be received from the soil, we may always consider it certain that these sorts of matter can only be absorbed by the roots in proportion as they are in a fluid or dissolved state in the soil. These sorts of matter, and particularly the different organic salts which we find in the ashes of vegetables, are not actually to be considered sources of nourishment, but stimulants to assist in digesting, as salt and spice are to the higher animals and man; we also not unfrequently observe that a superfluity or mixture of certain inorganic substances in the soil, prejudicial to certain families and species of plants, is the cause of disease when this inorganised matter is in a dissolved state and capable of being absorbed by them.

If we analyse the nourishment of plants, we shall find it is only the constituent parts of air, water, and charcoal. The experiments of Boussingault on the origin of nitrogen in organic bodies show, 1st, that no plant exists without a proportion of nitrogen; and 2d, that, while men and animals receive the portion of nitrogen of their bodies not from the air by breathing, but from food by assimilation, plants on the contrary draw their supply of nitrogen, not from manure or humus, but from the air. We come now to a very important point in the nourishment of plants, to which M. Payen has particularly called our attention in two treatises read before the Academy of Sciences at Paris, on the 8th and 14th of October, 1839: viz. that charcoal operates as a condenser, under the influence of water, on the constituent parts of the air, in the same manner as spongy platina on the elements of detonating gas; so that nitrogen and oxygen are dissolved, and, mixing with water, are absorbed by the spongioles, and carried to the cambium for assimilation. This property of condensing the air, and making it fit to be received by plants, does not exclusively belong to charcoal, for it is also more or less perceptible in other sorts of earth, chiefly in porous and pulverised bodies. We know that water, even when not distributed through charcoal or earth, absorbs some air, which becomes a watery fluid, and by heating is again expelled in the form of gas: but charcoal powder appears to possess this power in the highest degree; consequently, besides light and heat, is capable of carrying to the roots both air and water, i.e., nitrogen, hydrogen, and oxygen, in the greatest abundance.

3. Decomposition of the Charcoal, and Formation of a nourishing Substance for Plants. It is well known that manure, as such, does not nourish plants, and that, on the contrary, when it touches the roots it causes disease. We know that it is the constituent parts of the humus, i.e. the matter produced by decay, which nourish plants. This apparently takes place because the humus, with the cooperation of air and water, is continually forming oxide of charcoal, or carbonate and nitrogen, which, together with the saline particles, is absorbed and assimilated by the roots. For a long time it was generally believed that charcoal, as an inanimate body incapable of decay, contributed in no degree to the nourishment of plants, and that charcoal dust could only serve at most to make the earth looser and warmer. But M. Lucas found, from his experiments, that the charcoal in which plants grow by degrees undergoes decomposition, and at last becomes a sort of humus. This obviously takes place merely because the charcoal dust acts as humus, and, with the cooperation of water and air, continually gives out to the plants oxide of charcoal, or carbonate, together with the saline particles which are in the charcoal and remain in the ashes after burning. But, to prove this, some chemical experiments were necessary.

4. Comparative Chemical Examination of Charcoal Dust. The more perfectly to establish the theory of the effect of charcoal on vegetation, M. Lucas gave me for examination: —

1st. Ashes of fir charcoal in which no plants had grown.

2d. Ashes of fir charcoal in which plants had grown for half a year.*

3d. A portion of charcoal dust which had been used for another purpose for two years.†

* This charcoal was used for most of the experiments.
It was used to fill a bed, hence its impurity is easily accounted for. Lucas.

With these materials I made the following comparative experiments: — Two drachms of them were reduced to fine powder, and digested in three ounces of distilled water for 24 hours. All the three quantities, filtered oil from the charcoal, were uncoloured, and left the test paper unchanged. After the evaporation of the water, there remained only a very trifling yellowish residuum, of a saltish taste, which acted somewhat like an alkali, and, besides potash, contained also chlorine. No difference could be distinguished in this case between a, b, and c.

The portions of charcoal powder to which water had been applied were each separately digested in a sand-bath, with three ounces of water, to which a drachm of corrosive lie of potash was added. The liquid filtered from a was almost colourless, and was not the least muddy when saturated with muriatic acid. The liquid from b was brownish, and with muriatic acid yielded a flocky dark brown precipitate of humic acid, which, being carefully collected and dried, weighed 0.27 grains. The liquid from c was of a darker colour, and, with muriatic acid, yielded 0.45 grains of humic acid.

Two drachms of each of the three portions of charcoal were reduced to ashes in the platina crucible. The ashes of a weighed 22 grains, and lost, by shaking with distilled water, one grain in weight. The ashes of A yielded only 9 grains of ashes, of which only half a grain was dissolved by the water. The ashes of c, on the contrary, weighed 33 grains; apparently because the charcoal powder, while in use for two years, had become fouled with garden mould; of these 33 grains of ashes, two grains were dissolved in water. The constituent parts of the three portions of ashes retained their qualities; so that in the dissolvable parts were found potash, chalk, carbonic acid, sulphuric acid, muriatic acid, and phosphate. The portion indissoluble in water contained chalk, magnesia, traces of oxide of iron, carbonate, sulphuric acid, phosphate and silicic acid.

If the objection be made, with respect to these three portions of charcoal, that they are not all from the same tree, and might therefore yield a different weight of ashes, we may, with probability, suppose that this natural difference is very inconsiderable, as the charcoal was all of fir wood from the neighbourhood of Munich, where limestone debris is the general understratum of the woods.

The result is quite decisive and undisputed, that diluted lie of potash scarcely ever dissolves any thing from fresh fir charcoal, and that, on the contrary, charcoal in which plants have grown, being partly changed into humus and this being drawn out by diluted lie of potash, amounted in the charcoal b, after six months' use, to 2.25, and in the charcoal c, after being two years in use, to 3.75 of 1000. By this it is also proved, that charcoal, under the influence of light, air, water, and vegetation, is gradually decomposed, by losing carbon; in the place of which hydrogen and oxygen predominate, and concur with the remains of carbonate to form humic acid.

No less interesting is the further comparison of the ashes of, I may say, the virgin charcoal a and the charcoal b, which had been used half a year for vegetation; in this instance a and b were in the proportion of 122 to 75 of ashes from 1000 of charcoal. Undoubtedly the dissoluble salts were, in proportion to the increasing decomposition of the charcoal, absorbed by the roots. That the greater weight of the ashes of c is not decisive has been already mentioned. To make very correct experiments of this sort, charcoal from the same tree should be burnt, equally reduced to powder, and, in planting in this powder, all impurities of garden mould, &c., carefully avoided, and watering the plants with rain-water attended to.

5. Antiseptic Power of Charcoal. In judging of the effects of charcoal on vegetation, its antiseptic properties are of great importance, for it has very little power of retaining water, and the little it retains is partly absorbed by the roots and partly evaporated. This property deserves the greatest attention of gardeners, in respect to recovering the health of plants the roots of which have become injured by being in a clayey soil, and too freely watered, or after continued rain, or being in contact with manure not sufficiently decomposed. They should be immediately transplanted into charcoal powder, as the most effectual method of cure.

6. Literature. In all scientific examinations, if they have any pretension to be well-founded, the greatest assistance may be procured from historical and literary researches. In this instance, however, it is very remarkable that all research in books which were at command was only a loss of time and trouble. In Dietrich's Perfect Lexicon of Gardening and Botany, with the Supplements, I looked in vain for the article "Charcoal," or "Charcoal Dust." I did not find more in Piever's Encyclopedian Dictionary or in Brockhaus's Conversations Lexicon. Leopold's Economical Dictionary contains only the following short passage: '' Charcoal dust makes the earth light, and, when mixed with sand, is very useful in a clayey soil."

In Krünitz's Encyclopedia, vol. 43., the article Charcoal is very comprehensive with respect to its preparation and technical application; but, with regard to agriculture and gardening, it is only mentioned (p. 225.) that "charcoal ashes serve to improve soils, and earth becomes very light by it." M. Lucas says he has looked for it in London's Encyclopedia of Gardening, and found no further information there than in Krünitz.

From this I think I may conclude that all that has hitherto been known for the improvement of the soil by charcoal dust was only founded on casual observations, and that experiments made with a view to science on this very commonplace article, in the manner in which M. Lucas has pursued them with so much success, have not been hitherto made.