Manures and the principles of manuring

by Charles Morton Aikman

The great advance made in the direction of the improvement of the accuracy of old analytical processes and the discovery of numerous new ones have furnished us with elaborate analyses of the composition of plants. We now know that the plant-substance is made up of a large number of complex organic substances, formed out of carbon, hydrogen, oxygen, and nitrogen,[20] and that these substances form, on an average, about 95 per cent of the dry vegetable matter; the other 5 per cent being made up of mineral substances. As to the source of these different substances, our knowledge is, on the whole, pretty complete. With regard to the carbon of green-leaved plants, which amounts to from 40 to 50 per cent, subsequent research has confirmed Sénébier and de Saussure's conclusions, that its source is the carbonic acid gas of the air. The decomposition of the carbonic acid gas is effected by the leaves under the influence of sunlight. That a certain quantity of carbon may be obtained from the carbonic acid absorbed by plant-roots, is indeed probable. Especially during the early stages of plant-growth this source of carbon may be of considerable importance. Generally speaking, however, it may be said of all green-leaved plants, that the chief source of their carbon is the carbonic acid gas in the atmosphere.

Carbon Fixation by Plants.

The exact way in which this decomposition of carbonic acid gas is effected by the leaves is not yet clear. It seems to be directly dependent, in some way or other, on the chlorophyll, or green colouring matter. This decomposition of carbonic acid, and the fixation of the carbon by the plant with the formation of starch, takes place only under the influence of sunlight. During the night a reflex action takes place, which is commonly known as respiration, and which is exactly analogous to animal respiration.[21] The rate at which the fixation of carbon takes place depends on the strength of the sun's rays. It seems to take place very rapidly under a strong tropical sun.[22] The action of sunlight on the absorption of carbon has been studied by a number of observers, among others by Sachs, Draper, Cloez, Gratiolet, Caillet, Prillieux, Lommel, &c.

Action of Light on Plant-growth.

Experiments made by several observers, more especially Pfeffer, have shown that the yellow rays of the solar spectrum are the most potent in inducing this decomposition.

Some interesting experiments have been carried out by different observers on the possibility of growing plants under the influence of artificial light. While it would seem that the light from oil-lamps or gaslight is unable to promote growth, except in very exceptional cases, the electric light, or other strong artificial light, seems to be capable of taking the place of sunlight. Heinrich was the first to show that sunlight could be replaced by the magnesium light.

Experiments with the electric light have been carried out by Hervé-Mangon in France and Dr Siemens in England. The plants grown under the influence of the electric light were observed to be of a lighter green colour than those grown under normal conditions, thus indicating a feebler growth; in fact, Siemens was of the opinion that the electric light was about half as effective as daylight.[23]

These experiments are interesting from an industrial point of view; for it is conceivable that at some distant time electricity might be called to the aid of the agriculturist.

Source of Plants' Oxygen.

With regard to the source of the oxygen, which, next to carbon, is the element most largely present in the plant's substance—amounting to, roughly speaking, about 40 per cent—all evidence seems to indicate that it is chiefly derived from water, which is also the source of the plant's hydrogen. In addition to water, carbonic acid and nitric acid may also furnish small quantities. It has been pretty conclusively proved that the atmospheric oxygen, while necessary to plant-growth, and promoting the various chemical vital processes, is not a direct source of the plant's oxygen. The important function played by atmospheric oxygen in certain stages of the plant's growth has been long recognised. Malpighi, nearly two hundred years ago, observed that for the process of germination atmospheric air was necessary; and shortly after the discovery of the composition of the air was made, oxygen was identified as the important gas in promoting this process. Oxygen is also especially necessary during the period of ripening.