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.
Every person connected with the cultivation of the soil is aware that soils wear out, or become exhausted, by being constantly cropped with one kind of plant. It has been supposed by many, that the soil under such circumstances becomes impoverished by the plants. Others, again, conclude that it is poisoned by excrements thrown off by the roots. I propose to examine the arguments advanced by the latter, not with the object of showing that the soil may not be deteriorated by the excrements alluded to, but that they have hitherto failed to prove even the probability of the hypothesis.
It seems to be admitted that plants excrete indigestible matter from their roots. "The roots not only absorb fluid from the soil, but they return a portion of their peculiar secretions back again into it, as has been found by Brugmans, who ascertained that some plants exude an acid fluid from their spongioles; and also by Mr. Macare, who has proved that to excrete superabundant matter from the roots, is a general property of the vegetable kingdom." - (Lindley, In. Bot., Book, II, Chap. 2.) "If we place a growing bulb in a vessel of water, but do not change the water, in a few days a slimy substance appears in the water, evidently an excretion from the roots." - (Horticulturist, Vol. VII, p. 607.) "If you place a succory in water, it will be found that the roots will by degrees render the water bitter, as if opium had been mixed with it; a spurge will render it acrid; and a leguminous plant mucilaginous. And if you poison one half of the roots of any plant, the other half will throw the poison off again from the system." - (Theory of Horticulture, par. 40.) "An apple orchard will not immediately succeed upon the site of an old orchard of the same kind of fruit. No amout of manuring will enable it to succeed.
All the extracts, except the last, show that plants do excrete from the roots certain substances. The last extract is one of many observations which have been recorded to show that successive cropping deteriorates soil. That there is some change in it, is undoubted; but that it is caused by their excrements, is assumed. We know that each individual plant absorbs from the soil peculiar elements; or, at least, peculiar proportions of various elements. This has been proved by chemical analysis. Have the excrements of any given plant been subjected to the same ordeal, and the nature of the assumed poison been detected ? Would it prove to be arsenic, opium, or any of the metallic or alkaline poisons that are known to be as destructive to vegetable as to animal life? Whether with the idea of showing the injurious effects of excremen-titious matter or not, such an analysis would be highly interesting to those who cultivate the soil. The idea that it injures the soil, seems to originate from the apparent impossibility of a plant taking up again what it has once thrown off.
But has it been proved that the matter thus thrown off must necessarily remain unaltered in the soil, and may not be immediately changed, by conjunction or chemical affinity with other substances, to matter useful to the growth and existence of the plant ? It has been inferred by a correspondent of the Horticulturist, in the page quoted above, that many of the deaths which are supposed to result from "sour soil" and "overwater-ing," are rather caused by an accumulation of excrementitious discharges. Has there been anything discovered by which to prove that this "sourness" is not what most gardeners take it to be - the result of an acid? I, and no doubt the majority of my practical brethren, have frequently observed that the greater the amount of organic matter in soil, the easier does it "sour." We are particularly anxious to avoid the use of leaf mold in any other state than "well-rotted," on that account An abundance of water is favorable to the decomposition of organic matter; and "soils contain a peculiar acid analogous to humic acid, produced during the decay of vegetable matter, which is hurtful to the growth of plants." - (Solly. Bur. Chem., par. 313.) Lindley himself, one of the advocates of the excrementitious doctrine, suggests that " the subject has hitherto been so little investigated, that it is not safe, perhaps, to take it as the basis of a theory." - (Theory of Horticulture.) And since his time, Hen-fret, the most searching of modern physiologists, considers the theory as " entirely without foundation." - (Henfrey's Outlines of Veg. Phys. I quote from memory).
Although there seems, then, to be no evidence that the excrements of plants have the injurious effects on the soil they are charged with, the discussion of the subject has been productive of good. It has caused attention to be called to the nature of impoverished soils, and its causes, which must in the end produce knowledge which will be of undoubted interest to the cultivator of the soil.
[The question regarding the excretions of plants stands at the present moment as unsettled and debateable as ever. Duiiamel, we believe, was the first to call attention to it, being led to suspect excretions on account of the earth adhering to the young roots, or spongioles of plants. Brugmans, Decandolle, and others, took it up and collected a large number of facts (familiar to readers on this subject) bearing upon the point and furnishing a sort of circumstantial evidence in favor of such a theory. Macaire Prinsep, at the suggestion of Decandolle, made a series of experiments to test it, by growing a great variety of plants in phials of pure water; and the results were such as gave additional strength to the excretive doctrine, at the time. But practical men were not satisfied with experiments conducted upon plants in such unnatural conditions, and other experiments of a more reliable nature, subsequently conducted by some German phytologists, produced entirely opposite results. The theory, therefore, has never been fully established, although we believe that a majority of those who have given the subject attention regard it with a greater or less degree of favor. For our own part, we do not wholly discard the doctrine.
We almost daily meet with cases in practice that persuade us that at least it is not unreasonable. In turning up the soil in which certain plants have grown - the cabbage and turnip tribe, for instance - we find peculiar odors escape. So in potting or shifting house-plants, we find that different plants impregnate with different odors the soil in which they have grown. This may or may not be due to the process of excretion.
We do not, however, believe that this process is such, in any case, as to unfit the soil to reproduce the plants which grew in it. The fact, with which we all are familiar, of the failure of successive crops of any given tree or plant in the same soil, is unquestionably due, in the main, to an exhaustion of the soil, or its loss of the elements essential to the growth and perfection of such trees or plants. Our American agriculture exhibits thousands of instances where, owing to the extraordinary fertility of the land, twenty successive crops pf corn have been grown in the greatest perfection, without any renewal of the soil by deep tillage or the addition of manures. In other soils two such crops could scarcely be taken in succession. So in regard to other crops. In the case of nursery trees, we have known instances where a single crop of apple trees of four or five years growth so exhausted the soil, that a succeeding one of the same trees was a perfect failure, notwithstanding the most liberal culture.
Rotation of crops is an established principle in field, garden, and nursery culture; and this, not because plants excrete matters unfavorable to their growth, or favorable to the growth of a different class of plants; but chiefly, as we have already said, because they exhaust the soil of certain elements which are necessary and indispensable to their particular structure, composition, and mode of growth. Excretion to some extent is, however, possible and even probable.
The question is full of interest, not only in a scientific point of view, but as having a direct bearing upon one of the most important branches of culture - the nutrition of plants. - Ed].