In these processes it would seem that phosphoric acid plays a most important, and, as it would seem, an essential part. To this I shall return: at present I merely consider man’s influence on the soil or earth he lives on, what he derives from it, and what he returns to it, and in what form it is and ought to be returned. If it be true that without trees there would be no underwood, no corn, and no crops,—for trees attract the fertilizing rain, and cause the springs perpetually to flow which diffuse prosperity and comfort,—then assuredly man ought to be most careful in interfering with nature. It is the remark, I think of the illustrious Humboldt, that when the white man took possession of certain districts of North America, vast forests prevailed everywhere. On taking possession, experience showed that agues prevailed, and that wheat might be grown successfully. The forests have been now destroyed, and agues have disappeared; but phthisis pulmonalis prevails, and wheat no longer grows to maturity. We interfere with the soil as nature made it when we force it to produce from one acre the natural produce of ten; we interfere with the processes of nature when we load the air with the products of thousands of furnaces, manufactories, and the poison exhaled from poisonous rivers and brooks; and we interfere with nature when we alter the constitution of those streams and rivers from a natural to an artificial state, loading them with the refuse of our artificially-drained fields, &c.
Let us listen to Liebig on a matter to which he has given the utmost possible attention:—
“Experience in agriculture shows that the production of vegetables on a given surface increases with the supply of certain matters, originally part of the soil which had been taken up from it by plants—the excreta of man and animals. These are nothing more than matters derived from vegetable food, which in the vital processes of animals, or after their death, assume again the form under which they originally existed as parts of the soil. Now we know that the atmosphere contains none of those substances, and therefore can replace none; and we know that their removal from a soil destroys its fertility, which may be restored and increased by a new supply. Is it possible, after so many decisive investigations into the origin of the elements of animals and vegetables, the use of the alkalies of lime and the phosphates, that any doubt can exist as to the principles upon which a rational agriculture depends? Can the art of agriculture be based upon anything but the restitution of a disturbed equilibrium? Can it be imagined that any country, however rich and fertile, with a flourishing commerce, which for centuries exports its produce in the shape of grain and cattle, will maintain its fertility if the same commerce does not restore, in some form of manure, those elements which have been removed from the soil, and which cannot be replaced by the atmosphere? Must not the same fate await every such country, which has actually befallen the once prolific soil of Virginia, now in many parts no longer able to grow its former staple productions—wheat and tobacco? In the large towns of England the produce both of English and foreign agriculture is largely consumed. Elements of the soil indispensable to plants, do not return to the fields; contrivances resulting from the manners and customs of the English people, and peculiar to them, render it difficult, perhaps impossible, to collect the enormous quantity of the phosphates which are daily, as solid and liquid excreta, carried into the rivers. These phosphates, although present in the soil in the smallest quantity, are its most important mineral constituents. It was observed that many English fields exhausted in that manner, immediately doubled their produce as if by a miracle when dressed with bone earth imported from the Continent. But if the export of bones from Germany is continued to the extent it has now reached, our soil must be gradually exhausted, and the extent of our loss may be estimated by considering that one pound of bones contains as much phosphoric acid as a hundredweight of grain.”
Many practical farmers, I am aware, still doubt the facts and theories of chemistry as applied to agriculture; with them I am free to admit that agriculture is not a science as yet, but an experimental art. With this I have nothing to do directly, my object being to show in this chapter in how far civilized man modifies and influences the soil on which he lives. He, the practical farmer, clings to farmyard manure, which he collects in heaps in his farmyard, or by the roadside, exposed to every change of weather, to drenching rains, to summer heat, and winter’s cold; from it run in streams over the roads the liquid parts of the manure, carrying with them the soluble salts; out of what is left when it has become rotten he hopes to restore to the field what it lost during the previous crop, and to a certain extent he succeeds; on the other hand, the chemist argues that the grand object of modern agriculture is to substitute for farmyard manure, that universal food of plants, their elements, obtained from other and cheaper sources retaining its full efficacy; and this can only be done when we shall have learned, what as yet we know but imperfectly, how to give to an artificial mixture of the individual ingredients the mechanical form and chemical qualities essential to their reception, and to their nutritive action on the plant; for without this form they cannot perfectly supply the place of farmyard manure. All our labours must be devoted to the attainment of this important object.
However this may be, and however it may be explained by the chemists, it must be admitted that to the accidental discovery of bone manure England owes many turnip crops, and to the introduction of guano from Peru and Ichaboe crops of wheat which no other manure as yet known could have produced. Peruvian guano, the best of all, is the excreta of a sea bird; these excreta, placed in a clear and perfectly dry atmosphere, have been exposed for centuries to a tropical sun; no rain falls on the heaps, trodden down only by the gentle feet of the birds themselves.
That out of such a product there should arise so excellent a manure surpasses all previous reasoning derived from mere science.[38] It is obvious, then, that much still remains to be discovered. Were any proof of this required, we might refer to the agriculture of China, where, as has been reported, human excreta alone are used as manure, and with a success unequalled in any other part of the world. In that singular land they have discovered much, or using perhaps the discoveries of preceding races, have turned them to the best account. Their agriculture is said to be perfect.
With such a system of manure and such a population one might predicate a condition of earth, air, and water, incompatible with human life. Now the very reverse happens, at least, in so far as regards the Chinese themselves.
No land so teems with a population strong, active, and in robust health; true, it does not suit the European constitution; fever and dysentery sweep off the troops and sailors of European nations who visit the Celestial Empire for the purpose of trade or of plunder. There is a something unknown in the climate unsuitable to the European; the condition of the earth, air, and water of China, is fatal to him. In which of these does the noxious element reside—in all or in none? This is possible; but man in the meantime must decide by what he knows and sees. Here is a land teeming with life; on land, as on its waters, millions live; but that life, as regards man, is confined to the Chinese race, and is unsuited to the European; as regards the soil, manured in so strange a manner, it also is Chinese. Is it that we, generally speaking, spread the material in a liquid and vastly diluted form over the fields, whilst they manipulate and remove from it all moisture? There may be something in this, for it is known that organic compounds, above all, are most susceptible of change by the least perceivable alterations in their constituents. Agriculture is both a science and an art.
“The clearing of the primeval forests of America, facilitating the access of the air to that soil, so rich in vegetable remains, alters gradually, but altogether, its constitution; after the lapse of a few years no trace of organic remains can be found in it. The soil of Germany, in the time of Tacitus, was covered with a dense, almost impenetrable forest; it must at that period have exactly resembled the soil of America, and have been rich in humus and vegetable substances; but all the products of vegetable life in those primeval forests have completely vanished from our perceptions. The innumerable millions of molluscous and other animals, whose remains form extensive geological formations and mountains, have after death passed into a state of fermentation and putrefaction, and subsequently, by the continuous action of the atmosphere, all their soft parts have been transposed into gaseous compounds, and their shells and bones, their indestructible constituents, alone remain to furnish evidence of the existence of life continually extinguished and continually reproduced.”
If these facts are to be depended on, they explain much of the influence which man exercises over the soil, and of its reaction on himself; the hay ague or fever is the produce of his own hands; when he leaves on the surface millions of tons of fermentable and putrescible organic remains, he prepares for himself some at least of the diseases which are to follow. It is possible that epidemic influence, over which he neither has nor can have any control, might be greatly modified, and its evil effects abated by prudent action on his part. Typhus fever, the scourge of modern Europe, may not originate in any condition of the soil produced by man, but it sweeps thousands in the prime of life from the earth when placed in circumstances clearly dependent on man himself. Ten thousand young men are lodged in a barrack; speedily hundreds of these are swept off by typhus or consumption of the lungs; now something causes this, and the cause may rest with man himself. Pestilence and typhus follow in the train of famine; if they originate in fermentescible and putrescible substances, all these were present prior to the famine, and yet were not equal to the production of the maladies. Next comes famine, and prepares the way for malaria to do its work. The question, as may be already seen, is not so simple as chemists supposed it to be. The number of substances occurring in nature which are truly putrescible is singularly small;[39] but they are everywhere diffused, and form part of every organized being. To form an idea of what this amounts to, we have but to reflect on the life which naturally exists on the earth, and on that which is the result of man’s social condition. Let but the acre of heath or bog, even of pasture, which in its natural state supports so little of what lives, be converted into a garden, a wheat field, a nursery, and see what an amount of putrescible matter is the result. Let that spot on which nature has placed a single peasant’s family be converted into a city, and reflect on the influence man exerts on that soil. It is, I believe, a fact universally admitted, that all those substances which destroy the communicability or arrest the propagation of contagions and miasms, are likewise such as arrest all processes of putrefaction or fermentation; that under the influence of empyreumatic bodies, such as pyroligneous acid, which powerfully oppose putrefaction, the diseased action in malignant suppurating wounds is entirely changed; that in a number of contagious diseases, especially typhus, ammonia, free or combined, is found in the exposed air, in the liquid and solid excreta (in the latter as ammonio-phosphate of magnesia); such being the case, it seems impossible any longer to entertain a doubt as to the origin and propagation of many contagious diseases.