Liebig's Chemical Letters
LETTER XII
My dear Sir,
Having now occupied several letters with theattempt to unravel, by means of chemistry, some of the most curious functions ofthe animal body, and, as I hope, made clear to you the distinctions between the twokinds of constituent elements in food, and the purposes they severally subserve insustaining life, let me now direct your attention to a scarcely less interestingand equally important subject - the means of obtaining from a given surface of theearth the largest amount of produce adapted to the food of man and animals.
Agriculture is both a science and an art. Theknowledge of all the conditions of the life of vegetables, the origin of their elements,and the sources of their nourishment, forms its scientific basis.
From this knowledge we derive certain rulesfor the exercise of the ART, the principles upon which the mechanical operationsof farming depend, the usefulness or necessity of these for preparing the soil tosupport the growth of plants, and for removing every obnoxious influence. No experience,drawn from the exercise of the art, can be opposed to true scientific principles,because the latter should include all the results of practical operations, and arein some instances solely derived therefrom. Theory must correspond with experience,because it is nothing more than the reduction of a series of phenomena to their lastcauses.
A field in which we cultivate the same plantfor several successive years becomes barren for that plant in a period varying withthe nature of the soil: in one field it will be in three, in another in seven, ina third in twenty, in a fourth in a hundred years. One field bears wheat, and nopeas; another beans or turnips, but no tobacco; a third gives a plentiful crop ofturnips, but will not bear clover. What is the reason that a field loses its fertilityfor one plant, the same which at first flourished there? What is the reason one kindof plant succeeds in a field where another fails?
These questions belong to Science.
What means are necessary to preserve to a fieldits fertility for one and the same plant? - what to render one field fertile fortwo, for three, for all plants?
These last questions are put by Art, but theycannot be answered by Art.
If a farmer, without the guidance of just scientificprinciples, is trying experiments to render a field fertile for a plant which itotherwise will not bear, his prospect of success is very small. Thousands of farmerstry such experiments in various directions, the result of which is a mass of practicalexperience forming a method of cultivation which accomplishes the desired end forcertain places; but the same method frequently does not succeed, it indeed ceasesto be applicable to a second or third place in the immediate neighbourhood. How largea capital, and how much power, are wasted in these experiments! Very different, andfar more secure, is the path indicated by SCIENCE; it exposes us to no danger offailing, but, on the contrary, it furnishes us with every guarantee of success. Ifthe cause of failure - of barrenness in the soil for one or two plants - has beendiscovered, means to remedy it may readily be found.
The most exact observations prove that the methodof cultivation must vary with the geognostical condition of the subsoil. In basalt,graywacke, porphyry, sandstone, limestone, &c., are certain elements indispensableto the growth of plants, and the presence of which renders them fertile. This fullyexplains the difference in the necessary methods of culture for different places;since it is obvious that the essential elements of the soil must vary with the varietiesof composition of the rocks, from the disintegration of which they originated.
Wheat, clover, turnips, for example, each requirecertain elements from the soil; they will not flourish where the appropriate elementsare absent. Science teaches us what elements are essential to every species of plantsby an analysis of their ashes. If therefore a soil is found wanting in any of thoseelements, we discover at once the cause of its barrenness, and its removal may nowbe readily accomplished.
The empiric attributes all his success to themechanical operations of agriculture; he experiences and recognises their value,without inquiring what are the causes of their utility, their mode of action: andyet this scientific knowledge is of the highest importance for regulating the applicationof power and the expenditure of capital, - for insuring its economical expenditureand the prevention of waste. Can it be imagined that the mere passing of the ploughshareor the harrow through the soil - the mere contact of the iron - can impart fertilitymiraculously? Nobody, perhaps, seriously entertains such an opinion. Nevertheless,the modus operandi of these mechanical operations is by no means generally understood.The fact is quite certain, that careful ploughing exerts the most favourable influence:the surface is thus mechanically divided, changed, increased, and renovated; butthe ploughing is only auxiliary to the end sought.
In the effects of time, in what in Agricultureare technically called fallows - the repose of the fields - we recognise by sciencecertain chemical actions, which are continually exercised by the elements of theatmosphere upon the whole surface of our globe. By the action of its oxygen and itscarbonic acid, aided by water, rain, changes of temperature, &c., certain elementaryconstituents of rocks, or of their ruins, which form the soil capable of cultivation,are rendered soluble in water, and conseqently become separable from all their insolubleparts.
These chemical actions, poetically denominatesthe "tooth of time," destroy all the works of man, and gradually reducethe hardest rocks to the condition of dust. By their influence the necessary elementsof the soil become fitted for assimilation by plants; and it is precisely the endwhich is obtained by the mechanical operations of farming. They accelerate the decompositionof the soil, in order to provide a new generation of plants with the necesary elementsin a condition favourable to their assimilation. It is obvious that the rapidityof the decomposition of a solid body must increase with the extension of its surface;the more points of contact we offer in a given time to the external chemical agent,the more rapid will be its action.
The chemist, in order to prepare a mineral foranalysis, to decompose it, or to increase the solubility of its elements, proceedsin the same way as the farmer deals with his fields - he spares no labour in orderto reduce it to the finest powder; he separates the impalpable from the coarser partsby washing, and repeats his mechanical bruising and trituration, being assured hiswhole process will fail if he is inattentive to this essential and preliminary partof it.
The influence which the increase of surfaceexercises upon the disintegration of rocks, and upon the chemical action of air andmoisture, is strikingly illustrated upon a large scale in the operations pursuedin the gold-mines of Yaquil, in Chili. These are described in a very interestingmanner by Darwin. The rock containing the gold ore is pounded by mills into the finestpowder; this is subjected to washing, which separates the lighter particles fromthe metallic; the gold sinks to the bottom, while a stream of water carries awaythe lighter earthy parts into ponds, where it subsides to the bottom as mud. Whenthis deposit has gradually filled up the pond, this mud is taken out and piled inheaps, and left exposed to the action of the atmosphere and moisture. The washingcompletely removes all the soluble part of the disintegrated rock; the insolublepart, moreover, cannot undergo any further change while it is covered with water,and so excluded from the influence of the atmosphere at the bottom of the pond. Butbeing exposed at once to the air and moisture, a powerful chemical action takes placein the whole mass, which becomes indicated by an efflorescence of salts coveringthe whole surface of the heaps in considerable quantity. After being exposed fortwo or three years, the mud is again subjected to the same process of washing, anda considerable quantity of gold is obtained, this having been separated by the chemicalprocess of decomposition in the mass. The exposure and washing of the same mud isrepeated six or seven times, and at every washing it furnishes a new quantity ofgold, although its amount diminishes every time.
Precisely similar is the chemical action whichtakes place in the soil of our fields; and we accelerate and increase it by the mechanicaloperations of our agriculture. By these we sever and extend the surface, and endeavourto make every atom of the soil accessible to the action of the carbonic acid andoxygen of the atmosphere. We thus produce a stock of soluble mineral substances,which serves as nourishment to a new generation of plants, materials which are indispensableto their growth and prosperity.