Liebig's Chemical Letters
My dear Sir,
In the immense, yet limited expanse of the ocean,the animal and vegetable kingdoms are mutually dependent upon, and successive toeach other. The animals obtain their constituent elements from the plants, and restorethem to the water in their original form, when they again serve as nourishment toa new generation of plants.
The oxygen which marine animals withdraw intheir respiration from the air, dissolved in sea water, is returned to the waterby the vital processes of sea plants; that air is richer in oxygen than atmosphericair, containing 32 to 33 per cent. Oxygen, also, combines with the products of theputrefaction of dead animal bodies, changes their carbon into carbonic acid, theirhydrogen into water, and their nitrogen assumes again the form of ammonia.
Thus we observe in the ocean a circulation takesplace without the addition or subtraction of any element, unlimited in duration,although limited in extent, inasmuch as in a confined space the nourishment of plantsexists in a limited quantity.
We well know that marine plants cannot derivea supply of humus for their nourishment through their roots. Look at the great sea-tang,the Fucus giganteus: this plant, according to Cook, reaches a height of 360 feet,and a single specimen, with its immense ramifications, nourishes thousands of marineanimals, yet its root is a small body, no larger than the fist. What nourishmentcan this draw from a naked rock, upon the surface of which there is no perceptiblechange? It is quite obvious that these plants require only a hold, - a fasteningto prevent a change of place, - as a counterpoise to their specific gravity, whichis less than that of the medium in which they float. That medium provides the necessarynourishment, and presents it to the surface of every part of the plant. Sea-watercontains not only carbonic acid and ammonia, but the alkaline and earthy phosphatesand carbonates required by these plants for their growth, and which we always findas constant constituents of their ashes.
All experience demonstrates that the conditionsof the existence of marine plants are the same which are essential to terrestrialplants. But the latter do not live like sea-plants, in a medium which contains alltheir elements and surrounds with appropriate nourishment every part of their organs;on the contrary, they require two media, of which one, namely the soil, containsthose essential elements which are absent from the medium surrounding them, i.e.the atmosphere.
Is it possible that we could ever be in doubtrespecting the office which the soil and its component parts subserve in the existenceand growth of vegetables? - that there should have been a time when the mineral elementsof plants were not regarded as absolutely essential to their vitality? Has not thesame circulation been observed on the surface of the earth which we have just contemplatedin the ocean, - the same incessant change, disturbance and restitution of equilibrium?
Experience in agriculture shows that the productionof vegetables on a given surface increases with the supply of certain matters, originallyparts of the soil which had been taken up from it by plants - the excrements of manand animals. These are nothing more than matters derived from vegetable food, whichin the vital processes of animals, or after their death, assume again the form underwhich they originally existed, as parts of the soil. Now, we know that the atmospherecontains none of these substances, and therefore can replace none; and we know thattheir removal from a soil destroys its fertility, which may be restored and increasedby a new supply.
Is it possible, after so many decisive investigationsinto the origin of the elements of animals and vegetables, the use of the alkalies,of lime and the phosphates, any doubt can exist as to the principles upon which arational agriculture depends? Can the art of agriculture be based upon anything butthe restitution of a disturbed equilibrium? Can it be imagined that any country,however rich and fertile, with a flourishing commerce, which for centuries exportsits produce in the shape of grain and cattle, will maintain its fertility, if thesame commerce does not restore, in some form of manure, those elements which havebeen removed from the soil, and which cannot be replaced by the atmosphere? Mustnot the same fate await every such country which has actually befallen the once prolificsoil 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 bothof English and foreign agriculture is largely consumed; elements of the soil indispensableto plants do not return to the fields, - contrivances resulting from the mannersand customs of English people, and peculiar to them, render it difficult, perhapsimpossible, to collect the enormous quantity of the phosphates which are daily, assolid and liquid excrements, carried into the rivers. These phosphates, althoughpresent 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 doubledtheir produce, as if by a miracle, when dressed with bone earth imported from theContinent. But if the export of bones from Germany is continued to the extent ithas hitherto reached, our soil must be gradually exhausted, and the extent of ourloss may be estimated, by considering that one pound of bones contains as much phosphoricacid as a hundred-weight of grain.
The imperfect knowledge of Nature and the propertiesand relations of matter possessed by the alchemists gave rise, in their time, toan opinion that metals as well as plants could be produced from a seed. The regularforms and ramifications seen in crystals, they imagined to be the leaves and branchesof metal plants; and as they saw the seed of plants grow, producing root, stem andleaves, and again blossoms, fruit and seeds, apparently without receiving any supplyof appropriate material, they deemed it worthy of zealous inquiry to discover theseed of gold, and the earth necessary for its development. If the metal seeds wereonce obtained, might they not entertain hopes of their growth?
Such ideas could only be entertained when nothingwas known of the atmosphere, and its participation with the earth, in administeringto the vital processes of plants and animals. Modern chemistry indeed produces theelements of water, and, combining them, forms water anew; but it does not createthose elements - it derives them from water; the new-formed artificial water hasbeen water before.
Many of our farmers are like the alchemistsof old, - they are searching for the miraculous seed, - the means, which, withoutany further supply of nourishment to a soil scarcely rich enough to be sprinkledwith indigenous plants, shall produce crops of grain a hundred-fold.
The experience of centuries, nay, of thousandsof years, is insufficient to guard men against these fallacies; our only securityfrom these and similar absurdities must be derived from a correct knowledge of scientificprinciples.
In the first period of natural philosophy, organiclife was supposed to be derived from water only; afterwards, it was admitted thatcertain elements derived from the air must be superadded to the water; but we nowknow that other elements must be supplied by the earth, if plants are to thrive andmultiply.
The amount of materials contained in the atmosphere,suited to the nourishment of plants, is limited; but it must be abundantly sufficientto cover the whole surface of the earth with a rich vegetation. Under the tropics,and in those parts of our globe where the most genial conditions of fertility exist,- a suitable soil, a moist atmosphere, and a high temperature, - vegetation is scarcelylimited by space; and, where the soil is wanting, it is gradually supplied by thedecaying leaves, bark and branches of plants. It is obvious there is no deficiencyof atmospheric nourishment for plants in those regions, nor are these wanting inour own cultivated fields: all that plants require for their development is conveyedto them by the incessant motions of the atmosphere. The air between the tropics containsno more than that of the arctic zones; and yet how different is the amount of produceof an equal surface of land in the two situations!
This is easily explicable. All the plants oftropical climates, the oil and wax palms, the sugar cane, &c., contain only asmall quantity of the elements of the blood necessary to the nutrition of animals,as compared with our cultivated plants. The tubers of the potato in Chili, its nativecountry, where the plant resembles a shrub, if collected from an acre of land, wouldscarcely suffice to maintain an Irish family for a single day (Darwin). The resultof cultivation in those plants which serve as food, is to produce in them those constituentsof the blood. In the absence of the elements essential to these in the soil, starch,sugar and woody fibre, are perhaps formed; but no vegetable fibrine, albumen, orcaseine. If we intend to produce on a given surface of soil more of these lattermatters than the plants can obtain from the atmosphere or receive from the soil ofthe same surface in its uncultivated and normal state, we must create an artificialatmosphere, and add the needed elements to the soil.
The nourishment which must be supplied in agiven time to different plants, in order to admit a free and unimpeded growth, isvery unequal.
On pure sand, on calcareous soil, on naked rocks,only a few genera of plants prosper, and these are, for the most part, perennialplants. They require, for their slow growth, only such minute quantities of mineralsubstances as the soil can furnish, which may be totally barren for other species.Annual, and especially summer plants, grow and attain their perfection in a comparativelyshort time; they therefore do not prosper on a soil which is poor in those mineralsubstances necessary to their development. To attain a maximum in height in the shortperiod of their existence, the nourishment contained in the atmosphere is not sufficient.If the end of cultivation is to be obtained, we must create in the soil an artificialatmosphere of carbonic acid and ammonia; and this surplus of nourishment, which theleaves cannot appropriate from the air, must be taken up by the corresponding organs,i.e. the roots, from the soil. But the ammonia, together with the carbonic acid,are alone insufficient to become part of a plant destined to the nourishment of animals.In the absence of the alkalies, the phosphates and other earthy salts, no vegetablefibrine, no vegetable caseine, can be formed. The phosphoric acid of the phosphateof lime, indispensable to the cerealia and other vegetables in the formation of theirseeds, is separated as an excrement, in great quantities, by the rind and barks ofligneous plants.
How different are the evergreen plants, thecacti, the mosses, the ferns, and the pines, from our annual grasses, the cerealiaand leguminous vegetables! The former, at every time of the day during winter andsummer, obtain carbon through their leaves by absorbing carbonic acid which is notfurnished by the barren soil on which they grow; water is also absorbed and retainedby their coriaceous or fleshy leaves with great force. They lose very little by evaporation,compared with other plants. On the other hand, how very small is the quantity ofmineral substances which they withdraw from the soil during their almost constantgrowth in one year, in comparison with the quantity which one crop of wheat of anequal weight receives in three months!
It is by means of moisture that plants receivethe necessary alkalies and salts from the soil. In dry summers a phenomenon is observed,which, when the importance of mineral elements to the life of a plant was unknown,could not be explained. The leaves of plants first developed and perfected, and thereforenearer the surface of the soil, shrivel up and become yellow, lose their vitality,and fall off while the plant is in an active state of growth, without any visiblecause. This phenomenon is not seen in moist years, nor in evergreen plants, and butrarely in plants which have long and deep roots, nor is it seen in perennials inautumn and winter.
The cause of this premature decay is now obvious.The perfectly-developed leaves absorb continually carbonic acid and ammonia fromthe atmosphere, which are converted into elements of new leaves, buds, and shoots;but this metamorphosis cannot be effected without the aid of the alkalies, and othermineral substances. If the soil is moist, the latter are continually supplied toan adequate amount, and the plant retains its lively green colour; but if this supplyceases from a want of moisture to dissolve the mineral elements, a separation takesplace in the plant itself. The mineral constituents of the juice are withdrawn fromthe leaves already formed, and are used for the formation of the young shoots; andas soon as the seeds are developed, the vitality of the leaves completely ceases.These withered leaves contain only minute traces of soluble salts, while the budsand shoots are very rich in them.
On the other hand, it has been observed, thatwhere a soil is too highly impregnated with soluble saline materials, these are separatedupon the surface of the leaves. This happens to culinary vegetables especially, whoseleaves become covered with a white crust. In consequence of these exudations theplant sickens, its organic activity decreases, its growth is disturbed; and if thisstate continues long, the plant dies. This is most frequently seen in foliaceousplants, the large surfaces of which evaporate considerable quantities of water. Carrots,pumpkins, peas, &c., are frequently thus diseased, when, after dry weather, theplant being near its full growth, the soil is moistened by short showers, followedagain by dry weather. The rapid evaporation carries off the water absorbed by theroot, and this leaves the salts in the plant in a far greater quantity than it canassimilate. These salts effloresce upon the surface of the leaves, and if they areherbaceous and juicy, produce an effect upon them as if they had been watered witha solution containing a greater quantity of salts than their organism can bear.
Of two plants of the same species, this diseasebefalls that which is nearest its perfection; if one should have been planted later,or be more backward in its development, the same external cause which destroys theone will contribute to the growth of the other.
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