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Liebig's Chemical Letters



My dear Sir,

    You are now acquainted with my opinions respectingthe effects of the application of mineral agents to our cultivated fields, and alsothe rationale of the influence of the various kinds of manures; you will, therefore,now readily understand what I have to say of the sources whence the carbon and nitrogen,indispensable to the growth of plants, are derived.

    The growth of forests, and the produce of meadows,demonstrate that an inexhaustible quantity of carbon is furnished for vegetationby the carbonic acid of the atmosphere.

    We obtain from an equal surface of forest, ormeadow-land, where the necessary mineral elements of the soil are present in a suitablestate, and to which no carbonaceous matter whatever is furnished in manures, an amountof carbon, in the shape of wood and hay, quite equal, and oftimes more than is producedby our fields, in grain, roots, and straw, upon which abundance of manure has beenheaped.

    It is perfectly obvious that the atmospheremust furnish to our cultivated fields as much carbonic acid, as it does to an equalsurface of forest or meadow, and that the carbon of this carbonic acid is assimilated,or may be assimilated by the plants growing there, provided the conditions essentialto its assimilation, and becoming a constituent element of vegetables, exist in thesoil of these fields.

    In many tropical countries the produce of theland in grain or roots, during the whole year, depends upon one rain in the spring.If this rain is deficient in quantity, or altogether wanting, the expectation ofan abundant harvest is diminished or destroyed.

    Now it cannot be the water merely which producesthis enlivening and fertilising effect observed, and which lasts for weeks and months.The plant receives, by means of this water, at the time of its first development,the alkalies, alkaline earths, and phosphates, necessary to its organization. Ifthese elements, which are necessary previous to its assimilation of atmospheric nourishment,be absent, its growth is retarded. In fact, the development of a plant is in a directratio to the amount of the matters it takes up from the soil. If, therefore, a soilis deficient in these mineral constituents required by plants, they will not flourisheven with an abundant supply of water.

    The produce of carbon on a meadow, or an equalsurface of forest land, is independent of a supply of carbonaceous manure, but itdepends upon the presence of certain elements of the soil which in themselves containno carbon, together with the existence of conditions under which their assimilationby plants can be effected. We increase the produce of our cultivated fields, in carbon,by a supply of lime, ashes, and marl, substances which cannot furnish carbon to theplants, and yet it is indisputable, - being founded upon abundant experience, - thatin these substances we furnish to the fields elements which greatly increase thebulk of their produce, and consequently the amount of carbon.

    If we admit these facts to be established, wecan no longer doubt that a deficient produce of carbon, or in other words, the barrennessof a field does not depend upon carbonic acid, because we are able to increase theproduce, to a certain degree, by a supply of substances which do not contain anycarbon. The same source whence the meadow and the forest are furnished with carbon,is also open to our cultivated plants. The great object of agriculture, therefore,is to discover the means best adapted to enable these plants to assimilate the carbonof the atmosphere which exists in it as carbonic acid. In furnishing plants, therefore,with mineral elements, we give them the power to appropriate carbon from a sourcewhich is inexhaustible; whilst in the absence of these elements the most abundantsupply of carbonic acid, or of decaying vegetable matter, would not increase theproduce of a field.

    With an adequate and equal supply of these essentialmineral constituents in the soil, the amount of carbonic acid absorbed by a plantfrom the atmosphere in a given time is limited by the quantity which is brought intocontact with its organs of absorption.

    The withdrawal of carbonic acid from the atmosphereby the vegetable organism takes place chiefly through its leaves; this absorptionrequires the contact of the carbonic acid with their surface, or with the part ofthe plant by which it is absorbed.

    The quantity of carbonic acid absorbed in agiven time is in direct proportion to the surface of the leaves and the amount ofcarbonic acid contained in the air; that is, two plants of the same kind and thesame extent of surface of absorption, in equal times and under equal conditions,absorb one and the same amount of carbon.

    In an atmosphere containing a double proportionof carbonic acid, a plant absorbs, under the same condition, twice the quantity ofcarbon. Boussingault observed, that the leaves of the vine, inclosed in a vessel,withdrew all the carbonic acid from a current of air which was passed through it,however great its velocity. (Dumas Le‡on, p.23.) If, therefore, we supply doublethe quantity of carbonic acid to one plant, the extent of the surface of which isonly half that of another living in ordinary atmospheric air, the former will obtainand appropriate as much carbon as the latter. Hence results the effects of humus,and all decaying organic substances, upon vegetation. If we suppose all the conditionsfor the absorption of carbonic acid present, a young plant will increase in mass,in a limited time, only in proportion to its absorbing surface; but if we createin the soil a new source of carbonic acid, by decaying vegetable substances, andthe roots absorb in the same time three times as much carbonic acid from the soilas the leaves derive from the atmosphere, the plant will increase in weight fourfold.This fourfold increase extends to the leaves, buds, stalks, &c., and in the increasedextent of the surface, the plant acquires an increased power of absorbing nourishmentfrom the air, which continues in action far beyond the time when its derivation ofcarbonic acid through the roots ceases. Humus, as a source of carbonic acid in cultivatedlands, is not only useful as a means of increasing the quantity of carbon - an effectwhich in most cases may be very indifferent for agricultural purposes - but the massof the plant having increased rapidly in a short time, space is obtained for theassimilation of the elements of the soil necessary for the formation of new leavesand branches.

    Water evaporates incessantly from the surfaceof the young plant; its quantity is in direct proportion to the temperature and theextent of the surface. The numerous radical fibrillae replace, like so many pumps,the evaporated water; and so long as the soil is moist, or penetrated with water,the indispensable elements of the soil, dissolved in the water, are supplied to theplant. The water absorbed by the plant evaporating in an a‰riform state leaves thesaline and other mineral constituents within it. The relative proportion of theseelements taken up by a plant, is greater, the more extensive the surface and moreabundant the supply of water; where these are limited, the plant soon reaches itsfull growth, while if their supply is continued, a greater amount of elements necessaryto enable it to appropriate atmospheric nourishment being obtained, its developmentproceeds much further. The quantity, or mass of seed produced, will correspond tothe quantity of mineral constituents present in the plant. That plant, therefore,containing the most alkaline phosphates and earthy salts will produce more or a greaterweight of seeds than another which, in an equal time has absorbed less of them. Weconsequently observe, in a hot summer, when a further supply of mineral ingredientsfrom the soil ceases through want of water, that the height and strength of plants,as well as the development of their seeds, are in direct proportion to its absorptionof the elementary parts of the soil in the preceding epochs of its growth.

    The fertility of the year depends in generalupon the temperature, and the moisture or dryness of the spring, if all the conditionsnecessary to the assimilation of the atmospheric nourishment be secured to our cultivatedplants. The action of humus, then, as we have explained it above, is chiefly of valuein gaining time. In agriculture, this must ever be taken into account and in thisrespect humus is of importance in favouring the growth of vegetables, cabbages, &c.

    But the cerealia, and plants grown for theirroots, meet on our fields, in the remains of the preceding crop, with a quantityof decaying vegetable substances corresponding to their contents of mineral nutrimentfrom the soil, and consequently with a quantity of carbonic acid adequate to theiraccelerated development in the spring. A further supply of carbonic acid, therefore,would be quite useless, without a corresponding increase of mineral ingredients.

    From a morgen of good meadow land, 2,500 poundsweight of hay, according to the best agriculturists, are obtained on an average.This amount is furnished without any supply of organic substances, without manurecontaining carbon or nitrogen. By irrigation, and the application of ashes or gypsum,double that amount may be grown. But assuming 2,500 pounds weight of hay to be themaximum, we may calculate the amount of carbon and nitrogen derived from the atmosphereby the plants of meadows.

    According to elementary analysis, hay, driedat a temperature of 100ø Reaumur, contains 45ù8 per cent. of carbon, and1ù5 per cent. of nitrogen. 14 per cent. of water retained by the hay, driedat common temperatures, is driven off at 100ø. 2,500 pounds weight of hay, therefore,corresponds to 2,150 pounds, dried at 100ø. This shows us, that 984 pounds ofcarbon, and 32ù2 pounds weight of nitrogen, have been obtained in the produceof one morgen of meadow land. Supposing that this nitrogen has been absorbed by theplants in the form of ammonia, the atmosphere contains 39ù1 pounds weight ofammonia to every 3640 pounds weight of carbonic acid (=984 carbon, or 27 per cent.),or in other words, to every 1,000 pounds weight of carbonic acid, 10ù7 poundsof ammonia, that is to about 100,000, the weight of the air, or 60,000of its volume.

    For every 100 parts of carbonic acid absorbedby the surface of the leaves, the plant receives from the atmosphere somewhat morethan one part of ammonia.

    With every 1,000 pounds of carbon, we obtain-

    From a meadow . 32.7 pounds of nitrogen.

    From cultivated fields,

    Wheat . 21ù5 " "

    Oats . 22ù3 " "

    Rye . 15ù2 " "

    Potatoes . 34ù1 " "

    Beetroot . 39ù1 " "

    Clover . 44 " "

    Peas . 62 " "

    Boussingault obtained from his farm at Bechelbronn,in Alsace, in five years, in the shape of potatoes, wheat, clover, turnips, and oats,8,383 of carbon, and 250ù7 nitrogen. In the following five years, as beetroot,wheat, clover, turnips, oats, and rye, 8,192 of carbon, and 284ù2 of nitrogen.In a further course of six years, potatoes, wheat, clover, turnips, peas, and rye,10,949 of carbon, 356ù6 of nitrogen. In 16 years, 27,424 carbon, 858ù5nitrogen, which gives for every 1,000 carbon, 31ù3 nitrogen.

    From these interesting and unquestionable facts,we may deduce some conclusions of the highest importance in their application toagriculture.

    1. We observe that the relative proportionsof carbon and nitrogen, stand in a fixed relation to the surface of the leaves. Thoseplants, in which all the nitrogen may be said to be concentrated in the seeds, asthe cerealia, contain on the whole less nitrogen than the leguminous plants, peas,and clover.

    2. The produce of nitrogen on a meadow whichreceives no nitrogenised manure, is greater than that of a field of wheat which hasbeen manured.

    3. The produce of nitrogen in clover and peas,which agriculturists will acknowledge require no nitrogenised manure, is far greaterthan that of a potato or turnip field, which is abundantly supplied with such manures.

    Lastly. And this is the most curious deductionto be derived from the above facts, - if we plant potatoes, wheat, turnips, peas,and clover, (plants containing potash, lime, and silex,) upon the same land, threetimes manured, we gain in 16 years, for a given quantity of carbon, the same proportionof nitrogen which we receive from a meadow which has received no nitrogenised manure.

    On a morgen of meadow-land, we obtain in plants,containing silex, lime, and potash, 984 carbon, 32ù2 nitrogen. On a morgen ofcultivated land, in an average of 16 years, in plants containing the same mineralelements, silex, lime, and potash, 857 carbon, 26ù8 nitrogen.

    If we add the carbon and nitrogen of the leavesof the beetroot, and the stalk and leaves of the potatoes, which have not been takeninto account, it still remains evident that the cultivated fields, notwithstandingthe supply of carbonaceous and nitrogenised manures, produced no more carbon andnitrogen than an equal surface of meadow-land supplied only with mineral elements.

    What then is the rationale of the effect ofmanure, - of the solid and fluid excrements of animals?

    This question can now be satisfactorily answered:that effect is the restoration of the elementary constituents of the soil which havebeen gradually drawn from it in the shape of grain and cattle. If the land I am speakingof had not been manured during those 16 years, not more than one-half, or perhapsthan one-third part of the carbon and nitrogen would have been produced. We owe itto the animal excrements, that it equalled in production the meadow-land, and this,because they restored the mineral ingredients of the soil removed by the crops. Allthat the supply of manure accomplished, was to prevent the land from becoming poorerin these, than the meadow which produces 2,500 pounds of hay. We withdraw from themeadow in this hay as large an amount of mineral substances as we do in one harvestof grain, and we know that the fertility of the meadow is just as dependent uponthe restoration of these ingredients to its soil, as the cultivated land is uponmanures. Two meadows of equal surface, containing unequal quantities of inorganicelements of nourishment, - other conditions being equal, - are very unequally fertile;that which possesses most, furnishes most hay. If we do not restore to a meadow thewithdrawn elements, its fertility decreases. But its fertility remains unimpaired,with a due supply of animal excrements, fluid and solid, and it not only remainsthe same, but may be increased by a supply of mineral substances alone, such as remainafter the combustion of ligneous plants and other vegetables; namely, ashes. Ashesrepresent the whole nourishment which vegetables receive from the soil. By furnishingthem in sufficient quantities to our meadows, we give to the plants growing on themthe power of condensing and absorbing carbon and nitrogen by their surface. May notthe effect of the solid and fluid excrements, which are the ashes of plants and grains,which have undergone combustion in the bodies of animals and of man, be dependentupon the same cause? Should not the fertility, resulting from their application,be altogether independent of the ammonia they contain? Would not their effect beprecisely the same in promoting the fertility of cultivated plants, if we had evaporatedthe urine, and dried and burned the solid excrements? Surely the cerealia and leguminousplants which we cultivate must derive their carbon and nitrogen from the same sourcewhence the graminea and leguminous plants of the meadows obtain them! No doubt canbe entertained of their capability to do so.

    In Virginia, upon the lowest calculation, 22pounds weight of nitrogen were taken on the average, yearly, from every morgen ofthe wheat-fields. This would amount, in 100 years, to 2,200 pounds weight. If thiswere derived from the soil, every morgen of it must have contained the equivalentof 110,000 pounds weight of animal excrements (assuming the latter, when dried, atthe temperature of boiling water, to contain 2 per cent.).

    In Hungary, as I remarked in a former Letter,tobacco and wheat have been grown upon the same field for centuries, without anysupply of nitrogenised manure. Is it possible that the nitrogen essential to, andentering into, the composition of these crops, could have been drawn from the soil?

    Every year renews the foliage and fruits ofour forests of beech, oak, and chesnuts; the leaves, the acorns, the chesnuts, arerich in nitrogen; so are cocoa-nuts, bread-fruit, and other tropical productions.This nitrogen is not supplied by man, can it indeed be derived from any other sourcethan the atmosphere?

    In whatever form the nitrogen supplied to plantsmay be contained in the atmosphere, in whatever state it may be when absorbed, fromthe atmosphere it must have been derived. Did not the fields of Virginia receivetheir nitrogen from the same source as wild plants?

    Is the supply of nitrogen in the excrementsof animals quite a matter of indifference, or do we receive back from our fieldsa quantity of the elements of blood corresponding to this supply?

    The researches of Boussingault have solved thisproblem in the most satisfactory manner. If, in his grand experiments, the manurewhich he gave to his fields was in the same state, i.e. dried at 110ø in a vacuum,as it was when analysed, these fields received, in 16 years, 1,300 pounds of nitrogen.But we know that by drying all the nitrogen escapes which is contained in solid animalexcrements, as volatile carbonate of ammonia. In this calculation the nitrogen ofthe urine, which by decomposition is converted into carbonate of ammonia, has notbeen included. If we suppose it amounted to half as much as that in the dried excrements,this would make the quantity of nitrogen supplied to the fields 1,950 pounds.

    In 16 years, however, as we have seen, only1,517 pounds of nitrogen, was contained in their produce of grain, straw, roots,et cetera - that is, far less than was supplied in the manure; and in the same periodthe same extent of surface of good meadow-land (one hectare = a Hessian morgen),which received no nitrogen in manure, 2,062 pounds of nitrogen.

    It is well known that in Egypt, from the deficiencyof wood, the excrement of animals is dried, and forms the principal fuel, and thatthe nitrogen from the soot of this excrement was, for many centuries, imported intoEurope in the form of sal ammoniac, until a method of manufacturing this substancewas discovered at the end of the last century by Gravenhorst of Brunswick. The fieldsin the delta of the Nile are supplied with no other animal manures than the ashesof the burnt excrements, and yet they have been proverbially fertile from a periodearlier than the first dawn of history, and that fertility continues to the presentday as admirable as it was in the earliest times. These fields receive, every year,from the inundation of the Nile, a new soil, in its mud deposited over their surface,rich in those mineral elements which have been withdrawn by the crops of the previousharvest. The mud of the Nile contains as little nitrogen as the mud derived fromthe Alps of Switzerland, which fertilises our fields after the inundations of theRhine. If this fertilising mud owed this property to nitrogenised matters; what enormousbeds of animal and vegetable exuviae and remains ought to exist in the mountainsof Africa, in heights extending beyond the limits of perpetual snow, where no bird,no animal finds food, from the absence of all vegetation!

    Abundant evidence in support of the importanttruth we are discussing, may be derived from other well known facts. Thus, the tradeof Holland in cheese may be adduced in proof and illustration thereof. We know thatcheese is derived from the plants which serve as food for cows. The meadow-landsof Holland derive the nitrogen of cheese from the same source as with us; i.e. theatmosphere. The milch cows of Holland remain day and night on the grazing-grounds,and therefore, in their fluid and solid excrements return directly to the soil allthe salts and earthy elements of their food: a very insignificant quantity only isexported in the cheese. The fertility of these meadows can, therefore, be as littleimpaired as our own fields, to which we restore all the elements of the soil, asmanure, which have been withdrawn in the crops. The only difference is, in Hollandthey remain on the field, whilst we collect them at home and carry them, from timeto time, to the fields.

    The nitrogen of the fluid and solid excrementsof cows, is derived from the meadow-plants, which receive it from the atmosphere;the nitrogen of the cheese also must be drawn from the same source. The meadows ofHolland have, in the lapse of centuries, produced millions of hundredweights of cheese.Thousands of hundredweights are annually exported, and yet the productiveness ofthe meadows is in no way diminished, although they never receive more nitrogen thanthey originally contained.

    Nothing then can be more certain than the fact,that an exportation of nitrogenised products does not exhaust the fertility of acountry; inasmuch as it is not the soil, but the atmosphere, which furnishes itsvegetation with nitrogen. It follows, consequently, that we cannot increase the fertilityof our fields by a supply of nitrogenised manure, or by salts of ammonia, but ratherthat their produce increases or diminishes, in a direct ratio, with the supply ofmineral elements capable of assimilation. The formation of the constituent elementsof blood, that is, of the nitrogenised principles in our cultivated plants, dependsupon the presence of inorganic matters in the soil, without which no nitrogen canbe assimilated even when there is a most abundant supply. The ammonia contained inanimal excrements exercises a favourable effect, inasmuch as it is accompanied bythe other substances necessary to accomplish its transition into the elements ofthe blood. If we supply ammonia associated with all the conditions necessary to itsassimilation, it ministers to the nourishment of the plants; but if this artificialsupply is not given they can derive all the needed nitrogen from the atmosphere -a source, every loss from which is restored by the decomposition of the bodies ofdead animals and the decay of plants. Ammonia certainly favours, and accelerates,the growth of plants in all soils, wherein all the conditions of its assimilationare united; but it is altogether without effect, as respects the production of theelements of blood where any of these conditions are wanting. We can suppose thatasparagin, the active constituent of asparagus, the mucilaginous root of the marsh-mallow,the nitrogenised and sulphurous ingredients of mustard-seed, and of all cruciferousplants, may originate without the aid of the mineral elements of the soil. But ifthe principles of those vegetables, which serve as food, could be generated withoutthe co-operation of the mineral elements of blood, without potash, soda, phosphateof soda, phosphate of lime, they would be useless to us and to herbivorous animalsas food; they would not fulfil the purpose for which the wisdom of the Creator hasdestined them. In the absence of alkalies and the phosphates, no blood, no milk,no muscular fibre can be formed. Without phosphate of lime our horses, sheep andcattle, would be without bones.

    In the urine and in the solid excrements ofanimals we carry ammonia, and, consequently, nitrogen, to our cultivated plants,and this nitrogen is accompanied by all the mineral elements of food exactly in thesame proportions, in which both are contained in the plants which served as foodto the animals, or what is the same, in those proportions in which both can serveas nourishment to a new generation of plants, to which both are essential.

    The effect of an artificial supply of ammonia,as a source of nitrogen, is, therefore, precisely analogous to that of humus as asource of carbonic acid - it is limited to a gain of time; that is, it acceleratesthe development of plants. This is of great importance, and should always be takeninto account in gardening, especially in the treatment of the kitchen-garden; andas much as possible, in agriculture on a large scale, where the time occupied inthe growth of the plants cultivated is of importance.

    When we have exactly ascertained the quantityof ashes left after the combustion of cultivated plants which have grown upon allvarieties of soil, and have obtained correct analyses of these ashes, we shall learnwith certainty which of the constituent elements of the plants are constant and whichare changeable, and we shall arrive at an exact knowledge of the sum of all the ingredientswe withdraw from the soil in the different crops.

    With this knowledge the farmer will be ableto keep an exact record, of the produce of his fields in harvest, like the account-bookof a well regulated manufactory; and then by a simple calculation he can determineprecisely the substances he must supply to each field, and the quantity of these,in order to restore their fertility. He will be able to express, in pounds weight,how much of this or that element he must give in order to augment its fertility forany given kind of plants.

    These researches and experiments are the greatdesideratum of the present time. TO THE UNITED EFFORTS OF THE CHEMISTS OF ALL COUNTRIESWE MAY CONFIDENTLY LOOK FOR A SOLUTION OF THESE GREAT QUESTIONS, and by the aid ofENLIGHTENED AGRICULTURISTS we shall arrive at a RATIONAL system of GARDENING, HORTICULTURE,and AGRICULTURE, applicable to every country and all kinds of soil, and which willbe based upon the immutable foundation of OBSERVED FACTS and PHILOSOPHICAL INDUCTION.