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Liebig's Chemical Letters



My dear Sir,

    I treated, in my last letter, of the means of improving thecondition of the soil for agricultural purposes by mechanical operations and mineralagents. I have now to speak of the uses and effects of animal exuviae, and vegetablematters or manures - properly so called.

    In order to understand the nature of these, and the peculiarityof their influence upon our fields, it is highly important to keep in mind the sourcewhence they are derived.

    It is generally known, that if we deprive an animal of food,the weight of its body diminishes during every moment of its existence. If this abstinenceis continued for some time, the diminution becomes apparent to the eye; all the fatof the body disappears, the muscles decrease in firmness and bulk, and, if the animalis allowed to die starved, scarcely anything but skin, tendon, and bones, remain.This emaciation which occurs in a body otherwise healthy, demonstrates to us, thatduring the life of an animal every part of its living substance is undergoing a perpetualchange; all its component parts, assuming the form of lifeless compounds, are thrownoff by the skin, lungs, and urinary system, altered more or less by the secretoryorgans. This change in the living body is intimately connected with the process ofrespiration; it is, in truth, occasioned by the oxygen of the atmosphere in breathing,which combines with all the various matters within the body. At every inspirationa quantity of oxygen passes into the blood in the lungs, and unites with its elements;but although the weight of the oxygen thus daily entering into the body amounts to32 or more ounces, yet the weight of the body is not thereby increased. Exactly asmuch oxygen as is imbibed in inspiration passes off in expiration, in the form ofcarbonic acid and water; so that with every breath the amount of carbon and hydrogenin the body is diminished. But the emaciation - the loss of weight by starvation- does not simply depend upon the separation of the carbon and hydrogen; but allthe other substances which are in combination with these elements in the living tissuespass off in the secretions. The nitrogen undergoes a change, and is thrown out ofthe system by the kidneys. Their secretion, the urine, contains not only a compoundrich in nitrogen, namely urea, but the sulphur of the tissues in the form of a sulphate,all the soluble salts of the blood and animal fluids, common salt, the phosphates,soda and potash. The carbon and hydrogen of the blood, of the muscular fibre, andof all the animal tissues which can undergo change, return into the atmosphere. Thenitrogen, and all the soluble inorganic elements are carried to the earth in theurine.

    These changes take place in the healthy animal body duringevery moment of life; a waste and loss of substance proceeds continually; and ifthis loss is to be restored, and the original weight and substance repaired, an adequatesupply of materials must be furnished, from whence the blood and wasted tissues maybe regenerated. This supply is obtained from the food.

    In an adult person in a normal or healthy condition, no sensibleincrease or decrease of weight occurs from day to day. In youth the weight of thebody increases, whilst in old age it decreases. There can be no doubt that in theadult, the food has exactly replaced the loss of substance: it has supplied justso much carbon, hydrogen, nitrogen, and other elements, as have passed through theskin, lungs, and urinary organs. In youth the supply is greater than the waste. Partof the elements of the food remain to augment the bulk of the body. In old age thewaste is greater than the supply, and the body diminishes. It is unquestionable,that, with the exception of a certain quantity of carbon and hydrogen, which aresecreted through the skin and lungs, we obtain, in the solid and fluid excrementsof man and animals, all the elements of their food.

    We obtain daily, in the form of urea, all the nitrogen takenin the food both of the young and the adult; and further, in the urine, the wholeamount of the alkalies, soluble phosphates and sulphates, contained in all the variousaliments. In the solid excrements are found all those substances taken in the foodwhich have undergone no alteration in the digestive organs, all indigestible matters,such as woody fibre, the green colouring matter of leaves ( chlorophyle), wax, &c.

    Physiology teaches us, that the process of nutrition in animals,that is, their increase of bulk, or the restoration of wasted parts, proceeds fromthe blood. The purpose of digestion and assimilation is to convert the food intoblood. In the stomach and intestines, therefore, all those substances in the foodcapable of conversion into blood are separated from its other constituents; in otherwords, during the passage of the food through the intestinal canal there is a constantabsorption of its nitrogen, since only azotised substances are capable of conversioninto blood; and therefore the solid excrements are destitute of that element, exceptonly a small portion, in the constitution of that secretion which is formed to facilitatetheir passage. With the solid excrements, the phosphates of lime and magnesia, whichwere contained in the food and not assimilated, are carried off, these salts beinginsoluble in water, and therefore not entering the urine.

    We may obtain a clear insight into the chemical constitutionof the solid excrements without further investigation, by comparing the faeces ofa dog with his food. We give that animal flesh and bones - substances rich in azotisedmatter - and we obtain, as the last product of its digestion, a perfectly white excrement,solid while moist, but becoming in dry air a powder. This is the phosphate of limeof the bones, with scarcely one per cent. of foreign organic matter.

    Thus we see that in the solid and fluid excrements of man andanimals, all the nitrogen - in short, all the constituent ingredients of the consumedfood, soluble and insoluble, are returned; and as food is primarily derived fromthe fields, we possess in those excrements all the ingredients which we have takenfrom it in the form of seeds, roots, or herbs.

    One part of the crops employed for fattening sheep and cattleis consumed by man as animal food; another part is taken directly - as flour, potatoes,green vegetables, &c.; a third portion consists of vegetable refuse, and strawemployed as litter. None of the materials of the soil need be lost. We can, it isobvious, get back all its constituent parts which have been withdrawn therefrom,as fruits, grain and animals, in the fluid and solid excrements of man, and the bones,blood and skins of the slaughtered animals. It depends upon ourselves to collectcarefully all these scattered elements, and to restore the disturbed equilibriumof composition in the soil. We can calculate exactly how much and which of the componentparts of the soil we export in a sheep or an ox, in a quarter of barley, wheat orpotatoes, and we can discover, from the known composition of the excrements of manand animals, how much we have to supply to restore what is lost to our fields.

    If, however, we could procure from other sources the substanceswhich give to the exuviae of man and animals their value in agriculture, we shouldnot need the latter. It is quite indifferent for our purpose whether we supply theammonia (the source of nitrogen) in the form of urine, or in that of a salt derivedfrom coal-tar; whether we derive the phosphate of lime from bones, apatite, or fossilexcrements (the coprolithes).

    The principal problem for agriculture is, how to replace thosesubstances which have been taken from the soil, and which cannot be furnished bythe atmosphere. If the manure supplies an imperfect compensation for this loss, thefertility of a field or of a country decreases; if, on the contrary, more are givento the fields, their fertility increases.

    An importation of urine, or of solid excrements, from a foreigncountry, is equivalent to an importation of grain and cattle. In a certain time,the elements of those substances assume the form of grain, or of fodder, then becomeflesh and bones, enter into the human body, and return again day by day to the formthey originally possessed.

    The only real loss of elements we are unable to prevent isof the phosphates, and these, in accordance with the customs of all modern nations,are deposited in the grave. For the rest, every part of that enormous quantity offood which a man consumes during his lifetime ( say in sixty or seventy years), whichwas derived from the fields, can be obtained and returned to them. We know with absolutecertainty, that in the blood of a young or growing animal there remains a certainquantity of phosphate of lime and of the alkaline phosphates, to be stored up andto minister to the growth of the bones and general bulk of the body, and that, withthe exception of this very small quantity, we receive back, in the solid and fluidexcrements, all the salts and alkaline bases, all the phosphate of lime and magnesia,and consequently all the inorganic elements which the animal consumes in its food.

    We can thus ascertain precisely the quantity, quality, andcomposition of animal excrements, without the trouble of analysing them. If we givea horse daily 4ù5 pounds' weight of oats, and 15 pounds of hay, and knowingthat oats give 4 per cent. and hay 9 per cent. of ashes, we can calculate that thedaily excrements of the horse will contain 21 ounces of inorganic matter which wasdrawn from the fields. By analysis we can determine the exact relative amount ofsilica, of phosphates, and of alkalies, contained in the ashes of the oats and ofthe hay.

    You will now understand that the constituents of the solidparts of animal excrements, and therefore their qualities as manure, must vary withthe nature of the creature's food. If we feed a cow upon beetroot, or potatoes, withouthay, straw or grain, there will be no silica in her solid excrements, but there willbe phosphate of lime and magnesia. Her fluid excrements will contain carbonate ofpotash and soda, together with compounds of the same bases with inorganic acids.In one word, we have, in the fluid excrements, all the soluble parts of the ashesof the consumed food; and in the solid excrements, all those parts of the ashes whichare insoluble in water.

    If the food, after burning, leaves behind ashes containingsoluble alkaline phosphates, as is the case with bread, seeds of all kinds, and flesh,we obtain from the animal by which they are consumed a urine holding in solutionthese phosphates. If, however, the ashes of food contain no alkaline phosphates,but abound in insoluble earthy phosphates, as hay, carrots, and potatoes, the urinewill be free from alkaline phosphates, but the earthy phosphates will be found inthe faeces. The urine of man, of carnivorous and graminivorous animals, containsalkaline phosphates; that of herbivorous animals is free from these salts.

    The analysis of the excrements of man, of the piscivorous birds(as the guano), of the horse, and of cattle, furnishes us with the precise knowledgeof the salts they contain, and demonstrates, that in those excrements, we returnto the fields the ashes of the plants which have served as food, - the soluble andinsoluble salts and earths indispensable to the development of cultivated plants,and which must be furnished to them by a fertile soil.

    There can be no doubt that, in supplying these excrements tothe soil, we return to it those constituents which the crops have removed from it,and we renew its capability of nourishing new crops: in one word, we restore thedisturbed equilibrium; and consequently, knowing that the elements of the food derivedfrom the soil enter into the urine and solid excrements of the animals it nourishes,we can with the greatest facility determine the exact value of the different kindsof manure. Thus the excrements of pigs which we have fed with peas and potatoes areprincipally suited for manuring crops of potatoes and peas. In feeding a cow uponhay and turnips, we obtain a manure containing the inorganic elements of grassesand turnips, and which is therefore preferable for manuring turnips. The excrementof pigeons contains the mineral elements of grain; that of rabbits, the elementsof herbs and kitchen vegetables. The fluid and solid excrements of man, however,contain the mineral elements of grain and seeds in the greatest quantity.