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Liebig's Chemical Letters



My dear Sir,

    My recent researches into the constituent ingredientsof our cultivated fields have led me to the conclusion that, of all the elementsfurnished to plants by the soil and ministering to their nourishment, the phosphateof lime - or, rather, the phosphates generally - must be regarded as the most important.

    In order to furnish you with a clear idea ofthe importance of the phosphates, it may be sufficient to remind you of the fact,that the blood of man and animals, besides common salt, always contains alkalineand earthy phosphates. If we burn blood and examine the ashes which remain, we findcertain parts of them soluble in water, and others insoluble. The soluble parts are,common salt and alkaline phosphates; the insoluble consist of phosphate of lime,phosphate of magnesia, and oxide of iron.

    These mineral ingredients of the blood - withoutthe presence of which in the food the formation of blood is impossible - both manand animals derive either immediately, or mediately through other animals, from vegetablesubstances used as food; they had been constituents of vegetables, they had beenparts of the soil upon which the vegetable substances were developed.

    If we compare the amount of the phosphates indifferent vegetable substances with each other, we discover a great variety, whilstthere is scarcely any ashes of plants altogether devoid of them, and those partsof plants which experience has taught us are the most nutritious, contain the largestproportion. To these belong all seeds and grain, especially the varieties of bread-corn,peas, beans, and lentils.

    It is a most curious fact that if we incinerategrain or its flour, peas, beans, and lentils, we obtain ashes, which are distinguishedfrom the ashes of all other parts of vegetables by the absence of alkaline carbonates.The ashes of these seeds when recently prepared, do not effervesce with acids; theirsoluble ingredients consist solely of alkaline phosphates, the insoluble parts ofphosphate of lime, phosphate of magnesia, and oxide of iron: consequently, of thevery same salts which are contained in blood, and which are absolutely indispensableto its formation. We are thus brought to the further indisputable conclusion thatno seed suitable to become food for man and animals can be formed in any plant withoutthe presence and co-operation of the phosphates. A field in which phosphate of lime,or the alkaline phosphates, form no part of the soil, is totally incapable of producinggrain, peas, or beans.

    An enormous quantity of these substances indispensableto the nourishment of plants, is annually withdrawn from the soil and carried intogreat towns, in the shape of flour, cattle, et cetera. It is certain that this incessantremoval of the phosphates must tend to exhaust the land and diminish its capabilityof producing grain. The fields of Great Britain are in a state of progressive exhaustionfrom this cause, as is proved by the rapid extension of the cultivation of turnipsand mangel wurzel - plants which contain the least amount of the phosphates, andtherefore require the smallest quantity for their development. These roots contain80 to 92 per cent. of water. Their great bulk makes the amount of produce fallacious,as respects their adaptation to the food of animals, inasmuch as their contents ofthe ingredients of the blood, i.e. of substances which can be transformed into flesh,stands in a direct ratio to their amount of phosphates, without which neither bloodnor flesh can be formed.

    Our fields will become more and more deficientin these essential ingredients of food, in all localities where custom and habitsdo not admit the collection of the fluid and solid excrements of man, and their applicationto the purposes of agriculture. In a former letter I showed you how great a wasteof phosphates is unavoidable in England, and referred to the well-known fact thatthe importation of bones restored in a most admirable manner the fertility of thefields exhausted from this cause. In the year 1827 the importation of bones for manureamounted to 40,000 tons, and Huskisson estimated their value to be from 100,000to 200,000 sterling. The importation is still greater at present, but it is farfrom being sufficient to supply the waste.

    Another proof of the efficacy of the phosphatesin restoring fertility to exhausted land is afforded by the use of the guano - amanure which, although of recent introduction into England, has found such generaland extensive application.

    We believe that the importation of one hundred-weightof guano is equivalent to the importation of eight hundred-weight of wheat - thehundred-weight of guano assumes in a time which can be accurately estimated the formof a quantity of food corresponding to eight hundred-weight of wheat. The same estimateis applicable in the valuation of bones.

    If it were possible to restore to the soil ofEngland and Scotland the phosphates which during the last fifty years have been carriedto the sea by the Thames and the Clyde, it would be equivalent to manuring with millionsof hundred-weights of bones, and the produce of the land would increase one-third,or perhaps double itself, in five to ten years.

    We cannot doubt that the same result would followif the price of the guano admitted the application of a quantity to the surface ofthe fields, containing as much of the phosphates as have been withdrawn from themin the same period.

    If a rich and cheap source of phosphate of limeand the alkaline phosphates were open to England, there can be no question that theimportation of foreign corn might be altogether dispensed with after a short time.For these materials England is at present dependent upon foreign countries, and thehigh price of guano and of bones prevents their general application, and in sufficientquantity. Every year the trade in these substances must decrease, or their pricewill rise as the demand for them increases.

    According to these premises, it cannot be disputed,that the annual expense of Great Britain for the importation of bones and guano isequivalent to a duty on corn: with this difference only, that the amount is paidto foreigners in money.

    To restore the disturbed equilibrium of constitutionof the soil, - to fertilise her fields, - England requires an enormous supply ofanimal excrements, and it must, therefore, excite considerable interest to learn,that she possesses beneath her soil beds of fossil guano, strata of animal excrements,in a state which will probably allow of their being employed as a manure at a verysmall expense. The coprolithes discovered by Dr. Buckland, (a discovery of the highestinterest to Geology,) are these excrements; and it seems extremely probable thatin these strata England possesses the means of supplying the place of recent bones,and therefore the principal conditions of improving agriculture - of restoring andexalting the fertility of her fields.

    In the autumn of 1842, Dr. Buckland pointedout to me a bed of coprolithes in the neighbourhood of Clifton, from half to onefoot thick, inclosed in a limestone formation, extending as a brown stripe in therocks, for miles along the banks of the Severn. The limestone marl of Lyme Regisconsists, for the most part, of one-fourth part of fossil excrements and bones. Thesame are abundant in the lias of Bath, Eastern and Broadway Hill, near Evesham. Dr.Buckland mentions beds, several miles in extent, the substance of which consists,in many places, of a fourth part of coprolithes.

    Pieces of the limestone rock in Clifton, nearBristol, which is rich in coprolithes and organic remains, fragments of bones, teeth,&c., were subjected to analysis, and were found to contain above 18 per cent.of phosphate of lime. If this limestone is burned and brought in that state to thefields, it must be a perfect substitute for bones, the efficacy of which as a manuredoes not depend, as has been generally, but erroneously supposed, upon the nitrogenisedmatter which they contain, but on their phosphate of lime.

    The osseous breccia found in many parts of Englanddeserves especial attention, as it is highly probable that in a short time it willbecome an important article of commerce.

    What a curious and interesting subject for contemplation!In the remains of an extinct animal world, England is to find the means of increasingher wealth in agricultural produce, as she has already found the great support ofher manufacturing industry in fossil fuel, - the preserved matter of primeval forests,- the remains of a vegetable world. May this expectation be realised! and may herexcellent population be thus redeemed from poverty and misery!