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Liebig's Chemical Letters

LETTER XIII

   

My dear Sir,

    Having in my last letter spoken of the generalprinciples upon which the science and art of agriculture must be based, let me nowdirect your attention to some of those particulars between chemistry and agriculture,and demonstrate the impossibility of perfecting the important art of rearing foodfor man and animals, without a profound knowledge of our science.

    All plants cultivated as food require for theirhealthy sustenance the alkalies and alkaline earths, each in a certain proportion;and in addition to these, the cerealia do not succeed in a soil destitute of silicain a soluble condition. The combinations of this substance found as natural productions,namely, the silicates, differ greatly in the degree of facility with which they undergodecomposition, in consequence of the unequal resistance opposed by their integralparts to the dissolving power of the atmospheric agencies. Thus the granite of Corsicadegenerates into a powder in a time which scarcely suffices to deprive the polishedgranite of Heidelberg of its lustre.

    Some soils abound in silicates so readily decomposable,that in every one or two years, as much silicate of potash becomes soluble and fittedfor assimilation as is required by the leaves and straw of a crop of wheat. In Hungary,extensive districts are not uncommon where wheat and tobacco have been grown alternatelyupon the same soil for centuries, the land never receiving back any of those mineralelements which were withdrawn in the grain and straw. On the other hand, there arefields in which the necessary amount of soluble silicate of potash for a single cropof wheat is not separated from the insoluble masses in the soil in less than two,three, or even more years.

    The term fallow, in Agriculture, designatesthat period in which the soil, left to the influence of the atmosphere, becomes enrichedwith those soluble mineral constituents. Fallow, however, does not generally implyan entire cessation of cultivation, but only an interval in the growth of the cerealia.That store of silicates and alkalies which is the principal condition of their successis obtained, if potatoes or turnips are grown upon the same fields in the intermediateperiods, since these crops do not abstract a particle of silica, and therefore leavethe field equally fertile for the following crop of wheat.

    The preceding remarks will render it obviousto you, that the mechanical working of the soil is the simplest and cheapest methodof rendering the elements of nutrition contained in it accessible to plants.

    But it may be asked, Are there not other meansof decomposing the soil besides its mechanical subdivision? - are there not substances,which by their chemical operation will equally well or better render its constituentssuitable for entering into vegetable organisms? Yes: we certainly possess such substances,and one of them, namely, quick-lime, has been employed for the last century pastin England for this purpose; and it would be difficult to find a substance betteradapted to this service, as it is simple, and in almost all localities cheap andeasily accessible.

    In order to obtain correct views respectingthe effect of quick-lime upon the soil, let me remind you of the first process employedby the chemist when he is desirous of analysing a mineral, and for this purpose wishesto bring its elements into a soluble state. Let the mineral to be examined be, forinstance, feldspar; this substance, taken alone, even when reduced to the finestpowder, requires for its solution to be treated with an acid for weeks or months;but if we first mix it with quick-lime, and expose the mixture to a moderately strongheat, the lime enters into chemical combination with certain elements of the feldspar,and its alkali (potass) is set free. And now the acid, even without heat, dissolvesnot only the lime, but also so much of the silica of the feldspar as to form a transparentjelly. The same effect which the lime in this process, with the aid of heat, exertsupon the feldspar, it produces when it is mixed with the alkaline argillaceous silicates,and they are for a long time kept together in a moist state.

    Common potters' clay, or pipe-clay, diffusedthrough water, and added to milk of lime, thickens immediately upon mixing; and ifthe mixture is kept for some months, and then treated with acid, the clay becomesgelatinous, which would not occur without the admixture with the lime. The lime,in combining with the elements of the clay, liquifies it; and, what is more remarkable,liberates the greater part of its alkalies. These interesting facts were first observedby Fuchs, at Munich: they have not only led to a more intimate knowledge of the natureand properties of the hydraulic cements, but, what is far more important, they explainthe effects of caustic lime upon the soil, and guide the agriculturist in the applicationof an invaluable means of opening it, and setting free its alkalies - substancesso important, nay, so indispensable to his crops.

    In the month of October the fields of Yorkshireand Oxfordshire look as it they were covered with snow. Whole square miles are seenwhitened over with quicklime, which during the moist winter months, exercises itsbeneficial influence upon the stiff, clayey soil, of those counties.

    According to the humus theory, quick-lime oughtto exert the most noxious influence upon the soil, because all organic matters containedin it are destroyed by it, and rendered incapable of yielding their humus to a newvegetation. The facts are indeed directly contrary to this now abandoned theory:the fertility of the soil is increased by the lime. The cerealia require the alkaliesand alkaline silicates, which the action of the lime renders fit for assimilationby the plants. If, in addition to these, there is any decaying organic matter presentin the soil supplying carbonic acid, it may facilitate their development; but itis not essential to their growth. If we furnish the soil with ammonia, and the phosphates,which are indispensable to the cerealia, with the alkaline silicates, we have allthe conditions necessary to ensure an abundant harvest. The atmosphere is an inexhaustiblestore of carbonic acid.

    A no less favourable influence than that oflime is exercised upon the soil of peaty land by the mere act of burning it: thisgreatly enhances its fertility. We have not long been acquainted with the remarkablechange which the properties of clay undergo by burning. The observation was firstmade in the process of analysing the clay silicates. Many of these, in their naturalstate, are not acted on by acids, but they become perfectly soluble if heated toredness before the application of the acid. This property belongs to potters' clay,pipe-clay, loam, and many different modifications of clay in soils. In their naturalstate they may be boiled in concentrated sulphuric acid, without sensible change;but if feebly burned, as is done with the pipe-clay in many alum manufactories, theydissolve in the acid with the greatest facility, the contained silica being separatedlike jelly in a soluble state. Potters' clay belongs to the most sterile kinds ofsoil, and yet it contains within itself all the constituent elements essential toa most luxurious growth of plants; but their mere presence is insufficient to securethis end. The soil must be accessible to the atmosphere, to its oxygen, to its carbonicacid; these must penetrate it, in order to secure the conditions necessary to a happyand vigorous development of the roots. The elements present must be brought intothat peculiar state of combination which will enable them to enter into plants. Plasticclay is wanting in these properties; but they are imparted to it by a feeble calcination.

    At Hardwicke Court, near Gloucester, I haveseen a garden (Mr. Baker's) consisting of a stiff clay, which was perfectly sterile,become by mere burning extremely fertile. The operation was extended to a depth ofthree feet. This was an expensive process, certainly; but it was effectual.

    The great difference in the properties of burntand unburnt clay is illustrated by what is seen in brick houses, built in moist situations.In the town of Flanders, for instance, where most buildings are of brick, effloresencesof salts cover the surfaces of the walls, like a white nap, within a few days afterthey are erected. If this saline incrustation is washed away by the rain, it soonre-appears; and this is even observed on walls which, like the gateway of Lisle,have been erected for centuries. These saline incrustations consist of carbonatesand sulphates, with alkaline bases; and it is well known these act an important partin vegetation. The influence of lime in their production is manifested by their appearingfirst at the place where the mortar and brick come into contact.

    It will now be obvious to you, that in a mixtureof clay with lime, all the conditions exist for the solution of the silicated clay,and the solubility of the alkaline silicates. The lime gradually dissolving in watercharged with carbonic acid, acts like milk of lime upon the clay. This explains alsothe favourable influence which marl (by which term all those varieties of clay richin chalk are designated) exerts upon most kinds of soil. There are marly soils whichsurpass all others in fertility for all kinds of plants; but I believe marl in aburnt state must be far more effective, as well as other materials possessing a similarcomposition; as, for instance, those species of limestone which are adapted to thepreparation of hydraulic cements, - for these carry to the soil not only the alkalinebases useful to plants, but also silica in a state capable of assimilation.

    The ashes of coals and lignite are also excellentmeans of ameliorating the soil, and they are used in many places for this purpose.The most suitable may be readily known by their property of forming a gelatinousmass when treated with acids, or by becoming, when mixed with cream of lime, likehydraulic cement, - solid and hard as stone.

    I have now, I trust, explained to your satisfaction,that the mechanical operations of agriculture - the application of lime and chalkto lands, and the burning of clay - depend upon one and the same scientific principle:they are means of accelerating the decomposition of the alkaline clay silicates,in order to provide plants, at the beginning of a new vegetation, with certain inorganicmatters indispensable for their nutrition.



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