be predicated from a knowledge of the composition of the ashes of a plant, or from an analysis of the soil to be manured. Manures may act in one of three principal ways:-1. as direct food for the plant; 2. as what, for want of a better term, may be viewed as stimulants to its growth at a particular stage; and 3. as chemical agents which modify the constitution of other substances already present in the soil. Liebig has specially called attention to the first of these modes of the action of manures. The second of these methods has been particularly elucidated by Lawes, who showed that in the early stage of the growth of turnips, no manure is to be compared in efficacy with superphosphate of lime; although the plant itself contains so small a proportion of phosphates that it was precisely that plant fixed upon by Liebig, on theoretical grounds, as the one which from its non-requirement of phosphates was particularly well calculated to be grown upon land which might have been supposed to have been exhausted of its phosphates by a previous wheat-crop. It is now well known to the practical farmer, that the superphosphate when drilled in judiciously with the turnip seed, stimulates the growth of the plant in the earliest and most critical stage of its existence, and thus secures a far more abundant return than when the supply of superphosphate is omitted.*

The experiments of the same patient and indefatigable agriculturist, which have been carried on in conjunction with Dr. Gilbert, in a systematic manner, and upon a large scale for several consecutive years, have shown to the satisfaction of unprejudiced observers that the direct supply of phosphates to wheat crops frequently does not materially increase the produce, because ordinary wheat land already contains a far larger proportion of phosphates distributed through it than is necessary to the growth of many suc cessive crops of wheat. This circumstance appears to be in great measure dependent upon a peculiarity in the mode of growth of the wheat plant; it strikes down the fibres of its roots to a great depth into the earth, by which means it is enabled to appropriate the phosphates of a much larger extent of soil than can come within the reach of the short tuft of fibres which constitutes the root of a crop like the

It is, however, probable that no manure acts simply as a stimulant, but that all such manures enter into the composition of the plant itself. These stimulating manures appear to supply through certain stages of the plant's growth a constituent or constituents, which, though present in the soil, may not be so in a sufficiently concentrated form, or which, owing to the imperfect development of the root, may not be sufficiently within the reach of the plant at the particular time when they are most needed.

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turnip.* It is also found, and apparently for similar reasons, that the addition of silicate of potash as a manure does not materially strengthen the stem in wheat, but that the employment of ammoniacal manures, including the best varieties of guano, enables large crops of wheat to be raised for several consecutive years on the same piece of land, the ammonia appearing to supply nitrogen to the gluten of the wheat, and acting also as a general stimulant to the growth of the plant. But although this has been established as a scientific fact, it is not recommended that this practice be substituted for the system of rotation at present in use; though it appears that with the aid of nitrogenous manures wheat may be grown in larger quantities, and more frequently in the rotation, without injury to the fertility of the soil.

It is a matter of experience that land which will not grow two remunerative crops of the same kind in succession (wheat for instance) without intermediate manuring, will yet, after a succession of turnips, barley, and clover, again be fit for wheat with comparatively light manuring; and that this system of rotation produces effects similar to those obtained by fallowing, or allowing the land to lie uncropped for a year. Lawes and Gilbert consider that clover and some other leguminous crops are especially valuable to the land, by supplying a quantity of nitrogenous organic matter to the soil, in a form particularly well calculated for assimilation by the succeeding wheat crop.

It is generally supposed that all the nitrogen found in combination in plants, has been obtained by the plant either from nitric acid or from ammonia, or from some azotised compound present in the soil in the form of manure; and that plants have not the power of directly assimilating nitrogen from the atmosphere. It appears probable however, that the leguminous plants generally introduced by the farmer in rotation, have a particular aptitude for condensing and retaining the nitrogen which is presented to them during their growth, and that they store it up; so that by the debris of

*An interesting illustration of the influence of the development of the root upon the nature of the manure required, is afforded by a comparison of wheat and barley, which contain similar mineral constituents in nearly the same proportions:-wheat sown in autumn makes root all the winter, and is enabled to extract saline matters from the soil to a great depth; consequently it is seldom benefited by mineral manures directly applied, though it is greatly affected by the general mineral riches of the soil. Barley, on the other hand, is sown in spring; it throws out and depends upon root fibres nearer the surface, and is frequently much benefited by the use of manures containing phosphates.

their roots, and still more abundantly if ploughed into the land, they furnish a valuable source of nitrogen to succeeding crops. Wheat, on the contrary, accumulates but a comparatively small amount of nitrogen, and consequently thrives best when aided by the stimulus of manures containing that element. Lawes and Gilbert, however, found that the addition of 5 lb. of ammonia per acre in the form of manure, produced an increase of about a bushel of wheat; from which it would appear, calculating from the known composition of the corn and the straw, that not more than about two-fifths of the quantity of nitrogen supplied in the manure is retained in the gluten of the seed, in the straw, and in other parts of the plant.

Liebig accounts for the beneficial effects of the rotation of crops, by supposing that each plant in succession avails itself of saline matters which were unnecessary to the preceding crop. The process of fallowing appears to owe a certain portion of its efficacy to the gradual disintegration of the rocks, under the united influence of air, moisture, and alternations of temperature, in consequence of which, various substances, which were previously insoluble, are reduced to the soluble condition, and thus rendered available for the wants of the succeeding crop. It appears, however, probable, that the accumulation of compounds containing nitrogen within the soil during the period of fallow, has a material influence upon the following grain crop; but further experiments are still needed to elucidate the theory of fallowing.

The third mode in which manures act, viz. by the influence which they exert upon the components of the soil, is well exemplified in the application of lime, the most important uses of which are of an indirect character, although undoubtedly it is in some cases beneficial by supplying one of the mineral constituents essential to the healthy development of the plant.

Lime is applied to the land in the form either of slaked lime or of chalk. In the condition of slaked lime its chemical effects are exerted upon both the inorganic and the organic constituents of the soil. By its action upon the inorganic constituents, the insoluble silicates and the clays are decomposed: and potash, soda, alumina, and magnesia are set at liberty. Lime also decomposes salts of iron, rendering its oxide insoluble; it likewise partially decomposes common salt, forming carbonate of soda and chloride of calcium. The effects upon the organic constituents of the soil are even more important; for it is upon rich peaty soils which abound in organic matter, that the beneficial effects of lime are most evident. In such cases it

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occasions the decomposition of the organic matter, and gives rise to the formation of carbonic and nitric acids, as well as of ammonia; all of which bodies are important ingredients in the nutriment of the young plant; and at the same time the lime also liberates the saline constituents of a former vegetation, leaving them in a state well fitted for assimilation by the growing crop. This decay in the organic matter is produced by the simultaneous action of lime, atmospheric air, and moisture: it therefore takes place most rapidly near the surface, and is attended with the formation of nitric acid and ammonia, at the expense, in part, of the nitrogen of the atmosphere, just as occurs in the artificial nitre beds of Sweden, France, and other countries of the continent. Similar effects are produced, though much more slowly, by the action of chalk or carbonate of lime; and the beneficial results produced by the application of chalk are consequently less immediately perceived, and are diffused over a longer period of time.

In connexion with this indirect action of manures, it may be mentioned, that the beneficial effects of carbonic acid on the growth of plants are not limited to its direct action in supplying carbon to the crop; it performs a function not less important owing to its solvent power. The carbonates and phosphates of lime and of magnesia, and the oxide of iron are insoluble in pure water, but they are all dissolved to a small extent by water charged with carbonic acid, and are thus rendered available to the nutrition of the plant.

(1455) Varieties of Manures.-It has already been stated that the most important manures are those which are furnished by the excreta of animals; guano is one variety of this kind of manure ; it consists of the decomposing excreta of sea-fowl which have fed upon fish; from its origin it abounds in phosphates and in ammoniacal salts; farm-yard manure, both solid and liquid, and night-soil and urine are also manures of a similar kind. Phosphates are abundantly supplied by bone dust, which also contains a considerable amount of gelatin. In burnt bones the gelatin is destroyed. The so-called coprolites, which consist principally of phosphate of lime, are probably debris of extinct animals, found in the tertiary beds of the Norfolk 'Crag,' and when reduced to powder and mixed with about half their weight of sulphuric acid, furnish one source of the manure which is known as superphosphate of lime. In this compound, the sulphuric acid liberates a portion of phosphoric acid, under the influence of which a portion of the bone earth is rendered soluble.

In the foregoing facts we see strikingly displayed the mutual de

pendence of the vegetable and of the animal creation. If the plant be necessary to supply organized food for the animal, the animal in his turn ministers in a manner not less important to the support of the plant. The animal, with every breath that he draws, sends forth carbonic acid into the atmosphere, and supplies carbon, which may ere long form an integral portion of the growing plant; the excreta which he rejects as useless and noisome are greedily absorbed by the rootlets of countless tribes of vegetables; and when life no longer animates his frame, the muscular, the gelatinous, and the nervous tissues, during putrefaction, are returned to the atmosphere in the form of water, carbonic acid, ammonia, and sulphuretted hydrogen; whilst the hard and compact bony portions of his body gradually moulder into dust. These various compounds

supply, it may be, future crops of wheat, of turnips, or of potatoes, with their necessary carbon, ammonia, or phosphates, and the crops thus raised, in time, furnish sustenance to other generations of living beings; thus the very molecules of carbon, of nitrogen, or of hydrogen, which once formed part of the body of the beasts of the forest of former ages, may again become portions of living, breathing animals, or may at this moment constitute a portion of our own corporeal system.

The ashes of wood, of peat, and of coal are also valuable as manures, since they return to the soil the mineral ingredients which were removed from it by the vegetation of which they formed a part. Hence the burning of heath or of turf upon waste land commonly renders it more fertile; for it not only furnishes a considerable amount of mineral manure in an available form, but it also assists materially in disintegrating the felspathic constituents of the soil, which are much more readily acted upon by air and moisture after being thus exposed to a moderate ignition, than when present in their natural condition. It is in the same way that the burning of stiff clay, and subsequently distributing it upon the land, is sometimes beneficial.

Specific manures are in some cases required for particular crops upon certain soils. Gypsum or sulphate of lime is a highly important manure for leguminous crops, such as clover, in cases where lime or sulphates are deficient. Nitrate of soda in other cases forms a useful top-dressing to barley. Common salt, when applied judiciously to land where this ingredient is wanting, has been known to produce excellent results; and the importance of liming under various circumstances has been already alluded to.

For the further prosecution of this subject the reader is