Under conditions of natural growth and decay, when no crops are gathered in, or consumed on the land by live stock, the herbage on dying down and decaying returns to the atmosphere and the soil the elements taken from them during life; but under cultivation a succession of crops deprives the land of the constituents which are essential to healthy and luxuriant growth. Without an adequate return to the land of the matters removed in the produce, its fertility cannot be maintained for many years. In newly-opened countries, where old forestshave been cleared and the land brought into cultivation, the virgin soil often possesses at first a high degree of fertility, but gradually its productive power decreases from year to year. Gene-rdly speaking, it is more convenient to clear fresh forest land than to improve more or less exhausted virgin land by the application of manure, labour, and skill. In all densely peopled countries where such a mode of cultivation cannot be followed it is necessary to resort to artificial means to restore the natural fertility of the land and maintain and increase its productiveness.

The researches of Liebig, Wiegmann, Polstorff, and others have proved beyond doubt the important functions of the mineral constituents of the soil in relation to plant life. The gradual removal of phosphate of lime in the tillage of dairy districts, or the removal of other mineral matters essential to the healthy growth of farm crops, certainly impoverishes the land. The exhaustion of the soil is caused in a much more marked way, however, by the rapid loss in available nitrogenous plant food which soils sustain when under cultivation without manure.

Agricultural improvements manifest themselves in two different directions—the mechanical and the chemical. Under mechanical improvement the physical condition of the soil is bettered and its latent stores of plant food brought into action by mechanical means, such as ploughing, subsoiling, steam cultivation, <tc. The introduction of new and superior agricultural implements, good systems of drainage, and intelligent division of labour characterize the first stage of progress in agriculture. The second stage is marked by the application of chemical principles to practical agriculture, an application shown by the introduction of a rational system of feeding, a proper rotation of crops, and chiefly the use of chemical, or artificial, manures for the purpose of restoring the natural fertility of the soil and increasing its productive powers.

The aid which chemistry has rendered during the last twenty-five or thirty years to practical agriculture has greatly promoted agricultural improvements; and farming, which is in a large measure dependent for success upon an economical use of manures, is now being carried on much more rationally than in former times. The proper application of various kinds of manures is one of the most prominent features of successful modern farming.

In considering the economical use of manures on the land, regard must be had to the following points :—(T) the requirements of the crops intended to be cultivated; (2) the physical condition of the soil; (3) the composition of the soil; and (4) the composition of the manure. Briefly stated, the guiding principle of manuring economically and profitably is to meet the requirements of the crops intended to be cultivated, by incorporating with the soil, in the most efficacious states of combination, the materials in which it is deficient, or which the various crops usually grown on the farm do not find in the land in a sufficiently available condition to ensure an abundant harvest.

Soils vary greatly in composition, and hence it will be readily understood that in one locality or on one particular field a certain manure may be used with great benefit, while in another field the same manure has little or no effect upon the produce. Although increased attention has of recent years been paid to the chemical composition and properties of soils, there is still much room left for improvement, for many farmers disregard almost altogether the composition of their fields in buying artificial manures.

It has been pointed out by Sir John Lawes that in actual English farm-practice there is, speaking practically, a standard of natural produce which varies within certain limits, as influenced by seasons and management, and which cannot be permanently increased or reduced by cultivation; and further, when land is spoken of as being in “good condition,” reference is made only to the temporary rise of fertility by means of the manures employed, while by land “out of condition” is signified the exhaustion of the manures by the removal of crops, loss by drainage, <fcc., and that the soil has merely gone back to its standard of natural productiveness. Some soils, indeed, contain in their natural condition hardly a sufficient proportion of available elements of plant food to yield remunerative crops; such soils are naturally barren, and, although by the constant use of manures they may be improved and may attain some degree of fertility, they will, if left unrnanured, speedily revert to their former natural unproductive state.

The principal constituents of manures are—nitrogen, in the form of ammonia, nitrates, and nitrogenous organic matters; organic matters not containing nitrogen (humus); phosphoric acid, potash, soda, lime, magnesia, silica, sulphuric acid, and chlorine. Of these constituents by far the most important are the nitrogen, phosphoric acid, and potash, and these will be considered more in detail.

1. Nitrogen.— Nothing so much affects the productiveness of soils as nitrogen, when it is supplied to plants in a form in which it can be assimilated by them, and nothing is more readily removed from the land, either in the crop grown or, in the form of nitrates, by drainage. The “good condition” of land, to which allusion has been made already, is, in a great measure, the fertility which has been imparted to the soil by the nitrogen supplied to it, while the “natural productiveness” may be taken as that due to the phosphoric acid, potash, and other mineral constituents of the soil.

Sιipply of Nitrogenous Plant Food.— In the case of a crop growing under natural conditions, and not removed from the land, the mineral constituents taken from the soil, and carbonaceous as well as some nitrogenous organic matter, principally derived from the atmosphere, are returned to the land, and a rich carbonaceous soil or humus is produced in the course of years. Such a soil is capable of supplying for a considerable number of years, when under cultivation, the nitrogen needed by the crops grown on the land. This supply of nitrogen, however, gradually becomes exhausted by repeated cropping. The atmosphere, in addition to free nitrogen gas, which constitutes about four-fifths of its volume, contains but very little combined nitrogen in the forms of ammonia or nitric acid.

Μ. Boussingault’s experiments clearly show that plants do not possess the power of taking up by their leaves and of assimilating the free nitrogen of the air. This conclusion has been verified by the extensive researches of Messrs Lawes, Gilbert, and Pugh. The nitrogen contained as nitric acid and ammonia in the air, and descending upon the land in the shape of rain, dew, or snow, though, without doubt, adding to the supply of nitrogen in the soil, is altogether insufficient to meet the requirements of remunerative crops. The experiments of Messrs Lawes and Gilbert and Professor Way show that the average proportion of nitrogen deposited annually upon one acre of land at Rothamsted, St Albans, amounts to 7 ’ 21 lb, of which quantity 6 ’ 46 Ib occur as ammonia and ’ 75 ft) as nitric acid.

The further investigations of Dr Voelcker, Professor Frankland, and Messrs Lawes and Gilbert upon the drainage water from cultivated soils show that a considerable quantity of nitrogen in the shape of nitric acid j>asses into land drainage, and that this loss of nitrogen is much greater than the total amount supplied to the land by the rain and dew. The results which Messrs Lawes and Gilbert obtained in their experiments on the continuous growth of barley at Rothamsted from 1852 to 1875 afford, as is shown in the following table (I.), direct evidence of the insufficiency of the atmospheric supply of nitrogen, and of that present in the soil in the form of nitrogenous organic matter. [table]

The Rothamsted soil is a moderately stiff one, of considerable depth, and contains naturally the mineral elements of plant food in abundance; thus it has been possible to grow corn crops for over twenty-five years without any m inure (Plots 1). The crops in the second period of twelve years were, however, less than those of the first period, and in neither case were full crops obtained. While the application of mineral manures alone (Plots 2) produced only a slight increase in the case of the wheat, and rather better, though poor, results with barley, nitrogenous manures, applied to the land either in the shape of ammonia salts or nitrate of soda, produced a strikingly large increase. The experiments further show that while good crops, both of wheat and barley, can be grown by the annual application of 14 tons of farmyard manure per acre, the best results are obtained by the use of a mixture of mineral and nitrogenous manures.

In Messrs Lawes and Gilbert’s experiments the amount of nitrogen removed in different crops was determined with the following results :—over a period of thirty-two years (to 1875), wheat yielded an average of 20 - 7 ft) of nitrogen per acre per annum without any manure, but the annual yield has decreased from an average of over 25 ft) in the first eight to less than 16 lb in the last twelve years, and since 1875 it has been still less; over a period of twenty-four years, barley, when unmanured, yielded an average of 18 3 ft) nitrogen per acre per annum, but with a decline from 22 lb in the first twelve to 14 ’ 6 in the last twelve years. Experiments similar to those on wheat and barley have been made on oats, root crops, leguminous and grass crops, all showing the gradual decline in produce when grown continuously without nitrogenous manures, and proving that the soil and not the atmosphere is the chief source of nitrogen in plants. In face of these results the “mineral theory” of Liebig, which attached but small value to nitrogen applied to the soil in the form of nitrogenous manures, and maintained the sufficiency of the ammonia of the atmosphere for supplying the needs of the plant, cannot be accepted without reserve.

Notwithstanding the great effect produced by the nitrogenous manures, two-thirds of the nitrogen supplied was unrecovered in the increase of crops when the ammonia salts were supplied to wheat in autumn. When, however, nitrate of soda was used, which is always applied in the spring, the quantity left unrecovered was not much more than half that supplied.

The following table (II.), by Messrs Lawes and Gilbert, shows the amount of nitrogen recovered, and the amount not recovered, in the increase of the crop for 100 supplied in manure, to wheat and to barley respectively, the result being in each case the average over a period of twenty years :—

[table]

The question will naturally be raised, What becomes of the one-half or two-thirds of the nitrogen which is not recovered in the increase of the crops'? The examination of some seventy samples by Dr Voelcker, and a number of independent determinations by Dr Frankland, of the drainage-water from the experimental wheat plots which yielded the above results throw much light on this loss. The following table (III.) contains a summary of some of the more important results obtained.

Table III.—Nitrogen as Nitrates and Nitrites, per 100,000 parts of Drainage Water from Plots differently Manured, in the Experimental Wheat-field at E,othamsted, Wheat every year, commencing 1844. [table]

These experiments show how great may be the loss of nitrogen by drainage when ammonia salts or nitrates are liberally applied to the land in autumn, should there be much wet weather in winter, or even when applied in spring if there be much heavy rain; also, that the quantity of nitrogen in drainage water as nitrates is increased in proportion to the amounts of ammonia or nitrate employed on the land. Assuming that from one-quarter to nearly one-half the annual rainfall descends more than 40 inches below the surface, every inch of rain passing through the dι*ains and carrying with it one part of nitrogen in. 100,000 of water, there will be a loss of 2⅜ lb of nitrogen per acre from the manure. Dr Voelcker’s analysis of the drainage water of a wheat field manured in autumn by ammonia salts supplying 82 ft> of nitrogen per acre shows that for every inch of rain passing through the drains in January a loss took place of about 8⅜ lb of nitrogen, costing about ] s. per ∏> as manure. The loss of nitrogen thus is very large, and show's that by far the largest proportion of the nitrogen of manure which is not recovered in the crop is lost in drainage. In addition to the nitrogen removed in the crop and to that lost in drainage, some small proportion is found by analysis to be retained in the soil itself. The nitrogen may be of advantage to crops grown subsequently, according to the source from which it was derived; for while ammonia salts and nitrates yield but very small residues, and exert little or no effect beyond the first year, from bones, cake, and other such materials we get large residues of nitrogen in the soil, which tell markedly on future crops. The experiments on drainage water have further shown the absorbent power of soils, and that manuring matters when in contact with soils undergo remarkable changes, being taken up by plants, not in the simple state in which they are applied, but in quite different kinds of combination. Professor Way has the merit of having first proved that all soils possess, in different degrees, the power of absorbing ammonia from its solution in water, and that in passing solutions of salts of ammonia through soils the ammonia alone is absorbed, and the acids of the ammonia salts pass through in combination, generally with lime, or, if lime be deficient, with magnesia or other mineral bases of the soil.

In the drainage investigations at Dothamsted, it was found that, although large quantities of ammonia salts were applied to some of the plots, the drainage water from them contained mere traces of ammonia, but at all times of the year nitrates were present in quantity; from this it would appear that it is chiefly, if not solely, from nitrates that crops build up their nitrogenous organic constituents. Before leaving the subject of drainage water it is worthy of note here that phosphoric acid and potash, the most valuable mineral fertilizing constituents of the manures, passed but little into the drainage water, but were retained almost entirely in the land, while the more abundant and less important mineral matters, such as lime, magnesia, soda, chlorine, sulphuric acid, and soluble silica, passed in large quantities into the drainage water. It follows from these investigations, first, that much more nitrogenous material must be applied to the land than would be needed to produce a given increase in the crop, supposing all the nitrogen to be recoverable; and secondly, that nitrogenous organic matters when applied to the land undergo decomposition, and are gradually resolved into ammonia compounds, which, after being retained a short time by the soil, are finally oxidized into nitrates, in which form they are most available and beneficial to plants, but are not absorbed by the soil, and are readily washed out by rain. Nitrogen has great forcing properties, and is most beneficial when applied to crops in their early stages. Grass land on which nitrate of soda has been put as a top dressing shows very rapidly and markedly the effects of the manure.

Nitrate of soda, unless applied just at the time the crop is ready to take it up, will be largely wasted in drainage; hence it is usual to apply nitrate of soda as a top dressing in spring. Ammoniacal manures, such as Peruvian guano, soot, sulphate of ammonia, &c., when used for winter wheat, are best applied in autumn, either before the wheat is sown or when it is fairly above ground. On light land they are often used as top dressings for wheat early in spring. The gradual decomposition of farmyard manures gives a more constant supply of nitrogen than the manures already noticed, and as the fermentation of dung proceeds but slowly it is best to apply it, when quite fresh, in autumn or winter, allowing it to decompose in the land and to yield nitrogen as nitrates when required in spring by the fresh growth of vegetation. In the case of nitrogenous organic materials, such as wool or hair refuse, which take even longer than farmyard manure to decompose, it is necessary to apply them some three or four months before the seed is sown.

2. Phosphoric Acid.— Next in value to nitrogen as a constituent of manures comes phosphoric acid. Of all the mineral or ash constituents of plants, this is the most important, for the simple reason that it occurs in most soils in comparatively small proportions, and is required alike by corn and forage crops in larger quantities than lime, magnesia, and other mineral matters, which occur in most soils in almost inexhaustible quantities, or which, if deficient, can be easily and cheaply incorporated with the land.

Phosphoric acid occurs in soils principally in combination with lime as phosphate of lime, a constituent which enters largely into the composition of recent and fossil bones, coprolites, and guano, and of a variety of phosphatic minerals, such as Norwegian and Canadian apatite, Sombrero and Curaçoa rock phosphate, South Carolina phosphate, Spanish and Portuguese phosphorite, <fcc. These and other phosphatic materials are now used largely in the manufacture of superphosphate and other artificial manures.

Phosphate of lime is so sparingly soluble in pure water that, for all practical purposes, it may be considered to be insoluble in water; it is, however, attacked by the natural agencies at work in the soil, and rendered available as plant food. According to the more or less porous condition of the phosphatic manuring materials, phosphate of lime is rendered more or less readily assimilable. Thus, while bone dust and guano, on account of their porous condition, may be used with good effect as suppliers of phosphoric acid to the soil, others, such as hard and crystalline Canadian or Norwegian apatite, produce but little or no result when used as manures merely in a powdered state. It was in 1840 that Liebig suggested the treatment of bones with sulphuric acid in order to make their action more rapid. This treatment of bones by acid converts the phosphate of lime into a soluble lime salt called superphosphate, or, speaking chemically, transforms the original tribasic phosphate of lime into the monocalcic phosphate, sulphate of lime being at the same time produced. Following on this, Mr (now Sir) John Lawes treated mineral phosphates similarly with sulphuric acid, and with results which led to the establishment of a new industry, the manufacture of superphosphate and other artificial manures. The yearly importation of phosphatic minerals,...