Table shewing the Composition of some of the less common varieties of Guano.

Note.—The numbers in this Table are mostly derived only from a single analysis and have no value as determining the average composition of these Guanos, but they serve to give a general idea of their value.

On examining the tables given above, it is obvious that guanos may be divided into two classes, the one characterized by the abundance of ammonia, the other by that of phosphates; and which, for convenience sake, may be called ammoniacal and phosphatic guanos. Peruvian and Angamos are characteristic of the former, and Saldanha Bay and Bolivian of the latter class. The value of these two classes of guano differs materially, and they are also applicable under different circumstances, but to these points reference will afterwards be made.

Very special precautions are necessary on the part of the farmer in order to insure his obtaining a guano which is not adulterated, and of good quality if genuine. In the case of Peruvian guano, which is tolerably uniform in its qualities, it is possible to form some opinion by careful examination, and the following points ought to be attended to:

1st, The guano should be light coloured. If it is dark, the chances are that it has been damaged by water.

2d, It should be dry, and when a handful is well squeezed together it should cohere very slightly.

3d, It should not have too powerful an ammoniacal odour.

4th, It should contain lumps, which, when broken, appear of a paler colour than the powdery part of the sample.

5th, When rubbed between the fingers it should not be gritty.

6th, A bushel of the guano should not weigh more than from 56 to 60 lbs.

These characters must not, however, be too implicitly relied on, for they are all imitated with wonderful ingenuity by the skilful adulterator, and they are applicable only to Peruvian guano; the others being so variable that no general rules can be given for determining whether they are genuine. Neither are they so precise as to enable us to give any opinion regarding the relative values of several samples where all are genuine. The only way in which adulteration can with certainty be detected, and the value of different guanos be determined, is by analysis, and the importance of this can easily be illustrated.

In the table above, the average composition of the different guanos is given; but in order to shew how much individual cargos may differ from the mean, we give here analyses of samples of the highest and lowest quality of the genuine guanos of most importance:

The differences are here exceedingly large; and when the values of the two Peruvian guanos are calculated according to the method to be afterwards described, it appears that the highest exceeds the lowest in value by nearly £3 per ton. Of course, this is an extreme case, but it is no uncommon occurrence to find a difference of £1 or even £2 per ton between the values of cargos of Peruvian guano, which are sold at the same price.

The adulteration of guano is carried on to a very large extent; and though perhaps not quite so extensively now as it was some years since, it is only kept in check by the utmost vigilance on the part of the purchaser. The chief adulterations are a sort of yellow loam very similar in appearance to guano, sand, gypsum, common salt, and occasionally also ground coprolites and inferior guano. These substances are rarely used singly, but are commonly mixed in such proportions as most closely to imitate the colour and general appearance of the genuine article. The extent to which the adulteration is carried may be judged of from the following analyses taken at random from those of a large number of guanos, all of which were sold as first-class Peruvian.

In all those cases a very large depreciation in the value has taken place, and several of them are worth considerably less than half the price of the genuine guano, while they are generally offered for sale at about £1 under the usual price. The adulteration is chiefly practised in London, and cases occasionally occur which can be traced to Liverpool and other places; but it always takes place in the large towns, because it is only there that facilities exist for obtaining the necessary materials and carrying it out without exciting suspicion. The sophisticated article then passes into the hands of the small country dealers, to whom it is sold with the assurance that it is genuine, and analysis quite unnecessary. In other instances, adulterated and inferior guanos are sold by the analysis of a genuine sample, and sometimes an analysis is made to do duty for many successive cargos of a guano which, though all obtained from one deposit, may differ excessively in composition. In order to insure obtaining a genuine guano, it is above all things important to deal only with a person of established character, who will generally, for his own sake, satisfy himself that the article he vends is genuine and of good quality; and it is always important that the buyer should examine the analysis, and in all cases where there is the slightest doubt, should ascertain that the bulk sent corresponds with it. In the case of a Peruvian guano, a complete analysis is not necessary for this purpose; but an experienced chemist, by the application of a few tests, can readily ascertain whether the sample is genuine. Where the difference in value between different samples is required, a complete analysis is necessary, and this is indispensable in the case of the inferior guanos. Many of these are obtained from deposits of limited extent, and in loading it considerable quantities of the subjacent soil are taken up, so that very great differences may exist even in different parts of the same cargo. Nor must it be forgotten that, except in the case of Peruvian, the name is no guarantee for the quality of the guano, even if genuine. Peruvian guano is all obtained from the same deposits, those of the Chincha Islands, but the guanos which are brought into the market under the name of Patagonian, Chilian, etc., are obtained from a great variety of deposits scattered along the coasts of these countries, sometimes at a distance of several hundred miles from each other, and which have been accumulated under totally different circumstances. In illustration of this, it is only necessary to refer to the subjoined analysis of samples, all of which I believe to be genuine as imported, and which were sold under the name of Upper Peruvian Guano.

With the exception of Peruvian, the supply of good guanos of uniform composition is by no means large, and phosphatic guanos of good quality are now especially rare. The Saldanha Bay, and other similar deposits, have been exhausted, and few guanos of equally good quality have been lately discovered. There is no doubt, however, that such guanos are very useful, and if obtained in large quantity, and of uniform composition, would be used to a much larger extent than they at present are.

The value and use of guano are now so well understood, that it is scarcely necessary to enlarge on the mode of its application. Peruvian guano owes its chief value to its ammonia and phosphates, but it also contains potash, soda, and all the other constituents of plants in small quantity, although in a readily available condition, as is seen in the detailed analysis given in page 205.

In other guanos which have undergone more complete decomposition, and from which the soluble matters have been more or less completely exhausted by rain, the alkaline salts, or at least the potash they originally contained, have almost entirely disappeared. Hence an important difference between Peruvian guano and most other varieties. The former can be used as a complete substitute for farm-yard manure, and excellent crops of turnips and potatoes can be raised by means of it alone, and at a less cost than with ordinary dung. But though this may be done, and in many cases is attended with great economic advantages, it is a practice that cannot be recommended for general use, because the quantity of valuable matters contained in the usual application of guano is much smaller than in farm-yard manure, and the probability is that it would not, if used alone during a succession of years, be sufficient to maintain the soil permanently in a high state of fertility. Five cwt. of Peruvian guano, which is a liberal application per acre, contains about 95 lbs. of ammonia, and 130 of phosphates, while 20 tons of good farm-yard manure contain 312 of ammonia, and about the same quantity of phosphates, and when the other constituents, such as potash and soda, are compared with those in guano, the difference is still more striking. On the other hand, guano is a rapidly acting manure; its constituents are in a condition in which they are more immediately accessible by the plant, and its immediate effect is far more marked, as it is chiefly expended on the crop to which it is applied. It has indeed been alleged that it produces no effects on the subsequent crops, but this opinion can scarcely be considered as well founded. In no case does the crop raised by means of it contain the whole of the ammonia or phosphates present in the manure, and the unappropriated quantity, though it may, and probably does, escape from the lighter soils, must be retained and preserved for the use of subsequent crops by heavy and retentive clay soils. The general inference is, that though guano may at an emergency be used as an entire substitute for farm-yard manure, the practice is one to be generally avoided. When, however, as occasionally happens after a long continued use of farm-yard manure, organic matters have accumulated in the soil, and passed into an inert condition, then Peruvian guano may be used alone with very great advantage. In all cases the rapidity of the action of guano makes it an important auxiliary of farm-yard manure, and it is in this way that it may be most advantageously employed. Experience has shewn that one-half the farm-yard manure may be replaced by guano with the production of a larger crop than by the former alone in its full quantity. The proportion of guano usually employed is from three to five cwt., and it is alleged that a much larger quantity produces prejudicial effects on the subsequent crops, although it is not very easy to see on what this depends.

The variety of guano to be selected must depend to a great extent on the use to which it is to be put. Peruvian guano is most advantageously applied as a top-dressing to young corn and particularly to oats. For the turnip, the ammoniacal guanos were formerly preferred, and on strong soils, under good cultivation, their effects are excellent, but on light soils they are less applicable, their soluble salts being more rapidly washed out, and their effects lost, and in these cases they are surpassed by the phosphatic guanos.

No definite rules can be given for determining the soils on which these different varieties are most applicable, but each individual must determine by experiment that which best suits his own farm; and the inquiry is of much importance to him, as, of course, if the phosphatic guanos will answer as well as the ammoniacal, there is a large saving in the cost of the manure. A very excellent practice is to employ a mixture of equal parts of the two sorts of guano.

Pigeons' Dung.—The dung of all birds, which more or less closely resembles guano, may be employed with much advantage as a manure, but that of the pigeon and the common fowl are the only ones which can be got in quantity. Pigeons' dung, according to Boussingault, contains 8·3 per cent of nitrogen, equivalent to 10·0 of ammonia. Its value, therefore, will be more than half that of guano, but it varies greatly, and a sample imported from Egypt into this country, and analysed by Professor Johnston, contained only 5·4 per cent of ammonia. Hens' dung has not been accurately analysed, but its value must be about the same as pigeons'.

Urate and Sulphated Urine.—We have already discussed the urine of animals, in reference to farm-yard manure. But human urine, the composition of which was then stated, is of much higher value than that of the lower animals, and many attempts have been made to preserve and convert it into a dry manure. Urate is prepared by adding gypsum to urine, and collecting and drying the precipitate produced. It contains a considerable quantity of the phosphoric acid of the urine, but very little of its ammonia; and as the principal value of urine depends on the latter, it is necessarily a very inefficient method of turning it to account. A better method has been proposed by Dr. Stenhouse, who adds lime-water to the urine, and collects the precipitate, which, when dried in the air, contains 1·91 per cent of nitrogen, and about 41 per cent of phosphates. This method is subject to the same objection as that by which urate is made, namely, that the greater part of the ammonia is not precipitated. This might probably be got over to some extent by the addition of sulphate of magnesia, or, still better, of chloride of magnesium, which would throw down the phosphate of magnesia and ammonia. By much the best mode of employing urine is in the form of sulphated urine, which is made by adding to it a sufficient quantity of sulphuric acid to neutralize its ammonia, and evaporating to dryness. In this form all the valuable constituents are retained, and excellent results are obtained from it. Its effects, though mainly attributable to its ammonia, are also in part dependent on the phosphates and alkaline salts which it contains; and it is therefore capable of supplying to the plant a larger number of its constituents than the animal matters already mentioned.

Night-Soil and Poudrette.—The value of night-soil, which is well known, depends partly on the urine, and partly on the fæces of which it is formed. Its disagreeable odour has prevented its general use, and various methods have been contrived both for deodorising and converting it into a solid and portable form. The same difficulties which beset the conversion of urine into the solid form occur here, and in most of the methods employed the loss of ammonia is great. It is sometimes mixed with lime or gypsum, and dried with heat, and sometimes with animal charcoal or peat charcoal. The manufacture of a manure from night-soil, called "poudrette," has long been practised in the neighbourhood of Paris and other continental towns. The process employed at Montfauçon and at Bondy is very simple. The contents of the cesspools are conveyed to the work in large barrels, which are then emptied into tanks capable of containing the accumulation of several months. When filled they are allowed to stand for some time, during which the smell diminishes and the contents become nearly dry. The residue is then dug out and mixed with ashes, dry loam, charcoal powder, peat, peat-charcoal, saw-dust, and other matters, so as to deodorize it, and render it sufficiently dry for transport. Its general composition may be judged of from the subjoined analyses of samples from different places:—

These analyses shew sufficiently the extent to which the animal matters have been mixed with valueless driers, the second and third samples containing considerably more than half their weight of worthless matters.

Hair, Skin, and Horn.—The refuse of manufactories in which these substances are employed, are frequently used as manures. They are highly nitrogenous substances, and owe their entire value to the nitrogen they contain, their inorganic constituents being in too small quantity to be of any importance, wool and hair having only 2 per cent, and horn 0·7 per cent of ash. In the pure and dry state, and after subtraction of the ash, their composition is,—

It rarely if ever happens, however, that the refuse offered for sale as a manure is pure. It always contains water, sand, and other foreign matters. Woollen rags are mixed with cotton which has no manurial value, and the skin refuse from tan-works contains much lime. Due allowance must therefore be made for such impurities which are sometimes present in very large quantity.

Refuse horse hair generally contains 11 or 12 per cent of nitrogen. Woollen rags of good quality contain 12·7 per cent of nitrogen; woollen cuttings about 14; and what is called shoddy only 5·5 per cent. Horn shavings are extremely variable in their amount of nitrogen; when pure, they sometimes contain as much as 12·5 per cent, but a great deal of the horn shavings from comb manufactories, etc., contain much sand and bone dust, by which their percentage of nitrogen is greatly diminished, and it sometimes does not exceed 5 or 6 per cent.

All these substances are highly valuable as manures, but it must be borne in mind that they undergo decomposition very slowly in the soil, and hence are chiefly applicable to slow growing crops, and to those which require a strong soil. Woollen rags have been largely employed as a manure for hops, and are believed to surpass every other substance for that crop. As a manure applicable to the ordinary purposes of the farm they have scarcely met with that attention which they deserve, probably because their first action is slow and the farmer is more accustomed to look to immediate than to future results; but they possess the important qualification of adding permanently to the fertility of the soil.

Blood is a most valuable manure, but it is not much employed in this country, at least in the neighbourhood of large towns, as there is a demand for it for other purposes, and it can rarely be obtained by the farmer in large quantity, and at a sufficiently low price. In its natural state it contains about 3 per cent of nitrogen, and after being dried up, the residue contains about 15 per cent. It is best used in the form of a compost with peat or mould, and this forms an excellent manure for turnips, and is also advantageously applied as a top-dressing to wheat.

Flesh.—The flesh of all animals is useful as a manure, and is especially distinguished by the rapidity with which it undergoes decomposition, and yields up its valuable matters to the plant. It is rarely employed in its natural state, but horse flesh was at one time converted into a dry and portable manure, although, I understand, this manufacture is not now prosecuted. The dead animal after being skinned is cut up and boiled in large cauldrons until the flesh is separated from the bones. The latter are removed, and the flesh dried upon a flat stove. The flesh as sold has the following composition:—

The dried flesh and small bones of cattle, from the great slaughtering establishments of South America, was at one time imported into this country under the name of flesh manure. Its composition was—

But owing to the large proportion of phosphates contained in it, it may be most fairly compared with bones. It is not now imported, the results obtained from its use being said not to have proved satisfactory, although this statement appears very paradoxical.

Fish have been employed in considerable quantity as a manure. That most extensively employed in this country is the sprat, which is occasionally caught in enormous quantities on the Norfolk coast, and used as an application for turnips. They are sold at 8d. per bushel, and their composition is—

The refuse of herring and other fish-curing establishments, whales' blubber, and similar fish refuse, are all useful as manure, and are employed whenever they can be obtained. They are not usually employed alone, but are more advantageously made into composts with their own weight of soil, and allowed to ferment thoroughly before being applied.

Many attempts have been made to convert the offal of the great fish-curing establishments, and the inedible fish, of which large quantities are often caught, into a dry manure, which has received the name of "fish guano." The processes employed have consisted in boiling with sulphuric acid and other agents, and then evaporating, or sometimes by simply drying up the refuse by steam heat. A manure made in this way proved to have the following composition:—

The expense of manufacturing manures of this description has hitherto acted as a barrier to their introduction. In this country several manufactories have been established, but either owing to this cause, or to the difficulty of obtaining sufficiently large and uniform supplies of the raw material, some of them have not proved successful, but a manufactory is now in operation in Norway, which exports the manure to Germany. It is probable that most of the processes used in this country failed because they were too costly, and it is much to be desired that the subject should be actively taken up. It is said that the refuse from the Newfoundland fisheries is capable of yielding about 10,000 tons of fish guano annually; and the quantity obtainable on our own coasts is also very considerable.

Bones.—Bones have been used as a manure for a long period, but they first attracted the particular attention of agriculturists from the remarkable effects produced by their application on the exhausted pasture lands of Cheshire. During the present century they came into general use on arable land, and especially as a manure for turnips; and they are now imported in large quantities from the continent of Europe. The bones used in agriculture are chiefly those of cattle, but sheep and horse bones are also employed. They do not differ much in quality when genuine. The subjoined analysis is that of a good sample.

In general, bones may be said to contain about half their weight of phosphate of lime, and 10 or 12 per cent of water. But, in addition to their natural state, they are met with in other forms in commerce, in which their organic matter has been extracted either by boiling or burning. The latter is especially common in the form of the spent animal charcoal of the sugar refiners, which usually contains from 70 to 80 per cent of phosphate of lime, but when deprived of their organic matter, they may be more correctly considered under the head of mineral manures.

From the analysis given above, it is obvious that the manurial value of bones is dependent partly on their phosphates and partly on the ammonia they yield. It has been common to attribute their entire effects to the former, but this is manifestly erroneous; and although there are no doubt cases in which the former act most powerfully, the benefit derived from the ammonia yielded by the organic matter is unequivocal. When the phosphates only are of use, burnt bones or the spent animal charcoal of the sugar refiners are to be preferred.

At their first introduction, bones were applied in large fragments, and in quantities of from 20 to 30 cwt., or even more, per acre, but as their use became more general they were gradually employed in smaller pieces, until at last they were reduced to dust, and it was found that, in a fine state of division, a few hundredweights produced as great an effect as the larger quantity of the unground bones. Even the most complete grinding which can be attained, however, leaves the bones in a much less minute state of division than guano, and they necessarily act more slowly than it does, the more especially as they contain no ready-formed ammonia. They may be still further reduced by fermentation, which acts by decomposing the organic matter, and causing the production of ammonia; but not as is frequently, though erroneously supposed, by converting the phosphates into a soluble condition, for this does not occur to any extent, and their more rapid action is solely due to the partial decomposition of the organic matter, by which it is brought into a condition capable of undergoing a more rapid change in the soil. The rapidity of action of bones is still more promoted by solution in sulphuric acid, by which they are converted into the form of dissolved bones or superphosphate. At the present moment, however, very little of the superphosphates sold in the market are made exclusively from bones in their natural state, by far the larger portion being manufactured from mineral phosphates, or from bones after destruction of their organic matter, sometimes with the addition of small quantities of unburnt bones, but more frequently of sulphate of ammonia, to yield the requisite quantity of ammonia. These substances may therefore be best considered under the head of mineral manures.

CHAPTER XI.

COMPOSITION AND PROPERTIES OF MINERAL MANURES.

Mineral manure is a term which is now used with great laxity. In its strict sense, it means manures which contain only, and owe their exclusive value to the presence of, those substances which go to make up the inorganic part or ash of plants. It has, however, been usually taken to include all saline matters, and especially the compounds of ammonia and nitric acid, which are indebted for their manurial effects to the nitrogen they contain; and thus is so far incorrect. It would, however, be manifestly impossible to arrange these compounds with any degree of accuracy among either animal or vegetable manures, and hence the necessity of including them amongst those which are strictly mineral. The most important practical distinction between them and the substances discussed in the two preceding chapters is, that the latter generally contain the whole or the greater part of the constituents of plants. Even bones yield a certain quantity of alkalies, magnesia, sulphuric acid, and chlorine, and may in some sense be considered as a general manure. But those to which the term mineral manure is applied for the most part contain only one or two of the essential elements of plants, and hence cannot be applied as substitutes for the substances already discussed, although they are frequently most important additions to them.

Sulphate and Muriate of Ammonia.—These and other salts of ammonia have been tried experimentally as manures, and it has been ascertained that they may all be used with equal success; but as the sulphate is by much cheaper, it is that which probably will always be employed to the exclusion of every other. It contains, when pure, 25·7 per cent ammonia.

It is now manufactured of excellent quality for agricultural use, and when good, contains from 95 to 97 per cent of actual sulphate, the remainder consisting chiefly of moisture and a small quantity of fixed residue; but specimens are occasionally met with containing as much as 10 per cent of impurities, which, as its price is high, makes a material difference in its value. Inferior descriptions are also occasionally sold, among which is a variety distinguished by containing a large quantity of water and fixed salts, although it appears to the eye a good article. Its composition is—

An article called sulphomuriate of ammonia is also sold for agricultural use. It is obtained as a refuse product in the manufacture of magnesia, and is a mixture of sulphate and muriate of ammonia, with various alkaline salts. It differs somewhat in quality, and is sold by analysis at a price dependent on the ammonia it contains.

The quality of sulphate of ammonia may generally be judged of from its dry and uniformly crystalline appearance, and it may be tested by heating a small quantity on a shovel over a clear fire, when it ought to volatilize completely, or leave only a trifling residue. Some care, however, is necessary in applying this test, as in the hands of inexperienced persons it is sometimes fallacious. The salts of ammonia may be applied in the same way as guano; but they are most advantageously employed as a top-dressing, and principally to grass lands. In this way very remarkable effects are produced, and within a week after the application, the difference between the dressed and undressed portions of a field is already conspicuous. Experience has shewn that success is best insured when the salt is applied during or immediately before rain, so that it may be at once incorporated with the soil; as when used in dry weather little or no benefit is derived from it. It seems also to exert a peculiarly beneficial effect upon clover; and hence it ought to be employed only on clover-hay, as where ryegrass or other grasses form the whole of the crop we have better manures.

Ammoniacal Liquor of the Gas-Works, and of the Animal Charcoal Manufacturers.—Both of these are excellent forms in which to apply ammonia, when they can be obtained. The ammoniacal liquor of the gas-works is very variable in quality, but contains generally from 4 to 8 ounces of dry ammonia per gallon, which corresponds in round numbers to from 1 to 2 lb. of sulphate of ammonia. It is best applied with the watering-cart, but must be diluted before use with three or four times its bulk of water, as if concentrated it burns up the grass, and it is also advisable to use it during wet weather. The ammoniacal liquor of the ivory-black works contains about 12 per cent of ammonia, or about four or five times as much as gas liquor. It has been used in some parts of England, made into a compost, and applied to the turnip and other crops, and, it is said, with good effect. Bone oil, which distils over along with it, has also been used in the form of a compost; it contains a large quantity of ammonia and of nitrogen in other forms of combination; the total quantity of nitrogen it contains being 9·04 per cent, which is equivalent to 10·98 of ammonia. Only part of this nitrogen is actually in the state of ammonia; and some circumstances connected with the chemical relations of the other nitrogenous compounds in this substance render it probable that they may pass very slowly into ammonia, and may therefore be of inferior value; but the substance deserves a trial, as it is very cheap. It must be carefully composted with peat, and turned over several times before being used.

Nitrates of Potash and Soda.—Nitrate of potash has long been used as a manure, but its high price has prevented its general application, and its place has now been almost entirely taken by nitrate of soda, which is much cheaper and contains weight for weight a larger quantity of nitrogen. Both these salts are employed as sources of nitrogen; but nitrate of potash owes also a certain proportion of its value to the potash it contains. Nitrate of soda, on the other hand, must be considered to owe its entire value to its nitric acid, as soda is of little value to the plant; and, moreover, can be obtained in common salt at a price so low, as to make it a matter of no moment in the valuation of the nitrate. In its ordinary state, as imported from Peru, nitrate of soda contains from 5 to 10 per cent of impurities, and it bears a price proportionate to the quantity of the pure salt present in it. When of good quality it contains about 15 per cent of nitrogen, equivalent to 18 of ammonia, and is, therefore, richer in that constituent of plants than Peruvian guano. It is essentially a rapidly acting manure, and produces a marked effect within a very few days after its application; but owing to the fact that nitric acid cannot be absorbed and retained by the soil in the same manner as ammonia, it is liable to be lost unless it can be at once assimilated by the plant. For this reason it acts best when applied in small quantity as a top-dressing to grass-land, and to young corn. A large application has no advantages, and there can be no doubt that the best effect would be produced by several very small quantities, applied at intervals. In one experiment, Mr. Pusey found 42 lb. per acre to increase the produce of barley by 7 bushels, and very favourable results have been obtained by other experimenters. The beneficial effects of nitrate of soda appear to be almost entirely confined to the grasses and cereals. At least experience here has shewn that it produces little or no effect on clover; and one farmer has stated, that having recently adopted the practice of sowing clover with a very small proportion of ryegrass only, he has been led to abandon the use of nitrate of soda, which he formerly employed abundantly, when ryegrass formed a principal part of his crop. The action of nitrate of soda is very remarkable, not only in this respect, but also because a given quantity of nitrogen in it appears to produce a greater effect than the same quantity in sulphate of ammonia or guano. At the same time this statement must be taken as very general, definite experiments being still too few to admit of its being stated as an absolute fact. The probability is, that the same quantity of nitrogen, in the form either of ammonia or nitrate of soda, will produce the same effect, although the conditions necessary for its successful action may not be the same with the two manures. It is alleged that nitrate of soda is advantageously conjoined with common salt, which is said to check its tendency to make the grain crops run to straw, and to prevent their lodging, as they are apt to do, when it is employed alone. But considerable difference of opinion exists in this point, many farmers believing that salt produces no effect. When employed for hay, especially when mixed with clover, it is advisable to use it along with an equal quantity of sulphate of ammonia, which gives a better result than either separately.

Salts of Potash and Soda.—The substances just mentioned must be considered to owe their chief manurial value to nitric acid; but other salts have been used as manures in which the effect is undoubtedly due to the alkalies themselves. With the exception of common salt, most of the alkaline salts have only been used to a limited extent; and it is remarkable that, so far as our present experience goes, there is no class of substances from which more uncertain results are obtained.

Muriate and Sulphate of Potash have both been used, and the former has in some cases, and in particular seasons, produced a very remarkable effect in the potato; but in other instances it has proved quite useless. The cause of this difference has not been ascertained. Sulphate of soda has also been used to some extent, but apparently without much benefit; and there is no reason to expect that it should act better than common salt, which can be obtained at a much lower price.

Chloride of Sodium, or Common Salt, has at different times been employed as a manure, but its effects are so variable and uncertain, that its use, in place of increasing, has of late years rather diminished, it having frequently been found that on soils in all respects similar, or even on the same soil, in different years, it sometimes proves advantageous, at others positively injurious. Its use as an addition to nitrate of soda has been already alluded to, and it is said that it produces the same effect when mixed with guano and salts of ammonia. The accuracy of this statement is doubted by many persons, and the explanation which has been given of the cause of its action is more than dubious. It is supposed to enable the plant to absorb more silica from the soil; but this is a speculative explanation of its action, and has not been supported by definite experiment. Although little effect has been observed from salt, it deserves a more accurate investigation, as not withstanding the extent to which it has been employed, we are singularly deficient in definite experiments with it.

Carbonates of Potash and Soda have only been tried experimentally, and that to a small extent, nor is it likely that they will ever come into use, owing to their high price. The remarks we have made in the section on the ashes of plants regarding the subordinate value of soda, will enable the reader to see that greater effects are to be anticipated from the former than from the latter of these salts. They may, however, exert a chemical action on the soil, altogether independent of their absorption by the plant, but its nature and amount are still to determine.

Silicates of Potash and Soda have been employed with the view of supplying silica to the plant, but the results have been far from satisfactory. This may perhaps have been due to the doubtful nature of the commercial article, but now that silicate of soda can be obtained of good quality, it is desirable that the experiments should be repeated. It is said to have produced good effects on the potato.

Sulphate of Magnesia can be obtained at a low cost, and has been used as a manure in some instances with very marked success. It has been chiefly applied as a top-dressing to clover hay, but it seems probable that it might prove a useful application to the cereals, the ash of which is peculiarly rich in magnesia.

Many other saline substances have been tried as manures; but in most instances on too limited a scale to permit any definite conclusions as to their value. The experiments have also been too frequently performed without the precautions necessary to exclude fallacy, so that the results already arrived at must not be accepted as established facts, but rather as indications of the direction in which further investigation would be valuable. There is little doubt that many of these substances might be usefully employed, if the conditions necessary for their successful application were eliminated; and no subject is at present more deserving of elucidation by careful and well-devised field experiments.

Phosphate of Lime.—The use of bones in their natural state as a manure has been already adverted to, and it was stated, that though their value depended mainly on the phosphates, the animal matters and other substances contained in them were not without effect. The action of phosphates is greatly promoted by solution in sulphuric acid, and the application of the acid has brought into use many varieties of phosphates of purely mineral origin, or which have been deprived of their organic matters by artificial processes. Of these, the spent animal charcoal of the sugar-refiners, usually containing about 70 per cent of phosphates, and South American bone ash, are the most important. The latter is now imported in very large quantity, and has the composition shewn in the following analyses:—