Importance in Agriculture.

In the consideration of artificial manures, guano deserves the first place. This it does mainly on historical grounds, as it is now largely a manure of the past. Not merely has it been used in agriculture to an extent to which no other artificial manure has as yet ever approximated, but its influence on agricultural practice has been enormous. Introduced into this country about the middle of the present century, it was the first of artificial manures to be used in large quantities.[183] It may be thus described as having introduced the modern system of intensive cultivation, and given rise to the now almost universal practice of artificial manuring.

Influence on British Farming.

It is, indeed, difficult to over-estimate the important influence which the introduction of this most valuable fertiliser has exercised on British as well as, to a large extent, on European husbandry. Before its introduction the farmer was almost completely dependent on his farmyard manure. He was tied down to a great extent, by the exigencies of the then prevailing agricultural customs, to certain rotations of crops. He could do little in the way of enriching barren soils or of ensuring a heavy yield of crop. By the use of this very potent fertiliser, he quickly discovered that the most wonderful results ensued—results which must have seemed to him at first little short of miraculous. He found that by the application of a few hundredweights per acre, poor soils could be made to yield large returns, and that barren patches in a field could be brought up to the average of the surrounding portions by sprinkling merely a few handfuls of it; that by its means a good start could be ensured to every crop, and one slow of coming away could be hastened on. In short, in this wonderful brown powder, with such a characteristic odour, the astonished farmer discovered a manure which, for the speed of its action, and for the increase of crop it gave, completely threw into the shade both farmyard manure and bones. What wonder, then, that its fame as a manure should have become so quickly known and its use so extensive! It thus gave a most powerful impetus to intelligent farming by bringing home to the minds of those who used it the important position nitrogen and phosphates occupied as constituents of the soil, and the influence they exercised on plant-growth. It furnished, in fact, on an enormously large scale, a practical demonstration of the principles of manuring. The educational value which the use of guano thus exercised may be said to have been very great. It also led the way to the use of the various artificial manures so much used during the last fifty years. Impressed by the value of guano, farmers were favourably disposed towards the use of other fertilisers; and, largely owing to its widespread popularity, the new practice speedily gained ground.

Influence not wholly for Good.

But its influence, it must be admitted, was not wholly for good. In its very popularity lay the danger of its abuse. Had its value and the method of its action been more widely understood, and had the principles upon which the practice of artificial manuring depends been better realised, agriculturists would have been spared much of the needless pecuniary losses they sustained by being imposed upon by unscrupulous manure-dealers. Among the farming community the word guano soon became a name to conjure with, and under this title many spurious and worthless manures were attempted to be palmed off on the unwary farmer. Even the genuine article, there can be little doubt, was at one time largely adulterated; and as the farmer was almost invariably content to purchase the article not on any guaranteed chemical analysis, but simply on the ground of its appearance, colour, and more especially smell, every facility was given for the successful perpetration of such fraudulent imposition. Guano, it was very soon found, varied in its composition, but this variation in quality the farmer did not recognise. In the early days of its use all guano was in his eyes of the same value. Too often, as we have just pointed out, provided it had a good colour and a strong odour, it was all right. Under such conditions, it can scarcely be wondered at that its introduction should have proved not an unmixed blessing to agriculture.

Its Value as a Manure.

Guano derives its value as a manure from the nitrogen, phosphates, and the small amount of potash it contains. This at any rate is true of the great bulk of guano which has been used in the past. There are, as we shall immediately see, certain kinds of guano, known as phosphatic guanos, which only contain phosphates. The amount of such purely phosphatic guano directly used as a manure in this country is, however, inconsiderable, and guano may truly be described as owing its value chiefly to its nitrogen. Not a little of its value and popularity as a manure may be said to be due to the fact that it contains all of the three important manurial constituents, and that in this respect it may be regarded in a sense as a general manure, thus resembling most nearly, of all artificial manures, farmyard manure. Although its sources are now, to a very large extent, exhausted, and its total annual imports into this country are at present considerably less than what they were thirty or forty years ago,[184] it may be well, on account of its historical importance, to give a somewhat detailed account of its origin, occurrence, and value as a manure.

Origin and Occurrence.

Guano (which means dung)—or huano, as it is spelt in the Spanish language—was first used in Peru. It seems to have been used there long before that country was discovered by the Spaniards—probably as early as the twelfth century. Regarding its origin there can be little doubt. It is almost entirely derived from the excrements of sea-birds, such as pelicans, penguins, and gulls, as well as from the remains of the birds themselves, and of seals, walruses, and various other animals.[185] Under the influence of a tropical sun, and in a region in which rain scarcely ever falls, these excrements are soon dried, and remain little changed in their composition through centuries. Many of the Peruvian deposits must be extremely old, as they are covered up with sand and other dÉbris, and are of considerable depth. Especially is this the case with deposits occurring on the mainland, such as those at Pabellon de Pica, where the layer of sand or conglomerate covering up the deposit varies in depth from a few feet to over a hundred. The effect of this superficial covering has been to protect the guano, to a certain extent, from loss of nitrogen.

Although guano of the best class has been derived from the neighbourhood of Peru, deposits have also been found in many other parts of the world—viz., in North America, West Indies, Australia, Asia, Africa, and among the islands of the Pacific.[186]

Variation in the Composition of different Guanos.

The guano found in these different deposits varies very considerably in composition. This is due to the difference in the nature of the prevailing climate of the places where these deposits occur. Where the climate is dry and warm, as is the case in Chili and Peru, the excrements dry quickly and remain very little changed, as one very important condition of fermentation—viz., moisture—is absent.[187] In a damp climate, on the other hand, speedy fermentation ensues, resulting in the loss of nearly all the organic matter, including nitrogen, in such volatile forms as carbonate of ammonia, carbonic acid gas, water, &c. The soluble alkalies, the most important of which is potash, as well as the soluble phosphates, are also, under such conditions, lost to the guano by being washed out by the rain. We have thus a wide difference in the quality of the different deposits, depending on the extent to which decomposition has taken place. Guano thus ranges from the rich nitrogenous Peruvian kind, which has undergone little or no change from the time of its deposit, to the purely phosphatic kind (such as those of Malden and Baker islands), in which everything of manurial value has been lost except the insoluble phosphate of lime. Even among the nitrogenous guanos we find a considerable difference in quality, some deposits being partially impoverished by the action of the atmospheric moisture, dew, spray or sea-water, but still containing a considerable proportion of their nitrogen. Other deposits, again, are largely admixed with sand, which has been blown in upon them to such an extent as to make them unsaleable. We can divide guano, therefore, into two great classes—viz., nitrogenous and phosphatic.

I.—Nitrogenous Guanos.

(a) Peruvian.

By far the most valuable and abundant deposits as yet discovered have been those on the Peruvian and Chilian coasts. As already pointed out, guano seems to have been used in this country from a very early period; and so impressed were the Incas with its importance as a manure, that the penalty of death was imposed on any one guilty of killing the sea-fowl during the breeding season in the vicinity of the deposits.

The occurrence of guano in Peru seems first to have been made known in Europe in the beginning of the eighteenth century. It was not, however, till the beginning of the present century—viz., 1804—that A. Humboldt, the great German traveller, brought some of the wonderful fertiliser home with him, and that its composition was able to be investigated by chemical analysis. Shortly afterwards, its practical value was demonstrated by experiments carried out on potatoes by General Beatson in St Helena. To Lord Derby is due the credit of having first introduced it into this country, the earliest importation into Liverpool being in 1840. Experiments were shortly afterwards instituted in different parts of the country, prominent among which were those by Sir John Lawes and Sir James Caird; and so striking were the results obtained, that the manure rapidly found favour with the farming community—so much so, that ten years later the importations into this country amounted to no less than 200,000 tons, while in 1855 the total exports from the west coast of South America reached the enormous amount of 400,000 tons. In all, it has been estimated that since the year 1840 over 5,000,000 tons of Peruvian guano have been imported into this country.

Different Deposits.

Peruvian guano has been derived from various deposits occurring in different parts of the coast, and from a number of small adjacent islands. The richest of these was that found on Angamos, a rocky promontory on the coast of Bolivia. Samples of this guano contained as high as 20 per cent of nitrogen (equal to 24 per cent ammonia).[188] Unfortunately, however, the quantity of this deposit was extremely limited, and became rapidly exhausted. Next to this deposit in quality was the guano found on the Chincha islands, three little islands off the coast of Peru. These deposits were the largest which have ever been discovered, and for a period of nearly thirty years were almost the sole source of the Peruvian guano sold in commerce, over 10,000,000 tons having been exported from them alone. Some of this guano contained 14 per cent of nitrogen (equal to 17 per cent ammonia); and although part of the guano shipped from these islands was not quite so rich, yet it was all of a high-class order. The deposits on these islands were in many cases 100 to 200 feet in depth, and rested on rocks of granite. The lower layers were consequently found to be poorer in quality, and mixed with pieces of granite. The Chincha island deposits have been long exhausted,[189] and the chief deposits of Peruvian guano since worked have been those on Guanape and Macabi islands—a considerably inferior guano, containing only 9 to 11 per cent of nitrogen (equal to 11 to 13 per cent of ammonia)—which in their turn have become exhausted; from Ballestas, almost as rich as the Chincha island guano, also now exhausted; and from Pabellon de Pica, Punta de Lobos, Huanillos, Independence Bay, and Lobos de Afuera. Quite recently a deposit of very high-class guano was discovered in Corcovado, and a good many cargoes have already been shipped to this country. It is found to contain nitrogen equal to from 10 to 13 per cent ammonia, 30 to 35 per cent phosphates, and some potash, being thus a most valuable guano.

Appearance, Colour, and Nature.

In colour it varies from a very light to a very dark brown, the richer samples being generally lighter. Samples taken from even the same deposit have been found to differ very considerably in appearance, those taken from the lower and older layers being usually darker than those taken from the more recent upper layers. It was soon found also to vary very much in composition. After a deposit had been worked for some time, the quality of guano it yielded was found to be inferior and coarser, and in many cases mixed with pebbles or pieces of granite, porphyry, &c. This led to the custom of screening it on arrival in this country, before it was used as a manure. In the richer qualities—e.g., in the Chincha guano—little round concretionary nodules, varying in colour from pure white to dark brown, were occasionally found. Analysis showed these nodules[190] to be composed chiefly of potash salts. Sometimes, also, little crystals of almost pure ammonia salts were found. It soon became customary, therefore, to prepare guano for the market by separating the stones and reducing the whole to a fine uniform powder. One of its most characteristic properties, and the one which seems to have impressed the public most, was its pungent odour. Undue importance was attached to this property, in the belief that it was caused by the ammonia it contained. It may be doubted, however, whether the characteristic smell of guano is due so much to its ammonia as to certain fatty acids.

Composition.

In composition it is of a most complex nature. It contains its nitrogen in a great variety of forms, the chief of these being urate, oxalate, ulmate, humate, sulphate, phosphate, carbonate, and muriate of ammonia; and also in a rare form of organic nitrogen peculiar to guano, called guanine. According to Boussingault, some guanos contain small quantities of nitrates. Its phosphoric acid is present both in the soluble state—viz., as phosphates of the alkalies (ammonia and potash)—and in the insoluble state as phosphate of lime; and lastly, its potash is present as sulphate and phosphate. The proportion in which these different forms of nitrogen and phosphoric acid are present varies considerably in different samples. The richer a sample, as a rule, the more nitrogen in the form of uric acid it contains. The most of the nitrogen is present as uric acid and ammonia. Damp guanos contain more of their nitrogen as ammonia than dry ones, this being due to the fermentation which goes on in the former. On an average, about a third of its total nitrogen is soluble in water. Of its phosphates, on the other hand, only about a fourth are soluble in water.

The following analyses of a sample of Chincha island guano by Karmrodt[191] will illustrate this. (Sample dried at 212° Fahr.):—

1. Constituents easily soluble in Water.

2. Difficultly soluble in Water, soluble in Acids, Alcohol, and Ether.

In the above analysis it will be noticed that none of the ammonia is present as carbonate. In most samples, however, of Peruvian guano, the ammonia in this form amounted to from 1 to 2 per cent. In the inferior qualities, chiefly those which had been subjected to the action of water, and consequently of fermentation, to a certain extent, this form of ammonia was found to be most abundant. Such guanos were most liable to loss of nitrogen by volatilisation.

The older Peruvian guano contained as high as 14 per cent of nitrogen (equal to 17 per cent of ammonia), and of phosphoric acid 12 to 14 per cent (equal to 26 to 28 per cent of phosphate of lime). It, however, gradually deteriorated in quality as the deposits became worked out, the percentage of nitrogen becoming year by year less, until latterly Peruvian guano, as imported, contains only from 3 to 4 per cent of nitrogen (equal to 4 to 5 per cent of ammonia). This guano is, however, richer in phosphates, containing often 50 to 60 per cent of phosphate of lime, and 3 to 4 per cent of potash.[192]

(b) Other Nitrogenous Guanos.

The guanos, other than those which come from Peru, are chiefly purely phosphatic guanos, so that the term Peruvian has not unfrequently in the past been used as a generic term synonymous with the term nitrogenous, and consequently applied to all nitrogenous guanos independent of their source. There are, however, a few deposits other than the Peruvian which have yielded considerable quantities of valuable nitrogenous guano. Of those, the richest in quality—in fact, the richest of any deposits hitherto discovered—was the Angamos guano, which came from a rocky promontory on the coast of Bolivia. The few samples of this which have been analysed showed over 20 per cent of nitrogen. Unfortunately, the deposit proved to be comparatively insignificant in amount, and has long been exhausted.

Poorer in quality, but more abundant in quantity, were the deposits found on the Ichaboe and other islands off the south-west coast of Africa. These deposits were discovered shortly after the introduction of Peruvian guano, and for a few years supplied considerable quantities of valuable manure. The deposits first discovered were soon exhausted, so that for a number of years Ichaboe guano ceased to be procurable. Fresh deposits, however, were subsequently found, and considerable quantities have of late years been used in agriculture.[193] Ichaboe guano is inferior in value to Peruvian. It exemplifies the influence of small quantities of rain on guano deposits in impoverishing them in their nitrogen. In much of the Ichaboe guano imported into this country a large amount of feathers is found. It also contains an abnormally large quantity of insoluble matter.

Among the other nitrogenous guanos may be mentioned the Patagonian, Falkland, and Saldanha Bay. They are, like the Ichaboa, of comparatively recent origin, and are collected in small quantities after the breeding season every year.

II.—Phosphatic Guanos.

Phosphatic guanos, as already pointed out, are similar in origin to nitrogenous guanos. In their case, however, the nitrogen, alkalies, and soluble phosphates which they originally contained have been almost entirely lost by the decomposition of their organic matter and the action of water.[194] Most of them still contain very small quantities of nitrogen, amounting to a fraction of a per cent. Of these deposits there are very many occurring on islands in different parts of the world. In appearance the guano obtained from them is very different from nitrogenous guano, being much lighter in colour, and of a fine powdery nature. It forms a very rich phosphatic guano, containing in many cases between 70 and 80 per cent of insoluble phosphate of lime. Such guanos are largely used in the manufacture of high-class superphosphates, by treating them with sulphuric acid. Being of an insoluble nature, they are not very suitable for direct application to the soil. Of these phosphatic guanos the following are the chief—those marked in italics being still unexhausted:—

1. Baker, Jarvis, Howland, Starbuck, Flint, Enderbury, Malden, Lacepede, Browse, Huon, Chesterfield, Sydney, Phoenix, Arbrohlos, Shark's Bay, and Timor—all found on islands in the Pacific Ocean.

2. Mejillones, on the coast of Bolivia.

3. Aves, Tortola, Mona, and other deposits in the West Indies.

4. Kuria Muria islands, in the Arabian Gulf.

For further particulars as to the composition of these different guanos, the reader is referred to the Appendix, Note V., p. 329.

Inequality in Composition.

That guano was a substance of by no means uniform composition was a fact early recognised in the history of the trade. Not only did guano from different deposits show on analysis different percentages of the manurial ingredients, but different samples of guano from the same deposit were often found to differ very considerably from one another. It soon became the custom, therefore, to sell it on chemical analysis, each separate cargo being carefully analysed. But this custom did not wholly obviate the difficulty, as the guano in even one cargo might differ. In the case of the older and richer guanos, there was certainly more uniformity in quality, but they were liable to differ in their percentage of nitrogen.[195] As, however, the deposits became gradually worked out, their lower layers were found more or less largely admixed with stony and earthy matter, and their composition was naturally rendered very variable. This state of matters was unsatisfactory to buyers and sellers, and led to much friction between the two, as it was found wellnigh impossible on the part of the seller to guarantee the composition of his manure. The custom of preparing the material by reducing it to a fine powder before sending it into the market, and the custom, subsequently introduced, of treating it with sulphuric acid, have done away with this difficulty to a large extent.

"Dissolved" Guano.

The treatment of guano with sulphuric acid was first had recourse to in the case of cargoes damaged with water. In such guano, as has been already pointed out, fermentation has been permitted to take place, with the result of the formation of volatile carbonate of ammonia in greater or less quantity. By the addition of sulphuric acid the ammonia was fixed, and the guano was prevented from losing its most valuable constituent. It was soon found, however, that guano so treated possessed greater activity as a manure. The result of the sulphuric acid was to increase very materially the amount of its soluble phosphates, and also its soluble nitrogen compounds.[196] It had, moreover, the effect of producing a guano of uniform composition. The custom, first introduced in 1864 by Messrs Ohlendorff & Co., was soon largely practised. The guano is treated with 25 to 30 per cent sulphuric acid (sp. gr. 1.73). After a short time the resulting hard mass is, by means of disintegrators, reduced to a uniform powder.

"Equalised" or "Rectified" Guano.

As guano decreased in its quality the demand for a high-class article became more and more difficult to meet. This led to the custom of "fortifying" or "rectifying"—as it is variously called—the natural material with sulphate of ammonia. A manure closely resembling in the percentage of its manurial constituents the older rich guanos is thus obtained. Of these so-called "equalised" guanos, two qualities are at present sold, the first being guaranteed to contain nitrogen equal to 8 to 9 per cent ammonia, 30 to 35 per cent phosphates, and 2 to 3 per cent of potash; the second quality containing only about half as much nitrogen, but more phosphates.

However valuable this fortified guano may be—and it is, undoubtedly, a most valuable manure—its action cannot be supposed to be exactly similar to the old Peruvian guano, which it resembles in the percentage of its nitrogen, phosphates, and potash. Much of the distinctive value of guano as a manure, as will be pointed out immediately, lies in the fact that it contains its manurial ingredients in a variety of differently soluble compounds, which are gradually rendered available in the soil for the plant's needs. This undoubtedly is one of the reasons why the action of guano among manures is quite unique; and there are other reasons which we probably do not clearly understand. However skilfully the composition of the guano may be artificially simulated, it still remains an undoubted fact that the "equalised" guano is not exactly similar in its action to the genuine article. Nevertheless, that it is superior in its results to the poorer classes of guano at present available, and to ordinary compound manures, there can be little doubt. A great merit of the equalised guano is, however, that it is sold at a lower price than guano as imported; and as the guano is sold on a guaranteed analysis, the practice has done much to advance the true interests of agriculture.

Its Action as a Manure.

Next to farmyard manure, guano may be regarded as the most "general" of all the commonly used manures; for in addition to nitrogen, phosphoric acid, and potash, it contains nearly all the other plant ingredients, such as lime, magnesia, &c. Its special value as a manure, however, does not merely consist in the amount of valuable plant-food it contains. Like farmyard manure, it owes much of its characteristic action to the state of the intimate mixture of its manurial constituents, and also, as has already been pointed out, to the fact that it contains those constituents in a great variety of chemical forms, each of which differs in its solubility, and consequently availability for the plant's needs. Take, for example, the great number of different forms of nitrogen it contains. Some are in the condition in which plants can immediately absorb them, while the rest are in a series of less and less available forms, which, however, are gradually converted into available forms as the plant requires them. Like farmyard manure, again, it may be applied with almost equally good results to all kinds of crops and on all kinds of soils. We have in guano, in short, an admirable example of the value of applying our manurial ingredients in different forms. That this is no mere theory is abundantly proved by the large number of different experiments which have in the past been carried out with guano, more especially the well-known experiments made by Grouven, the German chemist. In those well-known experiments, guano was tested against a large variety of different fertilisers, and the tests were so arranged that in most cases the amounts of nitrogen, phosphoric acid, and potash were the same in the other manures used. In short, these experiments prove in a very striking manner that a manure artificially made up out of most valuable fertilisers, such as nitrate of soda, sulphate of ammonia, superphosphate, &c., so as to closely resemble in its composition guano, is by no means similar in its effects to the genuine article. As in farmyard manure, so in guano: we must look to the complexity of the composition of both these fertilisers in order to fully estimate their worth. There is in the action of both manures much that we cannot explain, or even, as yet, understand. The action of guano is merely one of many problems in the science of manuring which illustrate how unsatisfactory, despite the great amount of research already carried out, is our knowledge of this most important department of agriculture.[197]

Proportion of fertilising Constituents in Guano.

Guano must be regarded as a nitrogenous and phosphatic manure, as the quantity of potash it generally contains is small. In many soils, more especially in such a country as Scotland, this deficiency in potash is not of so much importance, as the value of potash as an artificial manure is less than is the case with the other two ingredients. In soils, however, lacking potash, guano ought to be supplemented with some potash manure. With regard to the nitrogen and phosphoric acid, we may ask if these two constituents are in the best proportions. This question does not admit of a direct answer. In the first place, the proportion in which these two ingredients are present is variable. In the old rich Peruvian guanos, as we have above shown, the nitrogen was more abundant than is the case at present. Such guanos, it was found, were best supplemented with phosphatic manure when applied to the field. In the "equalised" and "dissolved" guanos, which are now so largely sold, manufacturers attempt to adjust the percentage of nitrogen and phosphoric acid to what is considered the best proportion in most cases. As, however, we have again and again to point out, regard must be had both to the soil and the crop in determining what is the best proportion of the manurial ingredients in a manure. For cereals it may be well supplemented by nitrogenous manures, while for roots it may be well supplemented by phosphatic manures.

Mode of Application.

Like all manures, it is desirable to apply it in as fine a condition as possible, so as to ensure as thorough a mixture with the soil-particles as practicable. In order, furthermore, to prevent any risk of loss through volatilisation of the ammonia, as well as to ensure even distribution, it is best applied mixed with dry earth, ashes, sand, or some other substance,—not lime, however. The custom of applying along with the guano common salt, has been proved by numerous experiments to be highly beneficial to the action of the guano as a manure. The exact nature of the action of salt as an adjunct to manures is a point which has elicited much discussion. Its action is probably to be ascribed to a number of causes. For one thing, it probably acts as an antiseptic in retarding the fermentative action which has a tendency to go on so rapidly in such manures as guano. It further increases the power of the manure to attract moisture from the air—a most important property in the case of drought. Some experiments by Dr Voelcker illustrate this in a striking manner. Two lots of guano—one pure and one mixed with salt—were exposed to the action of the air for a month, and were then tested as to the amount of water they contained, when it was found that the lot containing the salt had absorbed 2 per cent more water than the other.

Much stress has been laid on the importance of having the guano buried a certain depth in the soil; and many experiments have been carried out to prove how much better it acts when so applied. This is probably due to the prevention of any loss of volatile ammonia, and the mixture of the manure with the soil-particles before it comes in contact with the plant-roots. This last precaution is an important one, for it has been found that the raw material is apt to have a bad effect on the seed or the plant's roots. This has been found to be especially the case in regard to potatoes, the quality of which has been found to suffer when the guano is brought into direct contact with the tubers. As guano is a manure which is speedily available, it is desirable to apply it as shortly before it is required by the plant as possible. It is therefore generally best applied in spring, shortly before seed-time, or indeed at the same time. Where farmyard manure is used, the guano has been recommended to be used as a top-dressing in small quantities. In the majority of cases it will be advisable, however, not to apply it as a top-dressing, for the various reasons above-mentioned.

Quantity to be used.

As to the quantity to be used, this of course will depend on the soil, the crop, and the amount and nature of the other manures employed: 1 to 4 cwt. per acre have been the usual limits, but even heavier dressings have been commonly resorted to, especially in Scotland, where 6 to 8 or even 9 cwt. for turnips are often used. Sir J. B. Lawes and Sir James Caird long ago, shortly after the introduction of guano, estimated, from the experiments they carried out, that the application of 2 cwt. per acre to the wheat crop gave an increase of 8 to 9 bushels in grain, and added a fourth to the quantity of straw. The former authority recommends 2 to 3 cwt. per acre for wheat, to be sown broadcast and harrowed into the land before sowing the seed. We have already stated that it may be used in all soils and for all kinds of crops. While this is so, it has been found to have specially favourable results when applied to the turnip crop, when it may be used in larger quantities than in the case of cereals. When applied to the turnip crop, it is well to use the more phosphatic guanos or to supplement it with superphosphates. By applying it in two lots, the larger portion before seed-time and the rest between the drills after the turnips are up, excellent results have been obtained. It has also proved an admirable manure for mangels. On the whole, it gives best results on heavy soils and in a dampish climate.

Adulteration of Guano.

Probably no artificial manure has been subjected to greater adulteration in the past than guano. This has been due to the fact that the practice of selling guano on analysis—especially among retail buyers—did not largely obtain in the early years of the trade. A good deal of this adulteration was probably caused by ignorant prejudice on the part of the farmer, to whom the pungency of its smell and its colour were too apt to be ranked as its most important properties. The variation in the quality of different kinds of guano was too often not sufficiently realised by the buyer, who not unfrequently was made to pay as high a price for guano of an inferior quality as he ought to have paid for that of the best quality. Indeed no manure illustrates the importance of chemical analysis more than guano. Among the different forms of adulteration practised may be mentioned the addition of such substances as sawdust, rice-meal, chalk, sulphates of lime and magnesia, common salt, sand, earth, peat, ashes of various kinds, and water. There can be no doubt, however, that such adulteration has now long ceased to be practised to any extent. Nevertheless, it may be of use to draw attention to one or two of the tests by means of which some of the commoner forms of adulteration may be detected. One or two are extremely easily detected—as, for example, adulteration with sand or other mineral substances. In such a case, the percentage of ash left on burning a small portion of the guano will be found to be excessive. The percentage of ash in a sample of genuine Peruvian guano should not exceed from 50 to 60 per cent. The colour of the ash is another important point, and may serve as a further indication of adulteration. In the case of genuine guano, this should be whitish or greyish. Red-coloured ash generally points to the adulteration of the guano with some mineral substance containing iron—such, e.g., as Redonda phosphate, a mineral phosphate of iron and alumina. Where the ash is white, but excessive in quantity, adulteration with common salt, sulphate of magnesia, gypsum, or chalk, may be suspected. The last-named substance is easily detected by treating it with any of the common acids, when brisk effervescence, due to the liberation of the carbonic acid, will ensue.[198] A further point of importance with regard to the ash is its solubility in water and in acids. A large insoluble residue may be taken as indicating adulteration with sand. Adulteration with water is also easily detected by heating a sample to the boiling temperature and determining the loss it sustains. Of course the amount of water varies in different samples. The appearance of the guano will serve fairly well to detect whether it is abnormally moist. It may be added, in conclusion, that Peruvian guano is extremely light; and while this by itself is not a sufficient test of genuineness, it may serve to confirm other tests.

III.—So-called Guanos.

Before concluding this chapter, reference may be made to certain manures which are commonly known under the name of guanos—such as "fish-guano," "flesh-guano," "meat-meal-guano," and "bat-guano,"—as well as to manures which may more conveniently be described here—viz., "fowl and pigeon dung."

Fish-Guano.

The application of fish, not suited for other purposes, to the fields as a manure is a practice which has obtained in certain parts of the country for a number of years. In many districts on the sea-coast, where fishing is the chief industry, the only way in the past of disposing of a superabundant catch of herrings, for example, has been to utilise them as a manure. From such a practice has sprung up what is now an important and ever-increasing trade—viz., the manufacture of fish-guano.

This manufacture was first started, and is still most largely practised, in Norway. The guano obtained varies very considerably in quality according to the nature of the process employed, and as to whether the guano is made from whole fish or merely from fish-offal. The latter source is the common one. The manufacture is carried on at the fish-curing stations, and the quality of the guano made from this source is somewhat different from that made from whole fish, as a large proportion of the fish-offal is made up of bones and heads. Large quantities of Norwegian fish-guano are exported to various parts of Europe.

The best quality of this guano may contain as much as 10 per cent of nitrogen, but as a rule it is nearer 8 per cent. A very considerable variation in the amount of phosphoric acid occurs for the reason above stated, the guano made from fish-scrap being naturally much richer in this ingredient than whole-fish guano. The phosphoric acid may be said to range from 4 to 15 per cent, and there is also a small quantity of potash present.

Guano is also manufactured in Norway from the carcasses of whales. Such guano contains from 7-1/2 to 8-1/2 per cent of nitrogen, and about 13-1/2 per cent of phosphoric acid.

In America fish-guano is manufactured to a considerable extent—one important source being the menhaddo, a coarse sort of herring. This fish is caught for the sake of its oil, which is extracted by boiling, the residue being manufactured, after pressing and drying, into guano.

In this country the manufacture of fish-guano is carried out to a considerable and increasing extent. Formerly it was imported from Norway to a larger extent than is now the case, the present annual imports amounting only to 1000 or 2000 tons. The total annual production in the United Kingdom is probably 7000 or 8000 tons.

Value of "Fish-Guano."

That fish-guano is a valuable manure there can be no doubt. What, however, impairs its value is the fact that, as a rule, it contains a certain amount of oil. The effect of this oil is to retard fermentation and decomposition when the guano is applied to the soil, and thus render its action slower than would otherwise be the case.

When applied to the soil, therefore, every opportunity ought to be given to promote its fermentation. It is best applied some time before it is likely to be used. It ought to be well mixed with the soil-particles, and not allowed to lie on the top of the soil. Its best effect will be on light well-cultivated soils, which permit of the access both of sufficient moisture and of sufficient air for rapid fermentation. Its value as a manure for hops, vines, grass, and strawberries has been found to be considerable. It has been recommended to be applied along with farmyard manure; and such a mode of application is no doubt well suited to promote its decomposition. It has also been used for mixing with superphosphate of lime. Professor Storer has advocated a more general use of fish as a manure than is at present the case. He suggests that even fish not suitable for edible purposes might be caught for the purpose of conversion into manure. The difficulty of preserving fish, however, is considerable; and he suggests the use of potash salts, such as muriate of potash, or lime for this purpose. The benefit of using potash would be twofold. In addition to acting as a preservative, it would considerably enhance the value of the resulting guano as a manure. There is much truth in Professor Storer's views; and no doubt, as our sources of artificial nitrogenous manures grow more limited, the manufacture of fish-guano will be carried on in the future on a larger and more systematic scale than hitherto.

Meat-meal Guano.

What is called "meat-meal guano" is generally that made from the refuse of the carcasses of cattle after they have been treated for their meat-extract according to Liebig's process. The meat-meal is used both for feeding and manurial purposes. Considerable quantities[199] of this guano are imported annually into this country from South America, Queensland, and New Zealand,—that coming from Frey Bentos, in Uruguay, being best known. It is a valuable manure, especially so for its nitrogen, which varies from 4 to 8 per cent, while it contains of phosphoric acid from 13 to 20 per cent. Some meat-meal guanos contain as much nitrogen as 11 per cent.

In some parts of the world, more especially in Germany, the carcasses of horses, as well as cattle, dogs, pigs, &c., which have died of disease, are converted into a guano. They are subjected to treatment by steam in digestors, by which means the fat and gelatine are separated and utilised, while the remaining portion of the animal is converted into guano. Other processes are also employed. The resulting manure contains from 6 to 10 per cent of nitrogen, and from 6 to 14 per cent of phosphoric acid.

Value of Meat-meal Guano.

Meat-meal guano is a valuable nitrogenous manure. The same remarks apply to it as to fish-guano, although it ferments probably very much more quickly than the latter, and is undoubtedly a more valuable manure.

Bat Guano.

In conclusion, we may consider bat guano. Bat guano, which is really a very rare curiosity, has been found accumulated in hot climates in caves.

The samples which have been analysed have differed very much in quality, some containing as much as 9 per cent of nitrogen and 25 per cent of phosphoric acid. Provided it could be obtained in any quantity, and of a quality even approximating to the above analysis, it need scarcely be pointed out that bat guano would be a most valuable manure.

A singular point about its composition is, that it has been found to contain a considerable proportion of its nitrogen (as much as 3 per cent) in the form of nitrates.

Pigeon and Fowl Dung.

Pigeon dung is a manure which historically is of great importance. The dung of pigeons was used as a manure by the ancient Romans; and even in modern times, more especially in France, it was considered a most important fertiliser. Despite these facts, pigeon dung is by no means a rich manure, and its composition compares most unfavourably with that of the guanos we have just been considering. According to Storer,[200] it only contains from 1-1/4 to 2-1/2 per cent of nitrogen, and from 1-1/2 to 2 per cent of phosphoric acid, and a little over 1 per cent of potash.

The dung of poultry is just about as poor, fowl dung containing from .8 to 2 per cent of nitrogen, 1-1/2 to 2 per cent of phosphoric acid, and a little under 1 per cent of potash; while that of ducks and geese is even poorer.[201]

From these statements it will be seen that the excrements of pigeons, hens, and ducks do not form a rich manure. One thing about pigeon dung which is to be noticed, is the fact that it ferments very quickly.

None of the pseudo-guanos, however rich they may be in manurial ingredients, can be regarded as equal in their action to the genuine article, for reasons which we have gone into already when considering the action of guano.

FOOTNOTES:

[183] Bones, it is true, were in use long before guano; but popular as they deservedly were, they had not been used, at the time of the importation of guano, to any very considerable extent.