Nitrate of soda,[202] or, as it is more correctly designated from a chemical point of view, sodium nitrate, now forms the chief artificial nitrogenous manure in use. Along with sulphate of ammonia, it has taken the place once held in the manure markets by the older Peruvian guano, and may without doubt be reckoned, at present prices, one of the cheapest and most valuable of the artificial sources of nitrogen for the plant. It is some sixty-two years ago since it was first exported from South America into this country. The total exports in that year amounted to about 800 tons, and some indication of the enormous extent to which the use of this valuable fertiliser has been developed since then will be obtained from the statement that the total exports at present amount to little less than 1,000,000 tons per annum, representing a monetary value of 6 to 7 millions sterling. Of this quantity about 120,000 tons are imported into Britain.[203] While its chief use is for manurial purposes, it must not be imagined that it is only used for this purpose. A certain amount is used in connection with various chemical manufactures—for instance, that of nitric and sulphuric acid—and also in the manufacture of saltpetre, the chief constituent of gunpowder.

Date of Discovery of Nitrate Deposits.

The exact date of the discovery of the nitrate deposits seems to be a point of considerable dubiety. The earliest published description of them was written by Bollaert about the year 1820, in which year, it is stated, the first shipment was made to England. It was not, however, till some ten or twelve years later that the Peruvian Government, to whom they then belonged,[204] seems to have recognised their value. The most important deposits are found in the vicinity of the town of Iquique, which is the chief nitrate port of South America. It is a somewhat striking fact that this substance, which has conclusively proved itself to be the most potent of all known artificial agents in the promotion of vegetable growth, should be found in a district utterly lacking the slightest traces of vegetation of any kind. Lest such a statement should seem to savour of irony, we hasten to explain that the singular barrenness of this part of the country is largely due to the character of its climate, the deposits occurring in the midst of sandy deserts,[205] on which rain never falls.

Their Origin.

The origin of these nitrate-fields is a geological problem of very considerable interest, the difficulty of which is greatly enhanced by their altitude—3000 to 4000 feet above the sea-level—and their distance inland, which amounts in some cases to eighty or ninety miles from the sea-coast. The nitrate deposits are not the only saline deposits found in Chili. According to the late David Forbes,[206] they are not to be confused with other saline formations, which appear at intervals scattered over the whole of that portion of the western coast, on which no rain falls. The latter stretch from north to south for a distance of more than 550 miles—their greatest development being between latitudes 19° and 25° south. The depth to which they extend downwards varies considerably. Most of them, however, are of a very superficial character, and "they always show signs of their existence by the saline efflorescence seen on the surface of the ground, which often covers vast plains as a white crystalline incrustation, the dust from which, entering the nostrils and mouth of the traveller, causes much annoyance, whilst at the same time the eyes are equally suffering from the intensely brilliant reflection of the rays of a tropical sun." These saline incrustations, or salinas, as they are generally called, are chiefly composed of salts of lime, soda, magnesia, alumina, and of boracic acid. Their composition would lead one to attribute their origin to the evaporation of salt water; for, with the single exception of boracic acid,[207] all the mineral substances are such as would be obtained by the evaporation of sea-water, or by the mutual reactions of its salts with the constituents of the adjacent rocks. As there is "indisputable evidence of the recent elevation of the whole of this coast," volcanic upheaval might be reasonably held to explain their altitude. Their comparative proximity to the coast would seem further to favour this theory. On these grounds, therefore, Forbes is inclined to think that they owe their origin to the evaporation, under the influence of a tropical sun, of lagoons of salt water, the communication of which with the sea had been cut off by the rising of the land.

Forbes and Darwin on the Theory of their Origin.

The obvious difficulty of accounting for the formation of the larger deposits by such a theory he meets by saying that it is only necessary to suppose that, even after the partial isolation of the lagoons by the elevations of the coast, they might still have maintained tidal or occasional communication with the sea by means of lateral openings in the chain of hills separating them from the ocean. In such cases there would be a gradual accumulation of salts, very much greater in amount than that due simply to the evaporation of the water originally contained in the lagoons. The above theory of the origin of the lower saline deposits may go to explain the mode of formation of the nitrate-fields; but in this case several difficulties present themselves. One is the much greater altitude of the latter, as well as their greater distance inland. This difficulty, however, may be met by assuming that they are of older origin than the lower deposits, and have been subjected to a correspondingly greater amount of volcanic upheaval. There is abundance of proof that this part of the continent has been the scene in the past of such volcanic upheaval. Forbes is of opinion that there is the fullest evidence to prove that, even since the arrival of the Spaniards, a very considerable elevation of the land has taken place over the greater part, if not the whole extent, of the line of coast; while Darwin states that he has convincing proof that this part of the continent has been elevated from 400 to 1200 feet since the epoch of existing shells. Furthermore, elevations of the coast-line, amounting in many cases to several feet, are known to have happened within recent times, while earthquakes and volcanic disturbances of a less striking nature are still of common occurrence. Successive lines, indicative of old sea-beaches, can be distinctly traced stretching inland, one behind the other; and patches of sea-sand and water-worn stone, found at a great distance from the coast, both in valleys and at altitudes much greater even than 4000 feet, point to the same conclusion.[208] The difficulty, therefore, of altitude and distance from the coast cannot be regarded as insuperable.

Source of Nitric Acid.

A difficulty, however, which is not so easily met, is afforded by the presence of the nitric acid which, in combination with the soda, forms the nitrate of soda. It is scarcely necessary to inform our readers that nitrogen—except, of course, in small quantities in the free state—is not a normal constituent of salt water. The question, therefore, of greatest interest in connection with the formation of these nitrate-beds is, Whence has the nitric acid been derived? Several theories have been put forward to account for it.

Guano Theory.

One is to the effect that it owes its origin to huge guano deposits, originally covering the shores of the large salt lakes which, by the subsequent overflowing of their shores, effected the mixture of the guano with the salts. In this way, by a slow process of decomposition, nitrate of soda would be ultimately formed.[209] This theory, apart from other considerations, seems at first sight extremely plausible, more especially when we remember that it is on this very coast that the greatest guano deposits have been found, and that the famous Chincha Islands, which alone have yielded over 10 million tons of this valuable fertiliser, are comparatively near the scene of the nitrate deposits. What seems further to support this theory, is the actual occurrence in the nitrate-fields themselves of small quantities of guano. But however plausible it may appear at first sight, it does not bear closer criticism. One very serious objection is the absence in these deposits of phosphate of lime, which is the largest constituent of guano. If they were really due to guano, how does it happen that the insoluble phosphate of lime should have disappeared, while the easily soluble nitrate of soda should alone be preserved? Again, assuming this theory to be correct, we should naturally expect to be still able to find evidence of the chemical changes which would under such circumstances have taken place, in the shape of portions of the guano in the transition stage. Such evidence, however, the most careful investigations have failed to detect. Apart, however, from the above objections, there seems to be little doubt, from evidence afforded by traces of birds' nests, &c., that the guano found in the nitrate-beds was deposited subsequent to the formation of the nitrate of soda.

Nitric Acid derived from Sea-weed.

The most probable theory seems to be that put forward by NÖllner. The origin of the nitric acid is, according to him, to be ascribed to the decay of great masses of sea-weed, which, by means of hurricanes such as are still prevalent in these districts, were driven into the lagoons. The chief difficulty in the way of accepting this theory is the enormous quantity of sea-weed required to produce the millions of tons of nitric acid these deposits contain. It must be remembered, however, as bearing upon this point, that the occurrence of gigantic masses of sea-weed in the Pacific Ocean[210] is by no means uncommon even at the present time. If, to understand the formation of coal, we must suppose the Carboniferous period to be one during which exceptionally luxuriant growth of vegetation took place, we may be permitted to suppose a similar luxuriant growth of sea-weed during the formation of the nitrate deposits. Very strong confirmation of the truth of this theory is further afforded by the presence in large quantities, in the raw nitrate of soda, of iodine, a substance characteristic of sea-weed; while pieces of sea-weed still undecomposed are met with here and there. On the whole, therefore, this theory, while not free from difficulties, seems to be the most worthy of acceptance as regards the origin of the nitrate deposits.[211]

Appearance of Nitrate-fields.

Having thus discussed the origin of the nitrate-fields, we may now give a more detailed description of their appearance. The chief deposits at present being worked are those lying in the Pampa de Tamarugal, in the province of Tarapaca. They stretch to a distance of thirty or forty miles inland, from Pisagua southwards to somewhat beyond the town of Iquique. This huge desert, as has been already indicated, seems to be entirely destitute of all vegetation and animal life. Even in the immediately adjoining country the only kind of vegetation that seems to grow is a species of acacia. The few streams that are found in this neighbourhood are entirely fed by the melting snow from the Cordilleras. Darwin describes the appearance presented by these pampas as resembling "a country after snow, before the last dirty patches are thawed." The caliche, or raw nitrate of soda, is not equally distributed over the pampas. The most abundant deposits are situated on the slopes of the hills which probably formed the shores of the old lagoons. An expert can tell from the external appearance of the ground where the richest deposits are likely to be found. The caliche itself is not found on the surface of the plain, but is covered up by two layers. The uppermost, known technically as chuca, is of a friable nature, and consists of sand and gypsum; while the lower, the costra, is a rocky conglomerate of clay, gravel, and fragments of felspar. The caliche varies in thickness from a few inches to 10 or 12 feet, and rests on a soft stratum of earth called cova.

The Method of mining the Nitrate.

The mode in which the caliche is excavated is as follows: A hole is bored through the chuca, costra, and caliche layers till the cova or soft earth is reached below. It is then enlarged until it is wide enough to admit of a small boy being let down, who scrapes away the earth below the caliche so as to form a little hollow cup. Into this a charge of gunpowder is introduced, and subsequently exploded. The caliche is then separated by means of picks from the overlying costra and carried to the refinery.

Composition of Caliche.

Both in appearance and composition it varies very much. In colour it may be snow-white, sulphur, lemon, orange, violet, blue, and sometimes brown like raw sugar.

The caliche found in the Pampa de Tamarugal contains generally about 30 to 50 per cent pure nitrate of soda; that in the province of Atacama contains from 25 to 40 per cent. The subsequent refining processes, which consist in crushing it by means of rollers and then dissolving it, need not here be described. It may be sufficient to mention that the process used is that known as systematic lixiviation, and is analogous to the method introduced by Shanks in the manufacture of soda. The chief impurity in the raw material is common salt: gypsum, sulphates of potassium, sodium, and magnesium, along with insoluble matters, are the other impurities. The manufacture of iodine, which, as has been already noticed, is found in the nitrate-beds, is also carried on at these oficinas.

Extent of the Nitrate Deposits.

The question of the extent of the nitrate of soda deposits is naturally one of very great interest, especially from the agricultural point of view. M. Charles Legrange, a French writer, estimated a few years ago that they still contained about 100,000,000 tons of pure nitrate of soda. Opinions on this point differ very considerably, and it seems wellnigh impossible to arrive at any very accurate estimate.

The number of years they will last will depend, of course, on the amount of annual exportation. This, at present, falls little short of 1,000,000 tons. If this amount is maintained, they should last, according to experts, some twenty or thirty years at least. A consideration which has an important influence on this question, is the price obtained for the article. If this should be increased, it may be possible to treat the larger quantities of the inferior raw material (which at present prices are allowed to accumulate) at a profit. Undoubtedly this is what will ultimately take place, when the richer quality of the caliche has been exhausted.

Composition and Properties of Nitrate of Soda.

As has already been pointed out, commercial nitrate of soda contains about 95 per cent of pure nitrate of soda, or about 15-1/2 per cent of nitrogen, which, if calculated as ammonia, would equal 19 per cent. It is, next to sulphate of ammonia (which contains 24-1/2 per cent of ammonia), the most concentrated nitrogenous manure, and further, contains its nitrogen in the form most readily available for the plant's use. Its most characteristic property is its great solubility, and consequent speedy diffusion in the soil, and the inability of the soil-particles to fix its nitrogen. In the latter respect it differs very considerably from other forms of nitrogen. Ammonia salts, though practically quite as soluble, do not diffuse in the soil so rapidly as nitrate of soda does; for the ammonia is more or less tenaciously fixed by the soil-particles, and retained till converted by the process of nitrification into nitrates.

Nitrate of Soda applied as a Top-dressing.

On this account nitrate of soda is chiefly employed—and rightly so—as a top-dressing. The risk of loss by drainage is thus minimised, and the valuable nitrogen finds its rightful destination—viz., in the plant's roots.

Encourages deep Roots.

A special benefit which the diffusibility of nitrate of soda has been held to confer on the plant, is to encourage the growth of deep roots, by inducing the growing plant to send down its roots into the lower layers of the soil after the nitrate of soda. The benefit of deep roots is, of course, very great. They enable the plant to withstand the action of drought, and at the same time increase the area whence the plant may derive its nourishment. Although the value of the manure is practically entirely due to the nitrogen it contains, it has been urged that the soda exercises a beneficial effect on the mechanical properties of the soil, by increasing its power of absorbing moisture, and in also rendering it more compact. This would partly explain how its results in dry seasons are so much better than those obtained from sulphate of ammonia. This mechanical action of nitrate can scarcely be very great when we remember the comparatively small quantity applied. Even in the driest of seasons there will always be sufficient moisture to secure the diffusion of the nitrate of soda, while the risk of loss by drainage will be reduced to a minimum. Much ignorance, as well as prejudice, has existed in the past as to the true nature of the action of nitrate of soda. Nor is this prejudice even yet entirely dispelled.

Is Nitrate an exhausting Manure?

The common charge brought against it is, that it is what has been termed an exhausting manure. This objection, to have any weight, must mean that nitrate of soda produces a crop which takes out of the soil an abnormal quantity of fertilising matter. But, so far as the writer is aware, no scientific evidence has ever been brought forward to support this contention. That the indiscriminate use of a manure may produce a crop in which the stem and leaves are unduly developed at the expense of the grain, or in which the quality of the crop may suffer from too rapid growth, is, of course, a well-known fact. But as this could also be produced by an overdose of soluble phosphoric acid as well as ammonia salts, it is not a property that belongs exclusively to nitrate of soda. Probably nitrate of soda has in the past been often used in this indiscriminate way so as to produce such results. The fault, therefore, lies not in the manure, but in the mode of its application. A few remarks, therefore, on this most important subject may prove serviceable.

Crops for which it is suited.

Opinions will naturally differ as to the crops to which it is profitable to apply nitrate of soda. Its value as a manure for cereals is pretty generally admitted. Its value as a manure for roots is not, however, so universally admitted. Experiments would seem to show that such a crop as the mangold derives just as much benefit as do the cereals; while in Germany practical experience on a very large scale has demonstrated its value as a manure for beetroots. It may be generally recommended as a manure for all crops, except, perhaps, the so-called leguminous crops, such as clover, beans, peas, &c., whose ability to obtain nitrogen for themselves renders the application of expensive artificial nitrogenous manures unadvisable.

An interesting point with regard to nitrate of soda is the curious effect it seems to have on the colour of the leaves of plants. This interesting fact has been strikingly demonstrated at the Rothamsted Experimental Station, in the contrast in the colour of the leaves of different experimental grass-plots, manured with nitrate of soda and sulphate of ammonia respectively—the plots manured with nitrate of soda being distinctly darker in hue, obviously owing to the greater production of chlorophyll or green matter. Such a depth of colour would seem to indicate a more healthy development.

Method of Application.

While opinions, therefore, will naturally differ as to the crops to which nitrate of soda will be most profitably applied, little difference of opinion exists as to the method of its application. The inability of the soil-particles to retain it, the frequency of rain, the costly nature of the manure itself, and its immediate availability as a plant-food, all point to the extreme advisability of using it as a top-dressing. Even when used as a top-dressing, it may be advisable not to apply the entire quantity all at one time. By applying it in instalments, little risk is run that, through inclemency of weather, the manure will be lost. Another point of importance in applying nitrate of soda is to secure uniform distribution. This of course is applicable to all artificial manures, but in a very special degree to nitrate of soda, because of its great value and the comparatively small quantity applied.As the uniform distribution of one cwt. of any material over an acre of soil is by no means an easy task, the mixing of nitrate of soda with some diluent, such as dry loam, is consequently highly advisable. Common salt is often applied along with nitrate of soda. The indirect value of salt as a manure is considerable, and when applied along with nitrate, ensures its more speedy diffusion in the soil, by increasing the soil's capacity for absorbing moisture from the air.

Must be a Sufficiency of other Fertilising Constituents.

A third point of importance in applying nitrate of soda, is to see that the soil is sufficiently supplied with the other plant-foods—phosphates and potash. This is a sine qua non, if the nitrate is to get a fair chance. If it is desired to apply nitrate of soda along with superphosphate of lime, a word of caution is necessary against making the mixture long before it is used. The reason of this is, that a chemical action is apt to ensue, resulting in the loss of the nitric acid in the nitrate of soda. The nature of the soil is another important consideration to be taken into account. In the case of extremely loose and sandy soils, it is scarcely to be recommended as the most suitable form in which to apply nitrogen. If applied to such soils, especial care ought to be taken to minimise risk of loss. No hard-and-fast rules can be laid down as to the quantity in which it ought to be applied. This must be regulated very much by the crop, the nature of the soil, and the quantity of other manures employed. From 1 to 1-1/4 cwt. may be recommended as a suitable quantity for corn crops which are otherwise liberally manured. On strong clay soils this quantity may be judiciously increased up to 2 cwt. Dr Bernard Dyer, who has experimented largely on its use as a manure for mangolds, is of opinion that an application of from 3 to 4 cwt. an acre is likely to prove thoroughly profitable; and the present writer has found in his experiments with turnips that a top-dressing of 1 cwt. amply repaid itself.

Conclusions drawn.

In conclusion, the nature and characteristics of nitrate of soda as a manure may be briefly summed up as follows:—

1. It is a whitish, crystalline salt, extremely soluble, and is quickly diffused in the soil. It should contain 95 per cent of pure nitrate of soda—i.e., 15-1/2 per cent of nitrogen, equal to about 19 per cent of ammonia.

2. Next to sulphate of ammonia, it is the most concentrated nitrogenous manure; the relative quantities of nitrogen these two manures contain being as three is to four.

3. It contains its nitrogen in the most valuable and readily assimilable form—i.e., as nitric acid, the form into which all other forms of nitrogen have first to be converted before they become available for the plant's uses.

4. That, at present market prices, nitrate of soda may be safely affirmed to be the cheapest form of nitrogenous manure.

5. That nitrate of soda, in addition to its direct value as a manure, probably exercises a slight influence on the mechanical properties of the soil, by increasing its compactness and water-absorbing capacities; that it further tends to promote deep roots, and thus to increase the soil area whence the plant may derive its nourishment, at the same time rendering the plant more able to withstand the injurious influence of drought.

6. That a plentiful supply of the other manurial constituents should be present in the soil, if nitrate of soda is to exercise its full value.

7. That it may be profitably applied in the case of nearly all kinds of crops, but that great care should be taken as to the mode of its application. That this should be almost invariably as a top-dressing, and that it should be applied in several doses if possible.

8. That its effects can be regarded as lasting only during the first year after application.

FOOTNOTES:

[202] This substance is also largely known under the name Chili saltpetre, to distinguish it from potassium nitrate or common saltpetre.