Manures and the principles of manuring

by Charles Morton Aikman

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.