NITRATE OF SODA.
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 deg. and 25 deg. 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 Noellner. 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.
[203] See Appendix, p. 351.
[204] We may remind our readers that these nitrate deposits were largely
the cause of the late war between Chili and Peru, which resulted in the
cession to Chili by Peru of the province of Tarapaca, where the most
important deposits are situated.
[205] The other nitrate deposits are found in the provinces of
Antofagasta and Atacama, and a certain amount of the refined article is
exported from these places. The amount, however, is inconsiderable as
compared with that which comes from the province of Tarapaca.
[206] See his elaborate article on the Geology of Bolivia and Peru,
published in the 'Quarterly Journal of the Geological Society' for
November 1860.
[207] The source of the boracic acid is probably volcanic.
[208] A friend of the present writer, who has visited this part of the
west coast of South America, informs him that at one point of the coast
at Mejillones (in Bolivia) he could trace the remains of no fewer than
twelve distinct sea-beaches, situated at different distances from the
sea, and rising to an altitude of 2500 feet.
[209] In this change, lime derived from the sea-shells would play an
important part. Modern researches have shown, as we have already said in
a previous chapter, that, in the conversion of organic nitrogen into
nitrates, the presence of carbonate of lime is a necessary condition.
[210] The Gulf weed is an instance in point. Huge masses of floating
sea-weed are sometimes found, 500 to 600 miles in length, forming the
so-called Saragossa Sea.
[211] A difficulty which has not been referred to is the belief
entertained by geologists that "there has been a change of climate in
Northern Chili, and that there must have been more rain there formerly
than there is at present. Traces of human habitations are found high up
in the Cordilleras to-day. Cobs of Indian corn, axes and knives of
copper tempered to exceeding sharpness, arrow-heads of agate, even
pieces of cloth, are dug up in arid plains now without any trace of
water for many leagues in or around them" (Russell, 'The Nitrate-Fields
of Chili,' p. 290).
APPENDIX TO CHAPTER IX.
NITRATE OF SODA.
_Total Shipments from South America, 1830-1892._
Year. Tons. | Year. Tons. | Year. Tons.
1830 800 | 1870 131,400 | 1886 437,500
1835 6,200 | 1875 321,000 | 1887 680,600
1840 10,100 | 1880 217,300 | 1888 745,700
1845 16,800 | 1881 344,600 | 1889 930,000
1850 22,800 | 1882 477,800 | 1890 1,030,000
1855 41,800 | 1883 572,400 | 1891 790,000
1860 55,200 | 1884 540,900 | 1892 790,000
1865 109,000 | 1885 423,100 |
The following tables exhibit the total imports into Europe, and into the
United Kingdom from the years 1873-92:--
NITRATE OF SODA, 1873-1892.
_Imports into Europe._ _Imports into United Kingdom._
Year. Tons. | Year. Tons.
1873 225,000 | 1873 124,000
1874 230,000 | 1874 108,200
1875 280,000 | 1875 164,900
1876 300,000 | 1876 166,800
1877 208,000 | 1877 69,600
1878 250,000 | 1878 104,400
1879 205,000 | 1879 55,300
1880 140,000 | 1880 48,300
1881 230,000 | 1881 54,800
1882 335,000 | 1882 96,000
1883 440,000 | 1883 103,700
1884 505,000 | 1884 103,700
1885 380,000 | 1885 109,400
1886 330,000 | 1886 75,100
1887 440,000 | 1887 83,100
1888 640,000 | 1888 103,100
1889 760,000 | 1889 120,000
1890 784,000 | 1890 114,000
1891 851,000 | 1891 121,000
1892 795,000 | 1892 115,000