THE MINOR MINERALINGREDIENTS OF SOILS; MINERAL FERTILIZERS; MINERALS
INJURIOUS TO AGRICULTURE.
(A.) MINERALS USED AS FERTILIZERS.
Of minerals important in soil-formation, not usually present in large
amounts in rocks, but extensively used in fertilization, the following
require mention:
_Apatite_; phosphate of lime containing more or less of the chlorids
and fluorids of the same metal; the mineral from which the phosphoric
acid of the soil is mostly derived. In the crystallized condition when
perfectly pure it is colorless; but it is mostly of a greenish tint
(hence “asparagus stone”). The pure crystalline mineral rarely occurs
in large masses (as in Canada); but small to minute crystals are found
widely disseminated in many rocks (granites, “basalts” of the Pacific
Northwest), thus passing into the soils formed from these rocks. These
crystals are readily recognized, being regular six-sided prisms with a
flat or obtusely pyramidal termination (distinction from quartz), and
do not effervesce with acids (distinction from calcite). By far the
largest deposits of this mineral occur in connection with carbonate of
lime, in the rock materials known as _phosphorites_. Lime phosphate
being, like the carbonate, soluble in carbonated water, the two
naturally frequently pass into solution, and are subsequently deposited
together. Most limestones contain a small proportion of lime phosphate,
being, as already stated, formed from the shells and the framework of
animal organisms usually containing also phosphates. But the content of
phosphates in limestones is not readily apparent to the eye, and the
richest deposits, save such as contain animal bones, have long passed
unsuspected as to their being anything else but limestone. Systematic
search has now revealed the presence of phosphate rock in numerous
localities, chiefly where limestone formations occur. In the United
States, in South Carolina, Florida, Alabama, Tennessee, Kentucky,
Nevada; in South America, on Curaçoa island, Venezuela; in the Antilles
on Sombrero, St. Martins and Navassa islands. In Africa, in Algiers and
Tunisia; in Europe, in Spain (Estremadura, one of the first deposits
known), France, Belgium and the adjacent parts of Germany; in Bohemia
and Galicia in Austria; and very extendedly in European Russia. Many
islands of Oceanica supply phosphorites derived from the decomposition
of bird guano by the coral limestone.
Unfortunately the percentage of phosphate in a large
proportion of these materials is not sufficiently high to
make their conversion into water-soluble superphosphate
economically possible at the present time; since all the
calcic carbonate present must also be converted into
comparatively worthless sulphate (gypsum) by the use of
sulfuric acid; and as yet no practicable method for avoiding
this difficulty has been found.
“_Thomas Slag._”—Probably the nearest approach to such
a method is indicated by the fact that a compound containing
four instead of three molecules of lime to one of P₂O₅, such
as is contained in the “Thomas slag” of the basic process of
steel manufacture, is nearly or in some cases (“sour” soils)
_quite_ as effective for the nutrition of plants as the
water-soluble superphosphate. This discovery has rendered
available for agricultural use the phosphoric acid contained
in the enormous deposits of limonite iron ore known as bog
ore, which contains a large proportion of ferric phosphate
and from that cause has until lately been excluded from the
manufacture of wrought iron and steel. It is reasonable
to hope that by some analogous process the low-grade
phosphorites, such as those of Nevada and the plains of
Russia, will also in the course of time become available
for agricultural use. Extremely fine grinding and washing
(producing “floats”) has been resorted to for the purpose of
rendering the raw phosphorites effective in fertilization.
But while this is successful on some soils, on others the
“floats” remain almost inert; so that this method has found
only limited acceptance.
_Animal bones_, which consist of from 24 to 30% of animal substance and
70 to 76% of “bone earth,” (or when fossil are free from the former),
are largely used for the manufacture of superphosphate. The bone-earth
consists in the main of tri-calcic phosphate with from one to two per
cent of calcium fluorid (much as in natural apatite), a small amount of
magnesic phosphate, and about 4 to 6% of calcic carbonate. Bone meal
can therefore supply to plants both phosphoric acid and nitrogen, and
the presence of the latter has been largely the cause of a material
overestimate of its efficacy as a fertilizer in the past. Wagner’s and
Maerker’s experiments have shown that at least in sandy soils poor in
humus, it cannot be considered an adequate source of phosphoric acid
for annual crops, and that in these soils its immediate effects are
almost wholly due to its nitrogen-content. The slow availability of
the phosphoric acid renders it unprofitable as a source of the latter,
outside of the heavier lands with abundance of humus; in “sour” lands
(notably on meadows) bone meal produces its best results. In soils
naturally calcareous, or in such as have received heavy dressings of
lime either as carbonate or in the caustic condition, the manurial
effects of bone meal are seriously diminished. Nagaoka (Bull. Coll.
Agr. Tokyo, Vol. 6, No. 3) shows that the crop of rice fertilized
with bone meal was reduced to less than half when limed, and that the
phosphoric acid taken up by the crop was reduced to one-sixth. In any
case it is most important that bone meal should be as finely ground as
possible, as in the case of the phosphorites; and this can best be done
when it has first been freed from fats by boiling with water, and then
steamed under pressure. It can then also be most readily converted into
superphosphate.
The phosphate minerals and the fertilizers manufactured therefrom are
of primary importance to agriculture. The phosphoric-acid content of
soils is mostly very small, and only a fraction of it is usually in
an immediately available form. Hence for permanent productiveness,
and especially for intensive farming or gardening, a cheap supply of
phosphate fertilizers is of first importance in all soils and climates.
Other phosphate minerals occur frequently, but as a rule only in small
amounts, in connection with the ores of most metals. The only ones
of these of interest to agriculture are _Vivianite_ and _Dufrenite_,
the phosphates respectively of the protoxid and peroxid of iron. The
former occurs in mineral deposits as small blue crystals, or more
frequently as blue earthy masses or streaks, in the substrata of rich
alluvial ground (Louisiana, California). Dufrenite sometimes results
directly from the oxidation of the protoxid mineral, which then turns
greenish and finally brown. Unfortunately these minerals, rich as
they are in phosphoric acid, cannot readily be utilized as sources of
phosphate fertilizers, because of the difficulty of getting rid of the
iron. Their occurrence usually suggests the presence of abundance of
phosphoric acid in the soil. But that which is actually combined with
the iron oxids is practically unavailable to plants; especially so in
the case of the peroxid compound, the formation of which is a common
source of loss of phosphoric acid when soils rich in iron are submerged
for any length of time; a point which is discussed below (chapt. 13).
Among the iron phosphate minerals, may also be mentioned “bog ore,”
which results from the reductive maceration of swamped ferruginous
soils, and accumulates in the subsoils and in the bottom of swamps or
moors, forming “moorbedpan”; a dark brown, rather soft mass, which is
sometimes used as an iron ore, especially since the invention of the
“basic process” of iron smelting, one of the products of which is the
phosphate or Thomas slag. (See above).
_Nitrate of Soda or Chile saltpeter._—This mineral being (like all
nitrates) easily soluble in water, can only occur in regions nearly
or quite destitute of rainfall. Such is the case in the Plateau of
Tarapacà in Northern Chile, where it occurs in large quantities; it
is likewise found, but to much smaller extent, in Nevada, southern
California, Egypt and India. By far its most extended occurrence is
that in Chile, where, together with common salt, it fills cavities and
crevices in a gravelly clay that forms the surface of a plateau from
three to six thousand feet above the sea. It is never pure, but always
mingled with a large proportion (up to 50% and over) of common salt;
also some Glauber’s salt (sulfate of soda) and some sodic perchlorate
and iodid; hence it forms an important commercial source of iodine.
The mixed mineral mass, called “Caliche,” when taken out
of the ground is dissolved in water; and the solution
boiled down, during which process the common salt is first
deposited and is raked out of the pans; the nitrate is
afterward farther purified by crystallization. As brought
into commerce for agricultural purposes it constitutes a
moist gray saline mass, somewhat resembling common salt,
of which substance it usually contains a few per cent;
occasionally also a small amount of sodic perchlorate (which
acts injuriously on vegetation). Aside from its use as a
fertilizer, Chile saltpeter serves for the manufacture
of nitric acid; and either directly, or after previous
transformation into potassic nitrate, for that of gunpowder.
The Chilean locality is the only one from which the
commercial article is derived; the deposits elsewhere are
too limited in extent to compete commercially with the South
American product. Caliche ranging as high as 80% of nitrate
of soda has been sent to the writer from the Colorado
Desert in Southern California, but the exact locality of
occurrence has not been divulged. Extended areas of clay
hills impregnated with nitrates exist in the Death Valley
region of California, but in the absence or extreme scarcity
of water in that region, it is doubtful whether these
impregnations can be made practically available. Another
locality is that near White Plains, Nevada, where Caliche
averaging about 50% purity is found in cavities and crevices
of a reddish volcanic rock. The rainfall in this region is
so slight that the greater part of the dust or sand blown
about by the wind consists of Glauber’s salt. Here also,
as in Chile, the niter deposits appear to be restricted to
within a short distance from the surface, and the total
amount thus far observed appears to be insufficient to
encourage large-scale exploitation.
_Origin of Nitrate Deposits._—The probable origin
of these niter deposits has given rise to a great deal of
discussion, and a wide difference of opinion exists as
to the source from which the nitrogen may reasonably be
supposed to have been derived. According to the present
state of our knowledge, it must be presumed that its
sources have been organic, and that the niter has been
produced by the activity of the same bacteria which now
produce nitrates in our soils, rendering the nitrogen of
humus available to plants. But it is by no means clear what
that organic material could have been; for at the present
time the plateau of Tarapacà is almost wholly destitute of
vegetation, if not of animal life. The latest and apparently
most reasonable suggestion is that of _Kuntze_, who
calls attention to the fact that the vicuñas and llamas
which are at home in this portion of the Andes, and are
known to have roamed over that region in countless herds,
have the curious habit of always depositing their manure in
one and the same place whenever at liberty. Each herd
of these animals has its definite dunging place at some
convenient point. That such herds have existed in the region
from time immemorial is obvious from historical as well as
collateral evidence; and as their manure accumulated, its
nitrification would progress rapidly under the prevailing
arid conditions. The common salt would naturally be derived
from the urine and excrements, and the alkaline salts
which exist throughout this region as the products of soil
decomposition, would be quite sufficient to account for
the alkaline bases in the caliche. On the other hand, the
presence of iodine points to seaweeds as the organic source.
_Intensity of Nitrification in Arid Climates._—Of the efficacy of
nitrification under arid conditions abundant evidence may be found
within the State of California. In the alkali lands of southern
California the nitrates of soda, lime and magnesia are almost
universally present; they form at times as much as one-fifth and even
more of the entire mass of alkali salts, and in one case the total
amount in the soil has been found to reach two tons per acre, with
an average of twelve hundred pounds over ten acres. In the plains of
the San Joaquin Valley, spots strongly impregnated with niter are
found, especially under the shadows of isolated oak trees, where the
cattle have been in the habit of congregating for a long time; a case
quite analogous to that supposed by _Kuntze_ to exist in the Chilean
locality. Of course it is only in arid climates that the accumulation
of nitrates can usually occur; for in the region of summer rains the
nitrates formed during the warm season will inevitably be washed into
the subdrainage, unless restrained by absorption by the roots of
vegetation. The heavy losses occasionally occurring from this cause in
the course of a rainy winter on summer-fallowed land have been amply
demonstrated by many investigations.
POTASH MINERALS.—By far the most abundant occurrence of potash in the
earth’s crust is that in silicates and notably in orthoclase or potash
feldspar, which contributes so largely to soil-formation. But in the
absence of any economically successful artificial method for producing
potash compounds from feldspars on a commercial scale, almost the
entire supply of potash salts was, until a comparatively late period,
derived from plant ashes, viz., the “potashes” of commerce. At the
same time, almost the entire demand for alkalies for industrial uses
bore upon the same product, until the invention, toward the end of the
last century, of _LeBlanc’s_ process for the manufacture of soda from
common salt; for until that time, soda in the various forms in which
it was imported from the Orient or prepared from seaweed ashes, was a
comparatively costly product. LeBlanc’s invention was most timely in
that it very quickly diminished materially the production of potashes
which, in view of the increased demand for alkalies for industrial
uses, seriously threatened the depletion of agricultural lands, and of
woodlands as well, of one of its most essential ingredients. Yet as
there are many industrial uses in which soda cannot replace potash,
the manufacture of potashes continued to a greater or less extent, as
no other available source except the ashes of land plants, was then
known. The production of potassic chlorid from the mother-waters of sea
salt in the spontaneous evaporation of sea water for the manufacture
of common salt, was on too small a scale to influence materially the
manufacture of potashes.
_Discovery of Stassfurt Salts._—The depletion of potash had become so
serious a matter in the agricultural lands of Europe, that for a time
much research was bestowed, and prizes offered for an economical method
of producing potash salts from feldspar, on a commercial scale. But
the problem had not been satisfactory solved when, in the year 1860,
attention was called to the fact that the saline deposits overlying
certain large rock-salt beds that had been developed by borings near
Stassfurt in Prussia, contained so large a proportion of potash
salts, as to render their purification and conversion into fairly
pure sulphate and chlorid technically feasible. The impulse having
been given, the potash industry developed rapidly in that region as
well as in the adjacent portions of Saxony, where the same formation
underlies; the production of “Stassfurt Salts” rapidly assumed a
greater development than that of the rock-salt which had originally
prompted the enterprise, and numerous additional boreholes demonstrated
an unexpectedly wide extension of the same beds. At the present time,
in consequence of such development, the manufacture of potashes from
plant ash has almost ceased, outside of Canada and Hungary; and the
production of potash salts in the Stassfurt region now supplies the
demand of the entire world, both for industrial and agricultural
purposes.
The cheapening of potash as a fertilizer has rendered possible the
profitable cultivation of large areas of land which were naturally
too poor in that substance for ordinary cultures; and has likewise
rendered possible the restoration to general culture of lands that
had ceased to produce adequately, on account of the depletion caused
by long-continued cropping. It has likewise served to intensify
agricultural production wherever desired; and between this supply and
that of phosphoric acid from the phosphorites (see above), and the
discovery of the nitrogen-absorbing power of leguminous plants, which
can be used for green-manuring, farmers have been enabled to dispense,
in many regions, with the production and use of stable-manure, which
until then had been considered an indispensable adjunct to agriculture
everywhere. Even within the last fifty years it was proclaimed by high
authority in Germany that stable-manure constituted, as it were, the
farmer’s raw material, from which he manufactured the various products
of the field through the intervention of the plant-producing power of
the soil.
_Origin of the Potash Deposits._—The manner in which
this accumulation of potash salts has been formed deserves
explanation. It is abundantly evident that nearly all
deposits of rock-salt thus far known have been formed by the
evaporation of sea-water at times when bays or arms of the
sea were cut off from open communication with the ocean.
The composition of sea-water has already been given and
discussed (chap. 2, p. 26); and by the slow evaporation of
sea-water on a small scale we can quite successfully imitate
the phenomena observed in natural rock-salt deposits. When
sea-water is heated a slight deposit of lime carbonate
(usually containing a little ferric oxid and silica) is soon
formed; and a corresponding thin deposit of ferruginous
limestone is commonly found at the base of rock-salt-bearing
deposits. Next above this we almost invariably find a
deposit of gypsum, sometimes of great thickness; in the
artificial evaporation of sea-water the same thing occurs
so soon as the brine has reached a certain degree of
concentration. It constitutes the major portion of the
“panstone” of salt-boilers. Next above follows a deposit of
rock-salt, at base somewhat mixed with gypsum; its thickness
varies greatly according to circumstances. Above it lie the
potash salts.
In the manufacture of sea-salt by evaporation in shore
lagoons or “saltpans,” the solution remaining after the salt
has been deposited (known as “mother-waters,” or “bittern”),
of course remains on the surface of the salt unless allowed
to drain off, as is done in the process of manufacture. When
not drained off, the water gradually evaporates, and there
remains a saline crust of a composition exactly resembling
that of the upper layers at Stassfurt, containing a large
proportion of potash salts.
If it be asked why the Stassfurt salts are not found
overlying every rock-salt deposit in the world, the answer
is that in a great many cases the concentrated mother-waters
have had an opportunity to flow off from the surface of the
rock-salt by the action of tides, the inflow of fresh water
from the land or from other causes. Their presence therefore
depends upon the fulfilment of accidental conditions not
nearly always realized in the natural evaporation of
sea-water, but which happened to occur on a very large scale
in that portion of the North-European continent.
_Nature of the Salts._—The potash is present in
the Stassfurt salts in the form of complex sulfates and
chlorids containing, besides, sodium, calcium and magnesium
in various proportions and modes of combination. The most
abundant of the potassic chlorid minerals is carnallite, a
hydrous chlorid of potassium and magnesium. The chlorids
characterize chiefly the upper portions of the deposit, the
sulfates the lower.
_Kainit._—Of the products derived from the Stassfurt salt industry
for agricultural use, the two requiring special consideration are
“kainit,” a natural mixture of the several chlorid minerals in varying
proportions; and “high-grade sulfate.” Being a natural product,
“kainit” is the cheapest source of potash available to the farmer;
but on account of its variability in composition it must be sold
and purchased on guaranteed assay. On account of its large content
of chlorin it is not desirable in the production of certain crops,
especially in the arid region, where alkali soils, and even those not
visibly alkaline, often contain already large amounts of chlorin.
Moreover, kainit usually contains a considerable proportion of
common salt. For the arid region therefore the sulfate is generally
preferable, although it is somewhat higher in price for the same
amount of potash. The potash content of commercial kainit (calculated
as K₂O) ranges from 16 to 35%, while the sulphate frequently ranges
from 80 up to 95% of the pure sulfate; thus costing materially less in
freight charges than the lower-grade kainit. Its potash content ranges
from 43 to over 50% of K₂O.
_Potash Salts in Alkali Soils._—The sulfates and chlorids of potassium,
however, occur not only in connection with rock-salt deposits, but are
also found in the alkali soils of the arid region. They are, in fact,
never absent where such salts occur at all, and their percentage in the
total of salts ranges all the way from about 4 to as much as 20% of
potash sulphate. In numerous cases it has been found that the content
of this salt to the depth of four feet amounts to from 1200 to 1500
pounds per acre. In such lands, of course, additional fertilization
with potash salts is totally uncalled for, the more as such soils
invariably contain, besides the water-soluble potash, an unusually
large percentage of the same in the form of easily decomposable
silicates, or zeolites.
_Farmyard or Stable Manure._—In connection with the subject of mineral
fertilizers, it will be proper to discuss briefly the uses and special
merits of stable manure, composts, etc. Up to within the last century,
these were practically the only fertilizers known and used, and the
exclusive use of this manure might have continued indefinitely but for
the discovery that as time progressed, stable manure and with it grain
crops, for the production of which it was necessary, became less and
less in amount, so as to threaten bread famines. The cause of this
diminution was, of course, the incompleteness of the return of the
soil-ingredients taken off by the crops, when these were exported to
feed the cities or foreign countries. Thus the attention of chemists,
and notably that of Liebig, was attracted to the solution of the
problem of keeping up production even with an insufficient supply of
stable manure; and the discovery of the use of mineral fertilizers was
the result of their activity.
The chemical composition of stable manure does not, alone, suffice to
explain its remarkable efficacy and the difficulty of replacing it
by any other material. The composition of manure of course differs
not only with different animals but also with the different feeds
consumed by them; but the average composition of farmyard manure is
approximately given thus by Wolff and others:
ANALYSES OF VARIOUS FARMYARD MANURES.
==========================================================
| 1. | 2. | 3. | 4. | 5.
| | | | |
Water | 71.00 | 75.00 | 79.00 | 79.95 | 72.33
Dry Matter | 29.00 | 25.00 | 21.00 | 20.05 | 27.67
Ash ingredients | 4.40 | 5.80 | 6.50 | | 5.87
Potash | 0.52 | 0.63 | 0.50 | 0.84[23]| 0.69
Lime | 0.57 | 0.70 | 0.88 | | 0.85
Magnesia | 0.14 | 0.18 | 0.18 | | 0.14
Phosphoric acid | 0.21 | 0.26 | 0.30 | 0.40 | 0.30
Ammonia | | | | | 0.02
Total Nitrogen | 0.45 | 0.50 | 0.58 | 0.78 | 0.46
----------------+-------+-------+-------+----------+-------
1. Average composition of fresh farm manure (Wolff).
2. Average composition of moderately rotted farm manure (Wolff).
3. Average composition of very thoroughly rotted farm manure
(Wolff).
4. Mixed cow and horse manure from a bed two feet thick, accumulated
during the winter in a large covered yard, and packed solid by
the tramping of cattle (The analysis by F. E. Furry).
5. “Box Manure,” consisting of mixed manure of bullocks, horses, and
pigs (Way, Royal Agric. Soc. Journ., 1850, II., 769).
[23] And soda.
It is thus seen that the percentage of the important plant-foods in
stable manure are minute when compared with those commonly found in
“commercial” fertilizers. Nor are they so much more available for plant
absorption than the latter; a very large proportion is not utilized
at all the first year, and unless the amount applied is very large it
hardly carries the supply needed for the usual crops.
It is now well understood that its efficacy is largely due to the
important physical effects it produces in the soil. It helps directly
to render heavy clay soils more loose and readily tillable. If well
“rotted” or cured it also serves to render sandy, leachy soils more
retentive of moisture; and the humus formed in its progressive decay
imparts to all soils the highly important qualities discussed later on
(chapt. 8). More than this, the later researches have shown that stable
manure acts perhaps most immediately upon the bacterial activity in the
soil, greatly increasing it not only directly by the vast numbers of
these organisms it brings with it, but also in supplying appropriate
food for those normally existing in the soil (see chapt. 9). In
so doing it serves indirectly to render the soil ingredients more
available, and to impart to the soil the loose condition required in a
good seed-bed—a “tilth” which cannot be brought about by the operations
of tillage alone.
The only possible substitute for the use of stable manure is found
in green-manuring with leguminous crops conjointly with the use of
commercial or mineral fertilizers. Unless this is done the use of the
latter, alone, ultimately leads to a depletion of humus substances,
which renders the acquisition of proper tilth by the seed-bed
impossible, and causes a compacting of the surface soil which no
tillage can remedy.
_Proper method of using stable manure in humid and arid climates._—In
the humid region it is a common practice to spread the stable manure
on the surface of the fields and leave it there without any special
operation to put it into the soil; trusting to the rains, earthworms
and subsequent tillage for its being brought into adequate contact
with the roots; it is rarely plowed in. In the arid region this
mode of using it is impracticable; it would remain on the surface
indefinitely without advancing in its decay because of the dryness, and
unless plowed in very deep the ordinary, strawy manure would ruin the
seed-bed by rendering it too pervious to the dry air, thus preventing
germination. Much of this valuable material has therefore been, and to
some extent is still being burnt, thus causing a severe depletion of
the land, both of humus and of mineral plant-food. The best way to deal
with stable manure in the arid regions is to thoroughly rot or cure it
before putting it on the land, and then plowing it in. To do this of
course it must be put in piles and wetted regularly; a procedure which
at the high prices of labor is thought to be too expensive, but which
in the end would be found eminently profitable, unless green-manuring
is regularly done. The very small proportion of humus generally present
in arid soils renders this precaution indispensable, if production and
proper tilth is to be maintained. The saving of stable manure and of
all composting material, even if less needful as a means of supplying
plant-food in the rich soils of the arid regions, is fully as essential
in order to maintain the humus supply.
(B.) MINERALS UNESSENTIAL OR INJURIOUS TO SOILS.
The minerals heretofore mentioned contribute to soil formation either
one or several ingredients, important to plant growth either by
their mechanical or chemical action. It remains to consider some not
intrinsically desirable, but frequently present in certain soils,
which should be known to the farmer in order that he may be enabled to
counteract or remove their injurious effects. Leaving aside such as are
of only casual or rare occurrence, the following may be mentioned as
among those which not unfrequently affect soils desirable for culture
to such extent as to make them unavailable for general farming purposes:
_Iron Pyrite_; sulphid of iron containing two molecules of sulphur
to one of iron; a mineral exceedingly common in deposits of metallic
ores, and whose deceptive gold-like color has caused it to be mistaken
for gold so often as to cause it to be designated as “fool’s gold”
among miners. While it frequently does contain some gold and is often
associated with valuable ores, it is practically valueless when
occurring outside of mineral veins, in rock-masses; and more especially
in sedimentary rocks, such as sandstones, limestones, shales and clays.
When present in soils it sometimes becomes a source of trouble to the
farmer, because in contact with air it is soon transformed into ferrous
sulfate or copperas, which, like the carbonate referred to above, is
injurious to plants. Sometimes indeed iron pyrite is actually _formed_
in badly-drained soils alongside of the carbonate of iron, when much
sulfate (such as gypsum) is present; and then its injurious effects
subside more slowly than do those of the carbonate (see above, p. 46).
_Recognition of Iron pyrite._—The mineral is easily
recognized by its golden or brass-yellow tint; the latter
color being the one most commonly shown in the “sulphur
balls” occurring in marls or soft limestones. A very easy
test is to pulverize it and then heat it on a shovel over
a fire, when it will soon itself take fire, burning with a
blue sulphur flame, and upon more complete roasting, leaving
behind a red powder, viz., “Venetian red” or red ochre;
that is, ferric oxid. In clays it commonly occurs in large,
well-defined cubes, which do not readily form copperas but
rather become covered with a crust of limonite or brown iron
ore.
When a subsoil is found to contain pyrite, or when “sulfur balls”
have been accidentally introduced with dressings of marl, the remedy
is thorough and persistent aeration of the material. In the case of
marls nothing more need be done; but in that of ill-drained subsoils
it is best to add lime in moderate dressings, to accelerate the
transformation into ferric hydrate or iron rust, and gypsum; whereby
the copperas becomes not only innocuous but adds two beneficial
ingredients to the soil. The same policy will render available manure
or other materials which have been disinfected by means of solution of
copperas.
_Halite_ (rock-salt), or common salt, has already been mentioned as
to its occurrence in connection with the Stassfurt potash salts (see
above, page 71); but as rock-salt it rarely exerts any injurious
influence upon lands. It is, however, a common ingredient of seashore
lands, and is also present to a certain extent in the alkali lands
of the arid countries. While it is true that occasionally small
quantities of common salt are used as an ingredient in fertilization,
its usefulness in that direction is exceedingly subordinate; and it
is far more generally to be considered as an injurious ingredient of
all cultivatable soils whenever present to a larger extent than a few
hundredths of one per cent It is usually considered that one-fourth
of one per cent of common salt renders lands unfit for most culture
plants. Only a few, such as asparagus, the beet, the saltbushes and
some others, succeed when it is present in this or in larger amounts.
In the case of sea water it is usually accompanied by a still more
injurious ingredient, magnesic chlorid or bittern; which is detrimental
to plant growth in much smaller quantities than the common salt itself.
_Recognition of Common Salt._—The presence of common
salt may, as a rule, be detected by the taste, well-known
to every one; when this taste is very intense or somewhat
bitterish, it indicates the presence of bittern. The
presence of salt, however, is easily verified without the
use of chemical reagents, by slowly evaporating some of
the clear water leached from the soil in a clean silver
spoon. If the last few drops are allowed to evaporate
spontaneously, it will be easy to distinguish, even with
the unaided eye, the square, cubical crystals, sometimes
combined into cross-shape, which are characteristic of
common salt. It is always an unwelcome addition to the land,
and as its action cannot be neutralized in any way, it can
be gotten rid of only by leaching-out. This process is
usually accomplished in seashore lands by the action of
rain, or by the overflow of fresh-water streams, after the
tide has been excluded by means of drains provided with
check-valves to prevent the inflow of tidewater; or else by
underdrainage, and flooding when possible.
_Mirabilite_, (Glauber’s salt) or sulfate of soda, exists not
unfrequently in the soils of the arid region and sometimes encrusts
extended areas of lowlands during the dry season. When present in the
soil it will commonly be seen blooming out on the surface after a rain,
in light, feathery, needle-shaped crystals, sometimes to such an extent
that it can be collected by the handful. Subsequently, when wafted
by the wind, it is reduced to a fine white dust, which constitutes a
goodly proportion and sometimes the entire mass of the “alkali dust”
that is so annoying on the plains of Nevada, and in the desert regions
generally, during the hot summer. Near White Plains, Nevada, it forms
a thick layer of “_white sand_,” in which the foot sinks deeply, and
which is carried about by the wind with great ease.
Glauber’s salt is never a desirable soil-ingredient. It is largely
produced as a by-product in several industries, but cannot be utilized
for agricultural purposes to any extent. It is, however, much less
injurious to plant growth than common salt; according to experience
in California it may be considered about three times less so. It
constitutes the major portion of what is commonly known as “white
alkali,” which is well known to be much less injurious to crops than
the “black” kind, which contains carbonate of soda.
_Trona and Urao_ are natural forms of carbonate of soda or salsoda.
Like Glauber’s salt, it commonly occurs as a surface efflorescence or
crust in dry or desert regions; either from the evaporation of standing
water, as in the case of the soda lakes of Nevada, Hungary and Egypt,
or as an efflorescence on the surface of the soil, as in the western
United States, Mexico (“urao”), North Africa (“trona”), and at many
points in the Old Continent. In the United States it is commonly known
as “black alkali,” because of the black spots formed on the surface by
evaporation; practically the same name (“kara”) is given it in Arabia
and Asia Minor, whence impure soda has long been imported into Europe;
while in north India it forms part of the “reh” salts that incrust
large areas (usar lands) in the Indo-Gangetic plain.
The
natural mineral always contains an excess of carbonic acid over
the “normal” salt, nearly in the proportion of four parts of
carbonic dioxid to three of soda; it is sometimes designated as
sesqui-carbonate. In hot sunshine it may lose most of this excess
for a time; while within the soil itself it may, in presence of
abundant carbonic acid, become temporarily converted wholly into
hydrocarbonate or “bicarbonate,” which is less corrosive than the
monocarbonate or common salsoda.
_Injury caused in soils._—Like common and Glauber’s salt, carbonate
of soda is always an unwelcome soil ingredient; more so, in fact,
than either of the other two, since less than a tenth of one per
cent is sufficient to render certain soils wholly untillable, by the
deflocculation or puddling of the clay; at the same time rendering it
impervious to water. It is by far the most injurious ingredient that
ordinarily occurs in otherwise good, arable soils; for in addition to
the physical effect just mentioned, it dissolves the humus-substance
of the soil, forming an inky-black solution which, especially when
evaporating on the surface and forming black spots, has given rise to
the popular name of “black alkali.” As will be more fully explained
hereafter, wherever such is the case, the first step necessary toward
reclamation is the transformation of the carbonate of soda, at least in
part, into the relatively innocuous sulfate, by means of gypsum in the
presence of water; while carbonate of lime remains in the soil.
In its direct action on the plants themselves, soda is also most
injurious; as when accumulated to any extent near the surface by
evaporation it will corrode the root-crown or stem, and sometimes
completely girdle the same, destroying the bark. Farther details on
this subject are given in chapter 22.
_Epsomite_, or Epsom salt, or sulfate of magnesia, is another one
of the water-soluble minerals frequently found efflorescent on the
surface of the ground; more commonly in saline seashore lands than
in the alkali region proper, although it is rather common in the
northeastern portion of the arid region of the United States. Whether
on the soil surface or in the crevices of rocks, its needle-shaped,
feathery crystals greatly resemble those of Glauber’s salt, but are
readily distinguished by the more intensely bitter taste. Epsom salt
is frequently the last remnant of sea-salts left in the soil after
reclamation. Though probably somewhat more injurious to plant growth
than Glauber’s salt, the mineral Kieserite, one of the Stassfurt
salts and consisting essentially of Epsom salt, is sometimes used as
an application to calcareous lands instead of gypsum, and with good
results. Yet gypsum is usually the safer, and equally effective.
_Borax_ (bi-borate of soda) occurs much more rarely than the salts just
described; most frequently in certain portions of California, forming
part of the “alkali” in the soil. It is injurious to plant growth, but
is as readily dealt with as is the carbonate of soda, by dressings of
gypsum, whereby inert borate of lime is produced.
It is hardly necessary to say that saline waters containing any of
the above salts in notable amounts must be used for irrigation very
cautiously. The measures to be observed in this respect will be
discussed later.
PART SECOND.
PHYSICS OF SOILS.