THE CHEMICAL PROCESSES OF SOIL FORMATION.
_Chemical Disintegration, or Decomposition._
It may be said that in general, the physical agencies of disintegration
are most intensely active in the dry or arid regions of the globe,
while chemical processes of decomposition are most active in humid
climates.
The chemical decomposition of rocks is primarily due to the action
of the atmosphere, the average composition of which may be stated as
follows:
================+==============================+==================
| VOLUME PER CENT. | WEIGHT PER CENT.
----------------+------------------------------+------------------
Nitrogen | 78.00 | 75.55
Oxygen | 21.00 | 23.22
Carbonic dioxid | .03-.04 | .045-.060
Ammonia | 1 to 4 millionths |
Water vapor | Variable; 48 to 83 grams per |
| cubic meter, when saturated |
| between O° and 50°C. |
----------------+------------------------------+------------------
In addition to the above, air contains minute amounts of the very
indifferent and therefore practically negligible elements, argon,
krypton, neon, xenon and helium, the aggregate amount of which in air
is somewhat less than one per cent, of which the greater part is argon.
So far as known these elements take no part whatever in vegetable or
animal life, and possess no known chemical action or affinity.
The primary active agents in effecting chemical changes in rocks by
which soils are formed, are water, carbonic acid,[8] and oxygen; all
therefore ingredients of the atmosphere. Hence the chemical changes so
brought about are in the most general sense comprehended within the
term weathering, as applied to rocks; while the corresponding but more
complex action within the soil itself is usually termed fallowing.
[8] Owing to the universal presence of water (H₂O) in air as well as in
soils, it is usual and convenient to speak of carbonic dioxid (CO₂) gas
when so occurring as carbonic acid (H₂CO₃), of which it produces the
effects (CO₂ + H₂O = H₂CO₃).
_Effects of Water._—Since but few substances, particularly among
those forming rocks, are totally insoluble even in pure water,[9] and
some (such as gypsum) may be considered easily soluble in the same,
the rain water must exert solvent action wherever it penetrates.
In nature, however, strictly pure water does not occur, it being
difficult to obtain it even artificially. Among the “impurities” almost
always contained in natural water, there are several that materially
increase its solvent power. Foremost among these, both because almost
universally present and on account of its great ultimate efficacy,
is _Carbonic dioxid_, in contact with water forming _carbonic acid_,
the acidulous ingredient of all effervescent waters, the gas which
is produced in nature by innumerable processes, such as decay,
putrefaction, fermentation, the slow or rapid combustion of vegetable
and animal substances, such as wood, charcoal and all other fuels; by
the respiration of animals; in the burning of limestone, etc. It is
therefore of necessity contained in air, on an average to the extent
of about 1-3000 of its bulk in the general atmosphere, but locally
in considerably higher proportions because of proximity to sources
of formation, and of its greater density as compared with air (1½ as
against 1). It may thus accumulate in inhabited buildings, in cellars,
wells, mines, caves; and it is contained in considerable proportion in
the air of the soil. Moreover, being easily soluble in water (to the
extent of an equal volume at the ordinary temperature and barometric
pressure) it is contained in all natural water, whether of rains,
rivers, springs or wells, and largely of course in that percolating the
soil. Such waters may therefore be considered as being acid solvents;
and as such, they exercise a far more energetic and far-reaching effect
than would pure water.
[9] See Chapter 18.
_Carbonated water a universal solvent._—While limestones are the rocks
most obviously acted upon by carbonated water, few if any resist it
altogether. Even quartz rocks of the ordinary kinds are attacked by
it; only the purest white crystalline quartzite may be considered as
sensibly proof against it. Granite and the rocks related to it are
rather quickly acted upon, because of the presence of the feldspar
minerals containing potash, soda and lime as bases[10] together with
alumina.
[10] The increase of solvent power on feldspar when carbonated instead
of distilled water is used, was well exemplified in an experiment
made by Headden (Bull. 65, Color. Exp’t Sta., p. 29), who allowed
pure distilled and carbonated water respectively to act on fresh but
finely pulverized feldspar, with frequent shaking, for five days. The
distilled water dissolved .0081 gram, the carbonated water, .0723 gram
of solids, or nearly nine times as much as the distilled water. Both
residues gave strong reactions for potash with platinic chlorid.
The results of this action are highly important; one being the
formation of clay, so essential as a physical ingredient of soils; the
other the setting-free of potash, one of the most essential nutrients
of plants. Hornblende and the related minerals are similarly acted
upon so far as they contain the same substances. In all cases, of
course, the silica (silicic acid) set free by the carbonic acid remains
partially or wholly in the resulting soils, as such. Lime also at first
mostly remains behind in the form of the carbonate; but potash and
especially soda compounds, being mostly readily soluble in water, are
largely carried away by the latter.
The effect of carbonated water upon silicate minerals is greatly
increased by the presence of ammonia (ammonic carbonate), which always
exists in atmospheric water to a greater or less extent. This effect
may readily be noted on the windows of stables, or other places where
animal offal decays, by the dimming of the glass surfaces; also in
glass bottles containing solution of ammonic carbonate.
_Action of Oxygen._—The effects of atmospheric _oxygen_ on rocks
are of course confined to those containing substances capable of
farther oxidation. Chief among these are ferrous (iron monoxid) and
ferroso-ferric oxid the latter imparting bottle-green, bluish and black
tints to so many minerals and rocks that these colors may usually
be taken as indicating its presence. By taking up more oxygen the
ferrous and ferroso-ferric oxids are converted into ferric oxid or its
hydrate (rust), the tints mentioned passing thereby into brick-red or
rust color, according as the former or the latter (or sometimes their
intermixtures) is formed. In either case there is an increase in bulk;
and this when taking place in the cracks or crevices of minerals or
rocks, tends, like the freezing of water, to widen the cracks and thus
to increase the surface exposed to attack. Since ferrous compounds,
when soluble in water, are injurious to plant growth, this oxidation
is of no little importance, and in soils must be carefully maintained
against a possible reversal.
It is hardly necessary to insist that the action of all these
chemical agents continues in the soils themselves, and that owing to
the fineness of the material, resulting in an enormously increased
surface exposed to attack, such action acquires increased intensity.
This is the more true as in soils bearing vegetation there are always
superadded the effects of the humus-acids resulting from the decay
of vegetable matter, as well as of the acid secretions of the living
plants.
_Action of Plants and their Remnants in Soil Formation._
(_a_) _Mechanical action._—The direct action of plants in forcing
their roots into the crevices of rocks and minerals and thus both
widening them by wedging, and by exposing new surfaces to weathering,
has already been alluded to. That the mechanical force exerted by root
growth is very great, may readily be judged from their effects in
forcing apart, even to rupture, the walls of rock crevices; but actual
measurement has shown the force with which the root, _e. g._, of the
garden pea penetrates, to be equal to from seven to ten atmospheres,
say from 200 to over 300 pounds per square inch. Such a force, exerted
under the protection of the corky layer protecting the root tips, often
produces surprising effects.
(_b_) _Chemical action._—Vegetation takes a most important part, from
a chemical point of view, both in the first formation of soils and
in their subsequent relations to vegetable life. The lower forms of
vegetation are usually the first to take possession of rock surfaces;
foremost among these are the lichens. In humid climates we find
these crust-like plants incrusting more or less all exposed rock
surfaces, sometimes with a solid mantle that can be peeled off in wet
weather, showing the corroded rock-surface, and the beginnings of soil
clustering amid the root-fibrils beneath. A microscopic examination of
the substance of these lichens often shows as a prominent ingredient,
crystals of oxalate of lime, the lime having of course been derived
from the rock, while the oxalic acid has been formed by the plant and
used in the corrosion of the rock minerals. When it is remembered that
this acid is comparable in strength to hydrochloric and nitric acids,
the energy of the attack of the lichens is explained. Its progress can
often be traced, even beyond the visible root fibers, by a change in
the color of the rock; _e. g._, from rust-color to brick red.
When by the action of the lichens a certain depth of loosened rock
or half-formed soil has been produced, the next step is usually the
advent of various mosses, which gradually shade out the crust-like
lichens, while the erect kinds persist for some time. Eventually
the mosses, after having increased still farther the soil layer on
the rock surface, are themselves partially or wholly displaced by
the hardier species of ferns; and with these the higher flowering
plants, such as the stonecrops and saxifrages (the latter deriving
their name from their “rock-breaking” effect), the heather, and many
other or shallow-rooted plants, gradually take possession. The roots
of all plants secrete carbonic acid; and many of them, much stronger
vegetable acids, such as oxalic and citric. In the crevices of rocks
we commonly find the roots forming a dense network over the surfaces,
the marks of which show plainly the solvent effect produced on the rock
by the root secretions. This is most readily observable on a polished
marble surface, or on feldspathic rocks. Of course the progress of
soil-formation is very much more rapid when, as in the case of powdered
lava (volcanic ash) and rock debris resulting from the effects of frost
etc., the surface is very much increased. In tropical climates, where
both vegetative and chemical action is most intense, it takes some of
the higher plants only a few years after a volcanic eruption to take
possession of portions of the “ash” surfaces; thus helping to form a
soil on which after a few more years agricultural plants such as the
vine and olive yield paying returns.
To this direct action of the higher plants is always added, to a
greater or less extent, that of innumerable bacteria, as well as molds;
whose vegetative and secretory action materially assists that of the
roots, and the weathering process in general.
_Humification._—While the mechanical action of the roots and the
chemical effect of the acids of their root secretions are very
efficient in promoting the transformation of mere rock powder into
soil material proper, the efficacy does not end with the life of the
plant. In the natural process of decay to which the roots are subject
after death, and which also affects the leaves, twigs and trunks
falling on the surface, the vegetable matter suffers a transformation
which must be considered more in detail hereafter, and results in
the formation of the complex mixture of dark-tinted substances known
as vegetable mold or _humus_; the remnant of vegetation that imparts
to surface soils their distinctive dark tint. Its functions in soils
are both numerous, and important to vegetable growth; as regards soil
formation, it assists disintegration of the rock minerals both by the
formation of certain fixed, soluble acids capable of acting on them
with considerable energy, and by the slow but continuous evolution of
carbonic acid under the influence of atmospheric oxygen, which has been
alluded to above.
_Causes influencing chemical action and decomposition._—The chemical
processes causing rock decomposition are of course continued in the
soil, and there also are materially influenced by climatic and seasonal
conditions, which bring about great differences in the kind and
intensity of chemical action.
Within the ordinary limits of solar temperatures it may be said that,
other things being equal, the higher the temperature the more intense
will be chemical action in soil formation. Since, however, water is
a potent factor in the majority of these processes, the presence or
absence of moisture at the same time with heat will cause material
differences in the kind and intensity of chemical action. In view of
the importance of carbonic acid as a chemical agent, the presence or
absence of vegetable matter or humus, from which by oxidation or decay
carbonic and humus-acids are formed, will likewise be of material
influence.
The presumption that climatic and seasonal conditions must greatly
influence both the kind and rapidity of the soil-forming processes, is
fully borne out by observation and practice. Especially is the amount
and distribution of rainfall of great importance in this respect, and
should therefore be first considered.
INFLUENCE OF RAINFALL ON SOIL FORMATION; LEACHING OF THE LAND.
In the general consideration of the soil-forming processes, it has been
stated that soils formed by the disintegration of rocks “in place,” _i.
e._, without removal from the original locality, are also designated as
“residual”; meaning thereby that only a portion of the original rock
remains to form the soil mass, while another portion has been removed.
To a slight extent this removal occurs by the partial washing-away of
the finest clay and silt particles; but the most important action from
the agricultural point of view is the removal by leaching with the
carbonated water of the atmosphere and soil, of certain easily-soluble
compounds formed in the process of chemical decomposition of rocks and
resultant soils. The nature of these compounds is exemplified in the
subjoined table giving the composition of some waters flowing from
drains in unmanured fields, laid at depths of from two to three feet;
and for comparison with these, the composition of the water of some of
the world’s large rivers, showing what these largest drains carry into
the ocean.
The analyses have in all cases, where necessary, been recalculated to
parts per million, and to oxids, from the published data.
The letter “c” indicates that the preceding figure has in the absence
of a direct determination been stoichiometrically calculated from the
data given, in order to complete the comparison.
COMPOSITION OF DRAINAGE WATERS FROM UNMANURED GROUND.
(PARTS PER MILLION.)
======================+=============+=============+==============+
| ROTHAMSTED. | PROSKAU. | MOCKERN. |
| (VOELKER.) | (KROCKER.) | (Ö. WOLFF.) |
+-------------+-------------+------+-------+
| | | Rye |Meadow.|
| | |Field.| |
----------------------+------+------+------+------+------+-------+
Potash, K₂O | 1.7 | 5.4 | 2.0 | 2.0 | 8.5 | 3.4 |
Soda, Na₂O | 6.0 | 11.7 | 15.1 | 13.7 | 23.3 | 8.2 |
Lime, CaO | 98.1 |124.3 |133.0 |118.1 |122.6 | 22.5 |
Magnesia, MgO | 5.1 | 6.4 | 33.3 | 22.4 | 14.9 | 6.7 |
Iron Oxid, Fe₂O₃ | 5.7 | 4.4 | 6.6 | 6.6 |} | |
Alumina, Al₂O₃ | | | | |} 8.0 | 6.0 |
Silica, SiO₂ | 10.9 | 15.4 | 7.0 | 6.0 | 7.0 | 4.0 |
Carbonic Acid, CO₂ | 48.1 | 44.4 | 75.8 | 82.6 | | 121.3 |
Phos’ Acid, P₂O₅ | .63| 9.1 |Trace.|Trace.|Trace.| 19.0 |
Sulfuric Acid, SO₃ | 24.7 | 66.3 |122.7 | 67.3 | | |
Chlorin, Cl | 10.7 | 11.1 | 4.8 | 4.2 | 14.0 | Trace.|
Nitrogenas, N₂O₅ | 3.90| 5.10| | | | |
Nitrogenas, NH₃ | .12| .13| | | | |
----------------------+------+------+------+------+------+-------+
Total Mineral Matter |215.9 |295.5 |400.3 |322.9 |198.3 | 191.1 |
Less O: Cl | 2.35| 2.4 | 1.1 | .9 | 3.1 | |
Corrected Total |213.3 |293.1 |399.2 |322.0 |195.2 | 191.1 |
Organic Matter | 22.9 | 19.3 | 25.0 | 16.0 | 26.0 | 26.0 |
----------------------+------+------+------+------+------+-------+
Total Solids |235.2 |312.4 |424.2 |338.0 |221.2 | 217.1 |
----------------------+------+------+------+------+------+-------+
| Farnham. | Munich. |
| (Way.) | (Zöller.) |
+------+------+-------------+---------
|Wheat | Hop | Lysemeter | Average.
|Field.|Field.| Drainage. |
----------------------+------+------+------+------+--------
Potash, K₂O |Trace.|Trace.| 6.5| 2.4 | 3.2
Soda, Na₂O | 14.3 | 45.7 | 7.1| 5.6 | 15.1
Lime, CaO | 69.3 |185.0 | 145.8| 57.6 | 107.6
Magnesia, MgO | 9.7 | 35.1 | 20.5| 8.9 | 16.3
Iron Oxid, Fe₂O₃ | } |} | .1| 6.3 |
Alumina, Al₂O₃ | }5.9 |} 7.1 | | |
Silica, SiO₂ | 1.35| 12.1 | 10.4| 11.3 |
Carbonic Acid, CO₂ | | | | |
Phos’ Acid, P₂O₅ |Trace.| 1.7 | 2.2|Trace.| 0.5
Sulfuric Acid, SO₃ | 23.5 |135.8 | 17.5| 27.1 | 60.8
Chlorin, Cl | 10.0 | 37.4 | 57.5| 9.5 | 17.7
Nitrogenas, N₂O₅ |102.4 |163.5 | | |
Nitrogenas, NH₃ | .25| .03| | |
----------------------+------+------+------+------+---------
Total Mineral Matter |248.8 |623.5 | 267.6|128.7 |
Less O: Cl | 2.2 | 8.2 | 12.7| 2.14|
Corrected Total |246.6 |615.3 | 254.9|126.6 | 285.7
Organic Matter |100.0 |105.7 | 20.5| 12.6 |
----------------------+------+------+------+------+---------
Total Solids |346.6 |721.0 | 275.4|139.2 | 352.6
----------------------+------+------+------+------+---------
COMPOSITION OF RIVER WATERS. PARTS PER MILLION.
====================+==========+==========+=============+===========
| Yukon, | Dwina, |St. Lawrence,| Missouri,
| Alaska. | above | Pointe des | Montana.
| |Archangel.| Cascades. |
---------------------+----------+----------+-------------+-----------
Potash, K₂O | Trace | 12.58 | 1.40 | 1.90
Soda, Na₂O | 8.10 | 23.38 | 6.90 | 30.10
Lithia, Li₂O | | | |
Lime, CaO | 30.40 | 37.30 | 45.30 | 58.00
Magnesia, MgO | 7.30 | 36.25 | 9.70 | 18.10
Manganese, Mn₃O₄ | | | |
Ferric Oxid, Fe₂O₃ | | 1.63 | |} 3.10
Alumina, Al₂O₃ | 1.80 | | |}
Silica, SiO₂ | 7.60 | 3.05 | 32.60 | 18.90
Carbonic Acid, CO₂ | 33.00 | 54.01 | 68.40 | 65.20c
Phosphoric Acid, P₂O₅| | .40 | Trace | .22
Nitric Acid, N₂O₅ | | | |
Sulfuric Acid, SO₃ | 8.50 | 29.62 | 47.70 | 21.90
Chlorin, Cl | .40 | 33.09 | 2.40 | 18.00c
Ammonia, NH₃ | | | |
| ----- | ------ | ------ | ------
Total Mineral Matter | 97.10 | 231.31 | 214.40 | 225.42
Less O : Cl | .10 | 7.33 | .55 | 4.10
| ----- | ------ | ------ | ------
Corrected Totals | 97.00 | 223.98 | 213.85 | 221.32
Organic Matter | | | |
| ----- | ------ | ------ | ------
Total Solids | 97.00 | | 213.85 | 221.22
---------------------+----------+----------+-------------+-----------
| F. W. | C. | T. S. |
| Clarke, | Schmidt, | Hunt, | Traphagen,
|Jour. Am. | Jahresb. | Geol. of |Bull. Mont.
|Chem. Soc.|d. Chemie,| Canada, | Expt. Sta.
|Feb. 1905,| 1873. | 1863. | No. 190.
| p. 112. | | |
---------------------+----------+----------+-------------+-----------
====================+============================================
| Mississippi near Carrollton, La.
|
| Average of one year. |
| |
+------+-----------+-------------+-----------
| | Min. | Max. | May 1905.
| | | |
| | | |
---------------------+------+-----------+-------------+-----------
Potash, K₂O | | | | 2.80
Soda, Na₂O |19.80c| | | 13.50
Lithia, Li₂O | | | |
Lime, CaO | | 49.75 Dec.| 33.81 Feb. | 41.20
| | | |
Magnesia, MgO | 40.80|16.39 March| 10.32 Feb. | 11.30
Manganese, Mn₃O₄ | 12.40| | | .16
Ferric Oxid, Fe₂O₃ |} 2.10| | | .11
Alumina, Al₂O₃ |} | | | .17
Silica, SiO₂ | 8.70|11.45 March| 5.94 April | 7.40
Carbonic Acid, CO₂ | 45.10| | | 33.16c
Phosphoric Acid, P₂O₅| | | | .33
Nitric Acid, N₂O₅ | .13| | | .23
Sulfuric Acid, SO₃ | 16.10| 24.72 July| 8.18 Jan. | 23.90
Chlorin, Cl | 9.60|1 4.50 June| 6.90 Dec. | 16.10
Ammonia, NH₃ | | | | .16
|------|---------- | ---------- | ------
Total Mineral Matter |154.73| | | 150.52
Less O : Cl | 2.10| | | 3.63
|------|---------- | ---------- | ------
Corrected Totals |152.63| 180.0 July| 110.0 Dec. | 146.89
Organic Matter | | | |
|------|---------- | ---------- | ------
Total Solids |152.63| | | 146.89
---------------------+------+-----------+-------------+-----------
| Porter, | Stone,
| Rep. New Orleans | U. S.
| Sewerage and Water | Reclamation
| Board. | Service.
| |
| |
---------------------+--------------------------------+-----------
====================+===========+================+========+==========
|Rio Grande,| |Average |Average
|Ft. Craig, |Nile near Cairo.|of the 7|19 Great
| N. M. | | rivers.|Rivers of
| | | |the World.
+-----------+-------+--------+--------+----------
| | High. | Low. | |
| | Aug. | May 13,| |
| | 1874. | 1875. | |
---------------------+-----------+-------+--------+--------+----------
Potash, K₂O | .80 | 15.01 | 4.04 | 4.80| 2.40
Soda, Na₂O | 43.40 | 5.87 | 13.01 | 18.20| 7.10
Lithia, Li₂O | | | | | .20
Lime, CaO | 22.80 | 44.22 | 51.78 | 43.50| 43.20
| | | | |
Magnesia, MgO | 2.10 | 10.33 | 10.29 | 13.10| 14.70
Manganese, Mn₃O₄ | | | | | 1.20
Ferric Oxid, Fe₂O₃ | | | 1.80 | 2.80|
Alumina, Al₂O₃ | | | 1.80 | 3.10|
Silica, SiO₂ | | 11.29 | 6.71 | 10.80| 16.40
Carbonic Acid, CO₂ | 10.25 | 42.81 | 40.91 | 38.10| 46.00
Phosphoric Acid, P₂O₅| | | | .24| .30
Nitric Acid, N₂O₅ | | | | .18| 3.80
Sulfuric Acid, SO₃ | 47.00 | 18.37 | 29.31 | 26.90| 8.00
Chlorin, Cl | 36.00 | 6.28 | 17.37 | 15.50| 3.70
Ammonia, NH₃ | | .043| .014| | .07
| ------ |-------| -------| ------| ------
Total Mineral Matter | 162.35 |154.223| 173.434| 173.12| 152.97
Less O : Cl | 8.05 | 1.40 | 4.13| 3.50| .72
| ------ |-------| -------| ------| ------
Corrected Totals | 154.30 |152.823| 169.304| 169.62| 152.25
Organic Matter | | | | | 16.4
| ------ |-------| -------| ------| ------
Total Solids | 154.30 |164.683| 200.594| 169.62| 168.65
---------------------+-----------+-------+--------+--------+----------
| O. Loew, | | | John
| U. S. | Letheby, Jour. | |Murray,
| Geogr. | of the Khediv.| |Scottish
| Survey | Agr. Society.| | Geogr.
| W. of | | | Mag.,
| 100th | | | Vol. 3,
| Merid. | | | 1887.
| Vol. 3. | | |
---------------------+-----------+----------------+--------+----------
It will be noted that in all the drain waters, lime is the ingredient
most abundantly leached out, and as reference to the acids shows,
mainly in the form of carbonate, also in that of sulfate. Magnesia is
next in amount among the bases; next in amount is soda, largely in
the form of sodium chlorid or common salt. Potash is present only in
small but rather uniform amounts. Of the acids the carbonic is the most
abundant, sulfuric next; chlorin and silicic acid come next, in about
equal amounts. Nitric acid passes off in small, but still relatively
considerable amounts.
Comparison of the drain waters with the river waters, while showing a
general qualitative agreement, also shows a marked diminution of total
solids (from 285.7 to 188.7; hence “soft river water”), and especially
of lime (from 107.6 to 43.2), together with the carbonic acid with
which it is mostly combined; indicating a deposition of lime carbonate
in the river deposits or alluvial lands. There is, on the other hand,
little if any general difference in the magnesia content of the two
classes of waters; nearly the same is true of soda, so that these two
bases really show a considerable relative increase when the diminished
total is considered. Potash remains about the same all through, viz.
two parts or a little more; phosphoric acid shows a fraction of one
millionth; nitric acid varies greatly but is usually higher in the
drain waters, sometimes showing a heavy depletion of the land by the
leaching-out of this important plant food.
It has been computed by John Murray, as quoted by Russell,[11] that the
volume of water flowing into the sea in one year, including all the
land areas of the earth, is about 6524 cubic miles. From the average
composition of river waters as given above, it would follow that nearly
five billions (4,975,117,588) of tons of mineral matter are annually
carried away _in solution_ from the land into the sea. The amount of
_sediment_ carried at the same time is many times greater; in the case
of the Mississippi river, it is more than five times the amount of the
matter carried in solution.
[11] Rivers of North America, p. 80.
Comparison of the river waters among themselves shows less of any
consistent relation to climatic conditions than might have been
anticipated. The waters of the arctic streams Yukon and Dwina show
wider differences than any two other waters in the list, unless it
be the St. Lawrence, another northern stream. The Missouri and Rio
Grande show by their high content of soda, chlorin and sulfuric acid
their origin in arid climates, where alkali lands prevail. The water
of the Nile is here represented by two analyses,[12] one showing the
season when the water is “red” and of high fertilizing quality because
of the sediment it brings down from the mountains of Abyssinia; the
other the “green” and relatively clear water which comes from the
great lakes and through the “sudd ” or grassy swamp region near the
junction of the Gazelle river with the Nile. Of the analyses given of
the Mississippi river water, the first represents the average of a full
year’s observations made weekly under the auspices of the New Orleans
Commission on Sewerage and Drainage, by J. L. Porter. The fourth is
an analysis made of water taken at the same point in May, 1905; the
analysis having been made in full by Mr. Stone, of the Reclamation
Service of the U. S. Geol. Survey, the direct determination of potash
and soda being in this case included. As will be seen, and might be
expected, the average of the Mississippi water corresponds quite nearly
to that of nineteen of the world’s great rivers as given by Murray. The
very great variation in the content of sulfates is evidently due to the
occasional heavy influx of the gypseous waters of the Washita and Red
rivers when in flood; while the minimum content (in January) agrees
almost precisely with the general average. Murray’s table would hardly
be changed if these analyses of Mississippi water were incorporated
therein, owing doubtless to the large and varied drainage area of the
great river.
[12] The correctness of Letheby’s analyses has been disputed, partly
because of their disagreement with former analyses in the very high
amount of lime, partly because of the high potash-content in the
Low-Nile water. The lime content is, however, confirmed by the partial
analyses made by Mathey in 1887, which gives an average of 44.1 for the
year, while the older analyses, made in Europe, of _transported_ water
gave only half as much. Letheby working on the spot was doubtless more
nearly right in this respect. His figure for potash in the “Low-Nile”
water agrees with former determinations, but that in the “High-Nile” is
approached only by that in the Dwina water. It may be suspected that
the soda is too low and potash too high in this analysis.
_Sea Water._—The nature of the substances permanently leached out is
also seen by considering the composition of sea water, since the ocean
is the final reservoir for all the leachings of the land. It might be
objected that the ocean may have received its salts from other sources;
but this objection is overborne by the fact that substantially the
same salts are found in landlocked lakes, in which, as they have no
outflow, the leachings of the adjacent regions are perforce, as a rule,
the only possible source of the salts. It is true that the nature of
the salts differs somewhat in different lakes, as might be expected;
but a general statement of that nature will, after all, be the same as
that made in regard to sea-water. The following table of the average
composition of sea-water, according to Regnault, illustrates these
facts.
MEAN COMPOSITION OF SEA-WATER.
Sodium Chlorid (common salt) 2.700
Potassium chlorid .070
Calcium sulfate (gypsum) .140
Magnesium sulfate (Epsom salt) .230
Magnesium chlorid (bittern) .360
Magnesium bromid .002
Calcium carbonate (limestone) .003
Water (and loss in analysis) 96.495
-------
100.000
The average saline contents of sea-water would thus be 3.505 per cent
In twenty-one determinations of the saline contents of the Atlantic
Ocean, the percentage ranged from 3.506 to 3.710 per cent Of this
mineral residue, common salt constitutes from about 75 to over 80 per
cent.
We see that most prominent among the ingredients mentioned
here is common salt (sodium chlorid), which forms nearly
four-fifths of the total solid contents. Next in quantity
are the compounds of magnesium, viz. Epsom salt and bittern,
with a very small amount of the bromin compound. Next come
the compounds of calcium (lime), of which gypsum is the
more abundant, while the carbonate, so abundant on the land
surface in the various forms of limestone, is present in
minute amounts only, yet enough to supply the substance
needed for the shells of shellfish, corals, etc. Least in
amount of the metallic elements mentioned is potassium.
Calculating the total amounts of chlorin, we find that it
exceeds in weight any one other element present in the salts
of sea-water, being two-sevenths of the whole solids.
Substantially the same result, with variations due to
local causes, as exemplified in the varying composition of
river and drain waters, is obtained when we consider the
saline ingredients of lakes having no outlet, and in which
therefore, the leachings of the tributary land area have
accumulated for ages. The Great Salt Lake of Utah, the
landlocked lakes of the Nevada basin, of California, Oregon,
and of the deserts of Asia, Africa, and Australia, all tell
the same tale, which may be summarized in the statement that
the chlorids of sodium and magnesium, and the sulfates of
sodium, magnesium and calcium constitute the bulk of the
leachings of the land; while of other substances potassium
alone is present in relatively considerable amount.
While the above analysis shows the ingredients of sea-water
so far as they can at present be directly determined
by chemical analysis, yet the presence of many others
is demonstrable, directly or indirectly, from various
sources. One is, the mother-waters from the making of
sea-salt, in which such substances accumulate so as to
become ascertainable by chemical means, and even become
industrially available in the cases of potash and bromin.
Another is the ash of seaweeds, which is indisputably
derived from the sea-water, and contains, among other
substances not directly demonstrable in the original
water, notable quantities of iodin (of which this ash is a
commercial source), iron, manganese, and phosphoric acid.
Again, the copper sheathing of vessels, as it is gradually
corroded, becomes more or less rich in silver, manifestly
thrown down from the sea-water, and the silver so obtained
is associated with minute amounts of gold. Copper, lithium,
and fluorin likewise have been found in sea water; and it
is probable that close search would detect very many of the
other chemical elements as ordinary ingredients in minute
amounts. This is what must be expected from the fact that
few mineral substances known to us are entirely insoluble in
pure water, and still fewer in water charged with carbonic
acid. The latter is always present in sea-water and holds
the lime carbonate in solution; on evaporation or boiling,
this substance is the first to be precipitated; and thin
sheets of limestone from this source are commonly found at
the base of rock-salt beds, which, themselves, are evidently
the result of the evaporation of segregated bodies of
sea-water in past geological ages.
Summing up the facts concerning the water of the sea and of landlocked
lakes, with reference to the ingredients of soils needful for the
nutrition of plants, it appears that the rock ingredients leached out
in the largest amounts (lime alone excepted) are those of which the
smallest quantities only are required by most plants; while of those
specially needful for plant nutrition, only potash is removed in
practically appreciable amounts by the stream drainage.
_Result of insufficient Rainfall; Alkali Soils._—When the rainfall
is either in total quantity, or in consequence of its distribution
in time, insufficient to effect this leaching, the substances that
otherwise would have passed into the drainage and the sea are wholly
or partially retained in the soil; and when the rainfall deficiency
exceeds a certain point, the salts thus retained may become apparent
on the surface in the form of saline efflorescences, or as it is
usually termed in North America, “alkali.”[13] Their continued presence
modifies in various ways the process of soil formation and the nature
of the soils as compared with those of regions of abundant rainfall
(“humid climates”); one of the most prominent and important results
being that, besides the easily soluble salts mentioned above, the
_carbonate of lime_ formed in the process of decomposition is also
retained, and imparts to the soils of regions of deficient rainfall
(“arid climates”) the almost invariable character of _calcareous_
lands. There is thus in the United States a marked and practically very
important contrast between the soils of the arid region west of the
Rocky Mountains and those of the “humid” region between the immediate
valley of the Mississippi and the Atlantic coast. These differences and
their practical bearings can be best discussed after first considering
more in detail the chemical decomposition of the several soil-forming
minerals.
[13] In some cases the soluble salts originate in rocks impregnated
with salts from marine lagoons or landlocked lakes, or directly from
their evaporation residues. But this is the exception rather than the
rule.