CHAPTER II.. The Chemical Processes of Soil Formation.

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, 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.

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, 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.

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 together with alumina.

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, 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.

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, 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.

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.” 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.

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.