Popular Science Monthly/Volume 48/March 1896/Steppes, Deserts, and Alkali Islands
|STEPPES, DESERTS, AND ALKALI LANDS.|
By Prof. E. W. HILGARD.
THE average reader feels but a moderate interest in the subject of steppes, which he usually associates with roving herds and Tartar or Indian tribes, whose periodic raids have in the past been a standing source of disquiet to civilization, whether in the Occident or Orient. The predatory habits of these people seemed to be proof sufficient of the fact that the countries occupied by them are not able to support permanently a population devoted to agricultural and industrial pursuits, and that their inhabitants are under more or less natural stress toward levying forced contributions upon their neighbors in order to eke out their existence in a satisfactory way, as in the case of the tribes of the deserts properly so called.
As to alkali lands, so far as they are known and considered at all, they are regarded only as obstacles to the settlement and cultivation of the otherwise desirable lands whose continuity they mar, aside from the discomfort their pungent dust and saline water causes to the overland traveler; while their Old World equivalents, the "salt steppes" of southeastern Russia, central Asia, and northern Africa, are among the most disconsolate images conjured up by the imagination of those who traverse or read about them. Moreover, it is currently supposed that these regions owe their saline soil to the evaporation of former salt lakes or seas, and that an indefinite amount of similar salts lurks below the surface, ready to replace whatever may be removed in any attempt to reclaim the lands for cultivation.
From this point of view it is hard to understand why the people foremost in ancient civilizations should have chosen for their abodes, and should have developed their civilization rather predominantly, in regions either adjacent to deserts, or having, during a considerable portion of the year, the aspect and character of the ill-reputed steppe. Egypt is the example nearest to Europe, but Asia Minor, Syria (including Palestine, the "land where milk and honey flows"), Persia, Arabia, and (crossing the Indus) northern India, the classic ground of the Vedas and Mahabharata, are more or less tainted with "the breath of the desert," as well as with its actual presence to a greater or less degree. Looking westward, we again find the old Carthaginian and later Moorish civilization on the borders of the desert. Crossing the Atlantic, we find the empire of the Incas on the steep, bare, uninviting western slope of the Andes, when just across the divide there lay the rich countries now forming the Colombian republics and typically exuberant Brazil. In North America, likewise, the civilization of the Aztecs and Toltecs was developed not in the wonderfully prolific tierra caliente, but on the arid plateaus of Old and New Mexico; persisting while the builders of Palenque and Copan had already passed into oblivion. It seems as though a strange infatuation had possessed these ancient nations in the preference given to bare, sun-scorched plains and mountains, as against the cool shades of the forest-clad countries, which in later times have almost monopolized the abodes of advanced civilization.
It may be said, of course, that forests offer the inconvenience of affording concealment to a lurking enemy; a serious consideration during the ages when a state of war was the normal condition of mankind. Again, it is said that the necessity of clearing away the forest before cultivation was possible offered an obvious inducement toward the utilization, first, of the treeless regions.
The former consideration doubtless weighed strongly in the first beginnings of settlement. And yet our Saxon forefathers, both in the old and new continents, have managed remarkably well in their forested countries, in the face of lurking enemies both animal and human. As regards the difficulty of clearing the forest lands for cultivation, it is amply offset by the necessity, almost universally existing in treeless countries, of providing irrigation if cultivation is to be anything more than a lottery. For forests are limited by a certain minimum of rainfall, below which regularity of crops is dependent upon artificial irrigation.
In other words, the countries which have harbored most of the ancient civilizations are regions of deficient rainfall and compulsory irrigation. And as irrigation means heavy investments of capital or labor, hence the co-operation of many and the construction of permanent works: it necessarily implies the correlative existence of a stable social organization, with protection for property rights, and (in view of the complexity of the problem of proper and equitable distribution of water) a rather advanced appreciation of the need and advantages of co-operative organization.
If, then, the general practice of irrigation is conditioned upon a not inconsiderable degree of advancement in social organization, shall we attribute the development of the ancient civilizations referred to only to its conservative influence, or are there other factors that have contributed toward the preference and long-continued permanence of these polities or populations?
The high cost of irrigation is usually found to be compensated by the high and regular production of the lands irrigated; it is almost a maxim that irrigated lands can support a much denser population than those of countries in which rainfall is relied upon for the production of crops, and where therefore frequent partial, or entire crop failures are to be looked for. The regularity of production, of course, results from command of the water supply as such; the high production has usually been ascribed
largely to the plant food brought to the land by the stream waters used in irrigation, whether in the form of suspended mud or in actual solution. Especially has the never-failing fertility of Egypt been ascribed to the mud carried down by the flood waters of the Blue Nile from the uplands of Abyssinia during the season when torrential rains prevail there.
Without denying a certain efficiency of this cause, a closer examination easily proves it to be inadequate to account for the millennially undiminished fertility of the Nile Delta. The average annual mud deposit of the Nile floods amounts to less than the thickness of a common pasteboard. Were this the best of stable manure well worked in, it could not produce the effect claimed. But examination proves it to be simply a rich soil, such as thousands of farmers could haul and spread upon their lands if it could produce the effect ascribed to this Nile sediment. Besides, perpetual fertility belongs equally to the lands of the neighboring Fayoom, which, being irrigated only with the clear water of Lake Mœris, do not receive the benefit of any sediment. But the analysis of that water, or of the clear Nile water itself, does not show it to contain any unusual amount of fertilizing matter in solution.
There are other examples, too, of lands perpetually productive for thousands of years without fertilization. One is the "regur" lands region of the Deccan, forming part of the plateau of south central India; another is the "loess" region of China, drained by the head waters of the Yellow River, and for ages the granary of that empire. In both these cases no alluvial fertilizing deposits come into play, and there is little or no irrigation. But in both cases we have a semiarid climate, the rainfall being close to the limit of actual deficiency; in the case of Egypt the deficiency is extreme during nine months out of the twelve.
What, then, is the effect of a deficient rainfall upon the nature of soils formed under its influence? And can these effects serve to explain, in any measurable degree, the choice of the ancient civilizations?
In the course of his investigations of the soils of the United States, the writer has had occasion to make extensive comparisons of the soils of the Atlantic humid region with those of the arid and semiarid West. A summary of the results of these comparisons was given in Bulletin No. 3 of the United States Weather Bureau in 1892. They led to important conclusions of a general nature, some of which could have been readily foreseen on general principles. Continued investigations made since have given additional confirmation, and have developed new facts having important bearings upon the possible utilization and productive value of vast land areas thus far considered either irreclaimable or adapted only to scanty pasturage.
Without going into technical details or figures, the case may be stated thus: Soils are formed from rocks by the physical and chemical agencies commonly comprehended in the term weathering, which includes both their pulverization and chemical decomposition by atmospheric action. Both actions, but more especially the chemical one, continue in the soil itself; the last named in an accelerated measure, so as to give rise to the farmers' practice of "fallowing"—that is, leaving the land exposed to the action of the air in a well-tilled but unplanted condition, with a view to increasing the succeeding year's crop by the additional amount of plant food rendered available, during the fallow, from the soil itself.
This weathering process is accompanied by the formation of new compounds out of the minerals originally composing the rock. Some of these, such as zeolites and clay, are insoluble in water, and therefore remain in the soil, forming a "reserve" of plant food that may be drawn upon gradually by plants; while another portion, containing especially the compounds of the alkalies, potash and soda, are easily soluble in water. Where the rainfall is abundant, these soluble substances are currently carried into the country drainage, and through the rivers into the ocean; which shows in its saline portion (about three and a half per cent) the average composition of the matters permanently leached out of the land. Most of this is common salt—chloride of sodium—but a large portion, if not all, of the other elements known are represented in sea water in a greater or less proportion. Among these, potash, lime, magnesia, sulphuric and a trifle of phosphoric acids require mention here.
Where, on the contrary, the rainfall is insufficient to carry the soluble compounds formed in the weathering of the soil mass into the country drainage, those compounds must of necessity remain and accumulate in the soil. They then constitute what in the western United States is now universally known as "alkali."
"Alkali" is not, then, as is popularly supposed, something foreign to the soil, imposed as a special affliction upon the dwellers in the arid or irrigation regions. It is the normal product of soil-formation and soil-weathering everywhere; but in the humid regions it appears only in the bottom and stream waters, and is not perceived in the soil itself.
Nor does it in either climatic region consist only of salts injurious or useless to vegetation. Its origin, as well as the chemical nature of sea water, proves that it should contain the useful or plant-food ingredients as well; and direct analysis amply confirms this induction. But it also shows that while there are great variations in the composition of the alkali salts in different regionsand even in contiguous localities, as a rule the useless ingredients are present in larger proportions than is the case in the original material from which the soil was formed, or in the latter itself.
The question as to what has become of the useful mineral plant food representing this difference is categorically answered by the analysis of the soils themselves. If we analyze, by identical methods, series of the soils of the arid and humid regions respectively, we find constant differences in their composition, that are manifestly due to the conditions under which they have been formed. They show in those of the arid regions, on the average, a markedly greater proportion of certain elements of plant food than in the soils that, under the influence of copious rainfall throughout the year, have been currently leached of whatever soluble matters were set free by weathering.
The explanation is, that when these soluble matters are retained in the soil for a length of time, they are given the opportunity of entering into the insoluble combinations already mentioned as repositories of "reserve" plant food—i. e., such as may be gradually drawn upon by plants, either by the direct solvent action of their acid root-sap, or by being again rendered watersoluble by a repetition of the weathering process.
Thus the soils of the arid region, whether containing a natural surplus of water-soluble salts in the objectionable guise of alkali or not, are found to be greatly superior, in the native stock of certain ingredients of plant food, to the average soils of the regions of abundant rainfall; their average being, in fact, equal to the most highly productive (usually alluvial) soils of the humid region.
The chief substances of which the arid soils thus retain considerable amounts that run to waste in the countries of abundant rainfall, are potash, lime, and magnesia. The average ratios of these as found in the United States, for the region east of the Mississippi, when compared with that west of the Rocky Mountains (or of the hundredth meridian), by the comparison of over a thousand analyses, are as one to three, one to fourteen, and one to six respectively.
But these numerical ratios do not adequately express some of the chief advantages enjoyed by the soils of arid regions. While the large amount of potash they contain relieves the farmer for a long time from supplying to his fields the potash fertilizers that prove so effectual and necessary in the East and in Europe, yet the almost universal presence of a surplus of lime (in the form of carbonate) is perhaps of even higher importance. To understand this it is only necessary to remind the reader of the common saying that "a limestone country is a rich country"—abundantly illustrated in the Atlantic States by the blue-grass region of Kentucky, the black prairies and "bluff"-lands of the Mississippi Valley, and hundreds of local examples. The common and beneficial practice of "marling" noncalcareous lands illustrates the same axiom. It logically follows that, inasmuch as actual examination shows practically all arid soils to be calcareous, "arid countries are rich countries" whenever irrigated; and the actual and concordant experience of mankind corroborates the conclusion.
In other words, the ancient civilizations have, consciously or unconsciously, chosen countries having naturally rich and durable soils, capable of supporting for a long time a denser population than the forested regions, without resort to artificial fertilization beyond irrigation. This seems to be the simple and rational explanation of their marked preference for arid countries; and unquestionably Egypt owes its perennially undiminished productiveness at least as much to its arid climate as to the alluvial deposits brought down by the Nile, as is shown in the oases of the Libyan Desert, as well as in India and China.
But if these things are true, then the steppes and alkali lands deserve the most earnest attention, both of agriculturists and of students of natural economy; for in them lie possibilities for the abundant sustenance and prosperity of the human race that have thus far been almost left out of account. While it is true that irrigation water may not be practically available for the whole of the arid regions of the globe, so much remains to be done in the study of the most economical use of water, of appropriate crops and methods of culture, that even an approximate estimate of actual possibilities in this direction can not yet be made. At all events, it is of the highest interest to study the problem of the reclamation of these intrinsically rich lands in all its phases.
Foremost in this problem is the question of the manner of dealing with the "alkali" salts, which, as experience proves, exist not only on the spots where they naturally show on the surface, for as soon as irrigation is practiced they appear at numerous points where no symptoms of alkali were noted before. Sometimes, indeed, the entire area of large farms may in the course of years become thus afflicted, so that orchards and vineyards that have been in bearing for a number of years become stunted, and in spots even perish.
Examination of the manner in which such injury comes about shows that it is rarely due to direct action of the salts on the roots. Almost always the injured part is at or near the root-crown, or base of stem or trunk; proving that it is the result of accumulation of the salts at the surface (so often obvious to the eyes), in consequence of evaporation.
It follows that the prevention of surface evaporation, to the utmost extent possible, is of the first importance.
Now, the simplest, cheapest, and most universally practicable mode of diminishing evaporation from a bare land surface is to
keep it constantly in loose, fine tilth, to a depth which varies with soil and climate. In humid climates four inches has been found sufficient. In the hot summers of arid countries even more than twice that depth may be necessary. In that case the alkali salts left behind by evaporation will be diffused through so large a mass of soil that no injury can ordinarily result.
But experience proves that some alkali soils are, in their natural state, incapable of being reduced to a proper condition of tilth; and this, as well as the tendency of the obnoxious salts to come to the surface more and more when land is irrigated, has been made the subject of extensive investigations by the California Experiment Station, in order to determine the proper methods for the permanent repression of these obstacles to cultivation.
These investigations proved, as far back as 1880, that the cause of difficult tillage in alkali lands is carbonate of soda (sal soda), the presence of which is recognized by the blackish spots and rings left on the soil when rain or irrigation water evaporates; hence the popular designation of "black" alkali, known to be specially injurious and corrosive. It was also shown that the peculiar ill effects of such alkali can be overcome by the use of sufficient dressings of gypsum or land plaster, which—acting in conjunction with water—transforms the corrosive sal soda into bland and relatively innocuous sulphate, or Glauber's salt. The latter, with more or less of common salt, forms the bulk of the salts in the cases of mild or "white" alkali, of which a much larger proportion is tolerated by all plants. Gypsum also has the effect of rendering insoluble the humus and phosphoric acid that had been dissolved by the salsoda.
But the rise of the alkali brought about by irrigation seemed to indicate that an indefinite amount of these salts might lurk in the depths of the soil; and that as irrigation wets the land more deeply than would the scanty natural rainfall, and correspondingly increases the surface evaporation, permanent reclamation of alkali lands seems difficult if not hopeless.
To decide this question, at first examinations of the natural bottom waters of such lands were made, which showed in most cases saline contents not greater than those of many waters long successfully used for irrigation. It then became important to determine just to what depth the impregnation of salts actually reaches.
For this purpose borings were made at the Culture Experiment Station near Tulare, Cal., samples being taken by means of a post-hole auger of each successive three inches of soil, down to the depth of two or four feet, as might be required. Each of these (twelve to sixteen) samples was then separately leached and analyzed, determining both the total amounts of salts present and the proportions of the three to five principal compounds. The results of this work in four typical cases are embodied in the curve diagrams below, which show the facts more plainly than would figures; the vertical line to the left representing the soil depth (in feet and inches), while on the horizontal lines the percentages of salts in the soil are entered at intervals representing two, or four one-hundredths of one per cent each. The area within the curve generated by connecting the points of actual observation represents, of course, the total of each ingredient indicated.
Diagram No. 1 represents the natural condition, the soil being at the time covered with the native spring growth of bright flowers. No alkali salts are seen on the surface at this point at any season.
It will be noted that the alkali salts are almost wholly accumulated between the depths of twenty and forty inches. Both above and below these limits impregnation is not strong enough to interfere with vegetable growth of any kind; within them, the subsoil is hardened into a sheet of hard pan, which not only prevents the passage of roots by its resistance, but would corrode them by contact with a mixture of salts containing up to ninety-four per cent of carbonate of soda. But as the native vegetation is mostly shallow-rooted and annual, this does not interfere with its welfare. The moisture imparted to the land by the scanty rainfall (about seven inches) evaporates through the roots and leaves of this vegetation during its growing period; when it dies off it leaves the ground completely dry, so that no rise of the salts to the surface by evaporation can take place during the season, and the seeds dropped can germinate when the rainy season comes, without injury from alkali.
Diagram No. 3 shows, on the other hand, what happens when irrigation is practiced on this land (or when the water rises from below by seepage from leaky ditches), and the ground is left bare. The abundant water then dissolves the alkali salts in the subsoil hardpan; and evaporation continuing through the whole year, the entire salts are in the course of a few seasons carried upward nearer to the surface. Diagram 3 shows the state of things under these conditions at the same date as Diagram 1 (May 3, 1895); No. 4, a and b, shows the condition existing near the surface at the end of the dry season, in September or October. It will be seen that at that time the salts have accumulated so near the surface that by taking the soil away to the depth of six inches, from five sixths to seven eighths of the total mass of salts would be removed, leaving the land with no more than almost any crop can easily resist.
Diagram 2, a and b, shows the state of the irrigated land when sown to barley, it being understood that these samples were taken within ten feet of No. 3. A glance reveals that we have here a case intermediate between Nos. 1 and 3. The upward movement of the alkali salts has been partially checked by the evaporation through the roots and leaves of the crop; whose thin foliage, however, could not act as effectually as the dense, leafy mat formed
by the dense growth of the native herbage on No. 1. Some evaporation from the soil surface itself continued, although it did not suffice to injure the crop by the rise of the salts: and as at harvest time most of the soil moisture was exhausted, only a moderate efflorescence of alkali was seen on this land even late in the season.
It is thus obvious that when by any means a good "stand" of a leafy perennial crop with deep roots, like alfalfa, can be obtained on alkali land, the rise of the salts resulting from irrigation is measurably checked, and may remain wholly innocuous so long as that crop occupies the soil. Experience amply confirms this conclusion; but the difficulty of obtaining such a stand is often very great, especially on "black" alkali land, which "rots" the seed as often as sown. It is then that the use of gypsum to neutralize the carbonate of soda often becomes the saving clause, enormous crops being then grown on land formerly considered worthless. Diagram No. 5 may illustrate this in the case of wheat, which was grown in 1892 at the Tulare Station on ground that, prior to the reclamation work, would hardly grow even "alkali weeds," but then yielded grain at the rate of forty-five bushels per acre.
The diagrams, however, convey unanimously the fundamentally important lesson that the amount of alkali salts in these soils is limited, and lies within such easy reach of the surface that ordinary underdrainage at the depth of from three to four feet will relieve these rich soils of their noxious surplus, once for all. Also that toward the end of the dry season the removal of a few inches of surface soil will go far toward relieving the land of the same.
A few words should be said in regard to the kind of the salts as well as their quantities. As regards the main ingredients, which may be considered as useless or harmful to vegetation, inspection of the diagrams shows that in no case is the noxious carbonate of soda as abundant near the surface as in the case of the subsoil hardpan in Diagram No. 1. Investigation has shown this to be due to the aëration which occurs near the surface; while, on the contrary, in a water-logged soil, the "black" alkali is constantly in progress of formation from the "white" or neutral salts. Hence we find the worst of the "black" cases in low or badly drained ground, and in close soils. Here, again, underdrainage affords radical relief.But underdrainage and washing-out of the salts would in many cases be like "throwing out the child with the bath." For, as has been stated at first, not only the useless but also the useful or plant-food ingredients, which the farmer purchases in the form of fertilizers, are present in them. They not unusually contain as much as twenty per cent of salts of potash, ten to twenty per cent of saltpeter, and several per cent of soluble phosphates. In one notable case the equivalent of one ton of Chile saltpeter (worth
|On medium alkali.||Partly reclaimed alkali soil.||On fully reclaimed alkali soil.|
Plate 5.—Wheat and Barley Grain on Alkali Land treated with Gypsum.
about four cents a pound) per acre of ground to the depth of one foot, was found to be present.
It would clearly be folly to wash out such quantities of fertilizing material unnecessarily; and this consideration emphasizes the importance of the less radical means of reclamation already referred to, viz., deep and thorough tillage to minimize evaporation, and in the case of "black" alkali the use of gypsum. In California the opening of numerous gypsum mines has already followed the latter recommendation.
When once the high productive value of alkali lands is generally realized, enormous areas will be added to the producing lands, not only in the arid region of the United States but in the Old World as well. The Russian investigators in central Asia are rapidly coming to this conclusion, and notably Von Middendorff reports that the inhabitants of Ferghana say that "the salt is the life of their soil" provided there is not too much of it; and that they actually sometimes carry the alkali efflorescences to the poor spots. In India, on the Ganges and Jumna, the typically rich lands of that anciently civilized region have had alkali salts made to rise to the surface in consequence of the establishment of high-lying irrigation canals by the English. Similar reports of high productiveness come from the alkali lands of the border and oases of the Libyan and Sahara Deserts, and from the pampas of Argentina.
But it must not be forgotten that the reclamation of these fertile lands requires the command of some pecuniary resources; and that the farmer or settler who depends on an annual crop for his subsistence should not undertake their cultivation at first. As in the case of mines, the wealth that lies within them is not yielded to mere scratching or prospecting, but requires the use of some capital and trained intelligence to become available.
- Taking this at one twenty-fifth of an inch, it would amount to about five tons per acre, or about two good two-horse loads. Three times that amount of stable manure is about the usual dressing for an acre.