Popular Science Monthly/Volume 73/July 1908/Soil Wastage
THE
POPULAR SCIENCE
MONTHLY
JULY, 1908
| SOIL WASTAGE |
By Professor THOMAS CHROWDER CHAMBERLIN, Ph.D., LL.D., Sc.D.
UNIVERSITY OF CHICAGO
THE invitation to give thought to the conservation of resources that affect our future, appeals to me with almost personal force, for my studies of the past decade have led to the belief that the era of the earth's future habitability is vastly greater than we have been wont to think. We have grown up in the belief that the earth sprang from chaos at the opening of our era and is plunging on to catastrophe or to a final winter in the near future. Quite at variance with this, I have come to believe that the earth arose from a regenerative process and that it offers a fair prospect of fitness for habitation for tens of millions of years to come. If this be true, it is eminently fit that our race should give a due measure of thought to the ulterior effects of its actions.
It is one of the latest conceptions of geology that climatic conditions have been of the same order as at present from early eras, in the large view, in spite of some notable variations, and that this uniformity is the result of a profound regulative system which has sufficed to keep the temperatures of the earth's surface and the constitution of the earth's atmosphere within the narrow range congenial to life for many millions of years. As a result there has been no break in the continuity of land life since it came into being eras ago. It appears, further, that the sources of supply of the vital elements are still adequate, and are likely to be so for long ages, that the regulative system is still in effective control, and that a vast future of habitability may fairly be predicted, subject only to some contingencies of collision or disturbing approach of celestial bodies. Whether you are prepared to accept so large a view of the habitable future or not, I trust you will strike hands with me in the conviction that the probabilities of the future are at least so great as to render imperative the serious consideration of our obligations toward it.
It is a familiar geologic deduction that for long eras rains have fallen on the lands and soils have grown in depth, while the surface has been washed away. Soil-production and soil-removal have run hand in hand, and yet they have been so controlled by the adjustments of nature that no large part of the surface has been swept bare enough to altogether exclude vegetation. More than this, it appears that the usual adjustments of nature make rather for increasing fertility of soil than depletion. It is true that at intervals deformations of the earth have intervened giving mountainous heights and precipitous surfaces from which the soil-product has been washed faster than it could be produced; and desert conditions have also intervened locally; but these diastrophic effects are perhaps rather rejuvenations necessary to the preservation of the continents than destructive episodes. Whenever such heights and slopes have been raised, the atmosphere and its waters have at once begun to grade them down, to cover them with soil, and to give to them a renewed habitability. So, in these and other ways, the gifts of the great past now present themselves to us as the product of a marvelous system of control which has checked excesses and forced movement toward the golden means in which have lain productivity and congeniality to life. Thus has come our inheritance of a land suitable for habitation, of a soil-mantle of great fertility, of a precipitation conducive to productiveness, and of a system of streams endowed with great possibilities of water-foods, of power and of navigation.
We do not hesitate to enter into the inheritance, but what part shall we take in the regulative system that produced and maintains it? How shall we cooperate with nature in rendering conditions still more serviceable to ourselves, and in transmitting a still greater inheritance for our successors? Clearly we may use the proper revenues of our inheritance, but surely we should not rob our successors of their share in it.
Let us turn at once to the basal factors in the problem, the rainfall, the soil-formation and soil-wastage, the special theme of this hour. The rainfall may be regarded as an inherited asset, the soil is clearly an inherited asset, even a little soil-removal is an advantage, but reckless soil-wastage is a serious error. Soils are the product of the atmosphere and its waters modifying the rock surface. When they have aided the air in producing soil by rock decay, the atmospheric waters may pass either into plants or back to the surface through the soil and out by evaporation, or they may pass on down to the groundwaters and thence into the streams, furnishing there the basis for water-foods, for power and for navigation. Here is a good work—soil-production—followed by advantageous courses of the water both up and down. On the other hand, the rainfall may rush away on the surface as a foul erosive flood, wasting soil and plant-food, gullying the surface, flooding the valleys, filling the reservoirs, sweeping out the dams, barring the streams and clogging the deltas. If it shall be found that nearly all the rainfall should go into the soil and thence into the under-drainage, coming out slowly and steadily by seepage and by springs into the streams, clear and pure, these streams should present nearly ideal conditions for water-food, for water-power and for stream-navigation. An ideal solution of the soil problem may therefore solve the greater part of the whole complex of problems of which navigation is the last term. It may thus prove to be the key problem. It is clearly the initial problem, for it attacks the rainfall when it first touches the earth.
To see more definitely if it be the key problem, we must turn to details, and yet, with the brevity that is imperative, we may only look at major details, passing by a multitude of special cases, some of which are even exceptions.
While soils are formed by the atmosphere and its waters acting upon rock (aided by plants and animals), soil surfaces are carried away by wind and wash. At any instant, then, the depth of the soil measures the lag of removal behind production. We hasten to note that the addition of new soil below and the loss of exhausted soil above are alike tributary to permanent fertility, and clearly the best results spring from the proper ratio of addition at the bottom, to wastage at the surface.
We have as yet no accurate measure of the rate of soil production. We merely know that it is very slow. It varies obviously with the kind of rock. Some of our soils are derived from material already reduced to a finely pulverized condition. Such are the lowland accumulations from highland wash. Such also is the glacial drift, rock-flour rasped from the face of the rock by the glacial file and ground up with old soils. Soils may be developed from such half-prepared material with relative rapidity, but observation shows that even in these cases, when the slope is considerable, wind, wash and cropping remove the surface much too fast for stable fertility. For average rock, under the usual conditions of our climate, the common estimate of natural loss and gain has been a foot in 4,000 to 6,000 years, which includes channel-cutting and bank-undermining. This seems to me too rapid a rate for ordinary soil production under normal conditions. Without any pretensions to a close estimate, I should be unwilling to name a mean rate of soil-formation greater than one foot in 10,000 years, on the basis of observation. If we allow 40,000 years for the formation of the four feet of soil next to the rock over our average domain, where such depth obtains, it will probably be none too conservative. To preserve a good working soil-depth, with such an estimate, surface wastage should not exceed some such rate as one inch in a thousand years. If one chooses to indulge in a more liberal estimate of the soil-forming rate, it will still appear, under any intelligent estimate, that surface wastage is a serious menace to the retention of our soils under present modes of management. Historical evidence enforces this danger. In the Orient there are large tracts almost absolutely bare of soil now, which formerly bore flourishing populations. Long-tilled lands generally bear testimony of like import. Much more than mere loss of fertility is here menaced; it is the loss of, the soil-body itself, a loss almost beyond repair. When our soils are gone, we too must go, unless we shall find some way to feed on raw rock or its equivalent. The immense tonnage of soil-material carried out to sea annually by our rivers, even when allowance is made for laudable wash, and for material derived from the river channels, is an impressive warning of the danger of excessive soil-waste. Nor is this all; the wash from one acre often buries the fertile portion of another acre, or of several. Sometimes one's loss is another's gain, but all too frequently one's loss is another's disaster.
If the atmospheric waters may not run off the surface freely without serious menace, where may they go and what may they do consistent with our welfare? The answer lies in a return to the study of the origin and internal work of soils. For necessary brevity, let us neglect all secondary soils, or overplacements, and consider simply the origin and activities of primary soils derived from primary rocks. The action of air and water in producing soil from such rock is partly chemical and partly physical. Certain rock substances are made soluble and become plant food or plant poisons, while others remain relatively insoluble, but are reduced to a finely divided state and form the earthy element of the soil.
Some of the soluble substances thus formed at the base of soils are necessary plant food, while some are harmful; but what is more to the point, all are harmful if too concentrated. There is need, therefore, that enough water pass through the forming soil, and on down to the ground-water and out through the under-drainage, to carry away the excess of these products. An essential part of the best adjustment is thus seen to lie in a proper apportionment of the amount of water which goes through the soils. If this be not enough, the plants will suffer from saline excess; if it be too much, the plants may suffer from saline deficiency.
When evaporation from the surface is active and prolonged, waters which had previously gone down to the zone of soil-formation and taken up soluble matter, may rise again to the surface bringing the soluble matter up and leaving it at the surface on evaporation. Up to a certain point this is favorable to the plant; beyond the critical point, it begins to be harmful, as abundantly shown in the "alkaline" efflorescences of arid regions.
Besides the water that goes through the soil into the subdrainage, and that which runs off on the surface, enough must be held at all times in the soil during the growing season to supply the plants, and yet not enough to water-log the soil.
Here, then, are a series of possible excesses and deficiencies, between which lie the golden means which give best results. The problem of soil-management thus appears to be a problem of proper balancings and adjustments.
The key to the problem lies in due control of the water which falls on each acre. This water is an asset of great possible value. It should be the habit of every acre-owner to compute it as a possible value, saved if turned where it will do good, lost if permitted to run away, doubly lost if it carries also soil values and does destructive work below. Let us repeat the story of its productive paths. A due portion of the rainfall should go through the soil to its bottomto promote soil-formation there; a due portion of this should go on into the under-drainage, carrying away harmful matter; a due portion should go again up to the surface carrying solutions needed by the plants; a due portion should obviously go into the plants to nourish them; while still another portion should run off the surface, carrying away a little of the leached soil matter. There are a multitude of important details in this complex of actions, but they must be passed by; the great features are clear and imperative.
Experimental studies have shown that, on the average within our domain, crops can use to profit all the rainfall during the growing season, and much or all of that which can be carried over from the non-growing seasons. This greatly simplifies the complex problem, for the highest crop-values will usually be secured when the soil is made to absorb as much of the rainfall and snowfall as practicable. There are, of course, many local exceptions. In securing this maximum absorption and internal soil-work, the run-off, and hence the surface wash, will be reduced to a minimum. It has already been seen that the wash of even this inevitable minimum is likely to be still too great to keep the proper slow pace with soil-generation, when the surface has much slope. Except on very level ground and on lodgment surfaces, there need be no solicitude about a sufficient removal of the soil surface. The practical problem then lies almost wholly in retaining and passing into the soil the maximum of the precipitation. Obviously this gives the minimum of wash to foul the streams, to spread over the bottom lands, to choke the reservoirs, to waste the water-power, and to bar up the navigable rivers. The highest solution of the problem for the tiller of the soil essentially solves the whole train of problems.
How is this control to be effected? All the known and tried methods of preventing wash and turning the rainfall into the soil should be duly employed. It is obvious that all methods of culture and all crops that increase the granularity and porosity of the soil contribute to the end sought. Deep tilth to promote soil granulation and deep-rooting plants to form root-tubes are specific modes of great value. Artificial underdrainage by preventing water-logging and promoting granulation aids the end sought. Contour cultivation by arresting and distributing the surface wash may also assist. Alternate strips of protected and cultivated land, reservoirs for catching and distributing concentrated rainfall, and other devices, serve to limit the wash of the slopes and give the surface waters the right direction.
It is possible that some of the more radical and permanent remedies will be found by a closer study of nature's methods. Nature has been working at this complex problem of balance between soil formation, soil waste, surface slope, plant growth and stream development, for millions of years. Looking closely at her methods, we note that she uses a much larger variety of plants to cover and protect the soil than we do, and that these plants have a wider range of adaptation to the special situations where protection is needed. We may, therefore, inquire whether we should not follow this precedent farther by developing more kinds of profitable plants and by using the protective varieties more freely on slopes especially subject to wash. Forest trees are a resource of this kind and should be employed as fully as practicable, as will, no doubt, be urged with great cogency by those who discuss the problem of forestry. We also have many shrubs, vines and fruit trees, whose employment to the maximum in covering areas subject to wash is likewise urged, either alone or in conjunction with trees. We are forced to recognize, however, that for the greater part the berries and fruits which render these profitable are perishable and have limitations of preservation, transportation, market, etc. But if shrubs and vines could be evolved by modern selective methods, whose nut-meats or dry seeds should be available for food in place of the watery pulp, and which could be treated much as cereals are, and have similar wide year-round markets, there would be a larger choice of crops to grow in soils subject to wash, and we might secure soil-protection with less crop-limitation. There would then be less need to press the culture of the cereals so far as we do now, and they could be limited more largely to surfaces less subject to harmful soil-loss.
Another of nature's marked methods is the formation of plantsocieties, or, from our point of view, combination-crops. No doubt there is much deleterious crowding and repressive rivalry among the natural mixtures of plants, but at the same time there seem to be associations that are mutually beneficial. No doubt man secures a great temporary advantage by isolating chosen plants and freeing them from competition, but this is clearly at some permanent disadvantage which is partially corrected by rotation, fertilizing and tilth. Can not a greater advantage be secured by a larger use of the combination method? It is clear that legumes and cereals are helpful associates in rotation and in some combinations. May not the principle be pushed much farther by the modern processes of selection and culture, so that legumes and cereals may be made more intimate companions in culture; so that, indeed, such helpful associates may replace weeds as the constant and spontaneous companions of the crops we cultivate? While kept in such subordination as to be servants of the chosen crop, may they not still aid effectively in covering and protecting the soil and thus guard against undue surface loss. Certainly much can be done by such congenial plants, used as fall, winter and spring crops, to cover the soil when specially exposed to wastage.
These and similar devices may be used to reduce the bare surfaces so much developed by present modes of cultivation, and may make it possible to cover permanently by profitable protecting crops the slopes where surface wash is most menacing.
But a critical question remains to be answered: Can such modes of soil-management and crop-selection be made to give reasonable profits? Before we can hope that the millions who till the soils will join effectively in a radical scheme of soil-conservation, it must be made to appear that the scheme will give reasonable returns at every large stage of its progress; must pay, let us say, in the long run of a lifetime. We may fairly assume that intelligent people will be guided by the total returns of a lifetime in lieu of beguilement by the ultra-quick returns of forced and wasteful cropping in total neglect of later results. It may be assumed that he who tills a farm from his twentieth to his sixtieth year will find more satisfaction in the summed profits of forty crops of increasing value, enhanced by the higher value of his land at the end, even though the margin above cost be no greater, than in the sum of forty crops of decreasing values with a debased land at the end. Our practical problem is, therefore, to so improve processes, to so increase intelligent management, and to so exalt the point of view, that every large step in the processes proposed shall give satisfactory returns for the labor involved. How far this is practicable just now, I must leave to those whose technical knowledge in the practical art of tillage fits them to answer; but it is clear that if such protective measures are not profitable now, they must soon become so; for, if the loss of soil proceeds at the present rate and the number of inhabitants continues to increase as now, the value of the residue of tillable land which will remain after a few centuries will so appreciate as to force extreme measures for its conservation. The pitiable struggles of certain oriental peoples to retain and cultivate the scant remnants of once ample soils is both an example and a warning. Our escape from such a dire struggle should spring from a clearer forevision, a deeper insight, greater technical skill and indefatigable industry.
Note.—Much valuable literature bearing on this and kindred subjects will be found in the numerous publications of the U. S. Department of Agriculture and the several State Agricultural Stations. Notable among these is the Farmers' Bulletin No. 20 on "Washed Soils," and a special contribution to "Soil Erosion" by W J McGee. The fundamental work on "Rocks, Weathering and Soils," by Dr. Geo. P. Merrill, of the National Museum, is also to be noted. Particularly valuable are the writings of Professor F. H. King on "Soils," "Soil Physics" and "Soil Management," and, especially for the south and west, the work on "Soils," by Professor E. W. Hilgard.