CHAPTER XVIII.. The Physico-Chemical Investigation of Soils in Relation to Crop

THE PHYSICO-CHEMICAL INVESTIGATION OF SOILS IN RELATION TO CROP PRODUCTION.

The chemical constituents of soils have been incidentally mentioned and discussed above, both in connection with the processes of soil-formation, and with the minerals that mainly participate therein. The manner of their occurrence and their relations to plant life, so far as known, must now be considered more in detail.

HISTORICAL REVIEW OF SOIL INVESTIGATION.

While the obvious importance of the physical soil-conditions has long ago rendered them subjects of close study by Schübler Boussingault and others, the chemistry of soils was very generally neglected for a considerable period, after the hopes at first entertained by Liebig that chemical analysis would furnish a direct indication and measure of soil fertility, had been sorely disappointed in respect to the only soils then investigated, viz., the long-cultivated ones of Europe. The results of chemical analysis sometimes agreed, but as often pointedly disagreed, with cultural experiences; so that after the middle of the nineteenth century, but few thought it worth while to occupy their time in chemical soil analysis.

The early work of Schübler on soil physics, published at Leipzig in 1838 under the title of “Grundsätze der Agrikulturchemie” and now almost inaccessible outside of old libraries, is remarkable as having anticipated very definitely much that has since been brought forward and elaborated anew. He is really the father of agricultural physics.

Popular Forecasts of Soil Values.—In newly-settled countries, and still more in those yet to be settled, the questions of the immediate productive capacity, and the future durability of the virgin land are the burning ones, since they determine the future of thousands for weal or woe. This need has long ago led to approximate estimates made on the part of the settler, by the observation of the native growth, especially the tree growth; and where this consists of familiar species, normally developed, such estimates on the part of experienced men, based on previous cultural experience, are generally very accurate; so much so that in many of the newer states they have been adopted in determining not only the market value, but also the tax rate upon such lands, their productiveness, and probable durability being a matter of common note.

Thus in the long-leaf pine uplands of the Cotton States, the scattered settlements have fully demonstrated that after two or three years cropping with corn, ranging from as much as 25 bushels per acre the first year to ten and less the third, fertilization is absolutely necessary to farther paying cultivation. Should the short-leaved pine mingle with the long-leaved, production may hold out for from five to seven years. If oaks and hickory are superadded, as many as twelve years of good production without fertilization may be looked for by the farmer; and should the long-leaved pine disappear altogether, the mingled growth of oaks and short-leaved pine will encourage him to hope for from twelve to fifteen years of fair production without fertilization.

Corresponding estimates based upon the tree growth and in part also upon minor vegetation, are current in the richer lands also. The “black-oak and hickory uplands,” the “post-oak flats,” “hickory bottoms,” “gum bottoms,” “hackberry hammocks,” “post-oak prairie,” “red-cedar prairie,” and scores of other similar designations, possess a very definite meaning in the minds of farmers and are constantly used as a trustworthy basis for bargain and sale, and for crop estimates. Moreover, experienced men will even after many years’ cultivation be able to distinguish these various kinds of lands from one another.

Cogency of Conclusions based upon Native Growth.—Since the native vegetation normally represents the results of secular or even millennial adaptation of plants to climatic and soil-conditions, this use of the native flora seems eminently rational. Moreover, it is obvious that if we were able to interpret correctly the meaning of such vegetation with respect not only to cultural conditions and crops, but also as regards the exact physical and chemical nature of the soil, so as to recognize the causes of the observed vegetative preferences; we should be enabled to project that recognition into those cases where native vegetation is not present to serve as a guide; and we might thus render the physical and chemical examination of soils as useful practically, everywhere, as is, locally, the observation of the native growths. To a certain extent, such knowledge would be useful in determining the salient characters of cultivated soils, also; and would be the more useful and definite in its practical indications the more nearly the cultural history of the land is known, and the less the latter has been changed by fertilization. For, so soon as the first flush of production has passed, the question of how to fertilize most effectually and cheaply demands solution.

It was from this standpoint, suggested by his early experience in the Middle West and subsequently most impressively presented to him in the prosecution of the geological and agricultural survey of Mississippi, that the writer originally undertook, in 1857, the detailed study of the physical characters and chemical composition of soils. It seemed to him incredible that the well-defined and practically so important distinctions based on natural vegetation, everywhere recognized and continually acted upon by farmers and settlers, should not be traceable to definite physical and chemical differences in the respective lands, by competent, comprehensively-trained scientific observers, whose field of vision should be broad enough to embrace concurrently the several points of view—geological, physical, chemical and botanical—that must be conjointly considered in forming one’s judgment of land. Such trained observers should not merely do as well as the “untutored farmer,” but a great deal better.

“Ecological” studies.—Yet thus far we vainly seek in general agricultural literature for any systematic or consistent studies of these relations. We do find “ecological” lists of trees and other plants, or “plant associations,” growing in certain regions or land areas, described in some of the general terms which may refer equally well to lands of profuse productiveness, or to such as will hardly pay for taxes when cultivated. Or when the productive value is mentioned, the probable cause of such value is barely alluded to, even conjecturally, unless it be in describing the “plant formations” as xerophytic, mesophytic or hydrophytic, upon the arbitrary assumption that moisture is the only governing factor; wholly ignoring such vitally important factors as the physical texture of the soil, its depth, the nature of the substrata, and the (oftentimes abundantly obvious) predominant chemical nature of the land. And on the other hand, we find even public surveys proceeding upon the basis of physical data alone, practically ignoring the botanical and chemical point of view, and inferentially denying, or at least ignoring, their relevancy to the practical problems of the farm.

Bull. 22, Bureau of Soils, U. S. Dept. of Agriculture.

Early Soil Surveys of Kentucky, Arkansas and Mississippi.—Among the few who during the middle of the past century maintained their belief in the possibility of practically useful results from direct soil investigation, were Drs. David Dale Owen and Robert Peter, who prosecuted such work extensively in connection with the geological and agricultural surveys of Kentucky and Arkansas; and the writer, who carried out similar work in the states of Mississippi and Louisiana, with results in many respects so definite that he has ever since regarded this as a most fruitful study, and has later continued it in California and the Pacific Northwest. This was done in the face of almost uniform discouragement from agricultural chemists until within the last two decades; with occasional severe criticisms of this work as a waste of labor and of public funds.

Investigation of Cultivated Soils.—All this opposition was largely due to the prejudices engendered by the futile attempts to deduce practically useful results from the chemical analysis of soils long cultivated, without first studying the less complex phenomena of virgin soils; and these prejudices persisted longest in the United States, even though in Europe the reaction against the hasty rejection of chemical soil work had begun some time before; as is evidenced by the methods employed at the Rothamsted Experimental Farm in England, the Agricultural College of France, the Russian agronomic surveys, and at several points in Germany. In none of these cases, however, more than the purely chemical or physico-chemical standpoint was assumed; although in Russia at least, virgin soils were easily obtainable and their native growth verifiable; and were actually in part made the subject of chemical investigation.

In the course of their work, Owen and Peter always carefully recorded the native vegetation of the soils collected; but neither seems to have formulated definitely the idea that such vegetation might be made the basis of direct correlation of soil-composition with cultural experience. Owen repeatedly expressed to the writer his conviction that such a correlation could be definitely established by close study; but early death prevented his personal elaboration of the results of his work. Peter likewise stoutly maintained to the last his conviction that soil analysis was the key to the forecasting of cultural possibilities; but not being a botanist he did not see his way to put such forecasts into definite form.

Change of Views.

In the United States as well, the ancient prejudices have now gradually given way before the urgent call for more definite information than could otherwise possibly be given to farmers by the experiment stations, most of whose cultural experiments, made without any definite knowledge of the nature of the soil under trial, were found to be of little value outside of their own experimental fields. Even the multiplication of culture stations in several states, unaccompanied by soil research, is found to be a delusive repetition of the same inconclusive, random experimenting, since it takes into consideration only the climatic differences, but leaves out of consideration the potent factors of soil quality and soil variations. At most these were usually mentioned by them in such indefinite terms as “a clay loam,” “a coarse sandy soil,” “gray sediment land,” and the like; frequently not even with a statement of the depth and character of the subsoil and substrata, much less of their geological derivation or correlations. Thus any one not happening to be personally acquainted with the land in question would be wholly without definite data to correlate the results with his own case. It is quite obvious that even if only to make possible the identification of new lands with others that have already fallen under cultural experience, and can therefore afford useful indications to the new settler, a close physical and chemical characterization of lands should be made the special object of study by the experiment stations and public surveys, particularly in the newer states.

Advantages for Soil Study offered by Virgin Lands.—Among the special advantages, then, offered by virgin soils for the study of the correlations of soils and crops, the usual existence of a native flora, representing the results of secular adaptation, is of fundamental importance. As it is at this time still historically known of most lands west of the Alleghenies what was their original timber growth, it is clear that their original condition can still be ascertained by comparison with uncultivated lands of similar growth, usually not very far away; and as their cultural history also is largely within the memory of the living generation, the behavior of such lands under cultivation is known or verifiable. Foremost among the data thus ascertainable is the duration of satisfactory crop production, and its average amount. To ascertain these surviving data by inquiry among the farming population should be among the foremost duties of those connected with soil surveys; and persons temperamentally unable to enlist the farmer’s sympathy and interest in such inquiries must be considered seriously handicapped, no matter what their scientific qualifications may be. In no quest is it more literally true that there is no one from whom the earnest inquirer may not learn something worth knowing.

Practical Utility of Chemical Soil-Analysis; Permanent Value vs. Immediate Productiveness.—In many existing treatises so much emphasis is given to the alleged proofs of the inutility of chemical soil examination in particular, that a special controversion of these arguments seems necessary, in connection with a detailed statement of what can, and in part has been, done in that direction. Hence the often-repeated allusion, in the sequel, to points bearing on this question. Hence, also, the detailed discussion of many points which in most agricultural publications are given only passing notice.

In all these discussions the difference between the ascertainment of the permanent-productive value of soils, as against that of their immediate producing capacity, must be strictly kept in view. The former interests vitally the permanent settler or farmer; the latter concerns the immediate outlook for crop production, the “Düngerzustand” of the Germans. The methods for the ascertainment of these two factors are wholly distinct, even though the results and their causes are in most cases intimately correlated. The failure to observe this distinction accounts for a great deal of the obloquy and reproach that has in the past so often been heaped upon chemical soil-analysis and its advocates.

PHYSICAL AND CHEMICAL CONDITIONS OF PLANT GROWTH.

While it is true that plants cannot form their substance or develop healthy growth in the absence or scarcity of the chemical ingredients mentioned on page xxxi of this volume, it is also true that they cannot use these unless the physical conditions of normal vegetation are first fulfilled. Both sets of conditions are intrinsically equally important and exacting as to their fulfilment; and the farmers’ task is to bring about this concurrence to the utmost extent possible. The chemical ingredients of plant-food can, however, be artificially supplied in the form of fertilizers, should they be deficient in the soil; but as has been shown in the preceding pages, it is not always possible to correct, within the limits of farm economy, physical defects existing in the land. Hence, however important is the natural richness of the soil in plant-food, the first care should always be given to the ascertainment of the proper physical conditions in the soil, subsoil and substrata. Without these, oftentimes, no amount of cultivation, fertilization and irrigation is effective in assuring profitable cultural results.

Condition of the Plant-food Ingredients in the Soil.—But even the abundant presence of the plant-food ingredients, as shown by analysis, will not avail, unless at least an adequate portion of the same exists in a form or forms accessible to plants. Of course this condition would seem to be best fulfilled by the ingredients in question being in the water-soluble condition. But in the first place, plants are quite sensitive to an over-supply of soluble mineral salts, as is evidenced by the injurious effects produced by the latter in saline and alkali lands. Furthermore, substances in that form would be very liable to be washed or leached out of the soil by heavy rains or irrigation, and would be lost in the country drainage. It is therefore clearly desirable that only a relatively small proportion of the useful soil-ingredients should be in the water-soluble or physically absorbed condition, but that a larger supply should be present in forms not so easily soluble, yet accessible to the solvent action which the acids of the soil and of the roots of plants are capable of exercising. This virtually available supply we may designate as the reserve food-store.

Finally, there is practically in all soils a certain proportion of the soil-minerals in their original form, as they existed in the rock-materials from which the soil was formed. These minerals being usually in a more or less finely divided or pulverulent condition, they are attacked much more rapidly by the chemically-acting “weathering” agencies, viz., water, oxygen, carbonic and humus acids, than when in solid masses; and thus, transformation of the inert rock-powder into the other two classes of mineral soil-ingredients progresses in naturally fertile soils with sufficient rapidity to produce, in a single season, sensible and practically important results, known as the effects of fallowing.

The Reserve.—The nature of these processes has been discussed in chapters 1 to 4; and it will be remembered that two of their most prominent results are the formation of clay, and of zeolitic-compounds, the latter being, as heretofore stated (pp. 36 ff) hydrous silicates of earths and alkalies, easily decomposable by acids, and also capable of exchanging part or the whole of such basic ingredients with solutions of others that may enter the soil. These zeolitic compounds therefore fulfil two important functions in the premises, viz.: a ready yielding-up of part of their ingredients to acid solvents, and a tendency to fix, by exchange, a portion or the whole of the soluble compounds that may be set free in, or brought upon the land. The first-mentioned property is of direct avail in that the soil-humus forms, and the roots of plants exude, acid solvents on their surface, and can thus draw upon the reserve store of food; the second tells in the direction of preventing the waste of water-soluble manurial ingredients supplied to, or formed in the soil. (See above,