Chapter III.. Soil Analysis and the Historical Methods of Soil Investigation.

SOIL ANALYSIS AND THE HISTORICAL METHODS OF SOIL INVESTIGATION.

Owing to the labors of Davy, Boussingault, de Saussure, Liebig, Sachs, Knop, Salm-Horstmar, and other scarcely less distinguished savants, it has been clearly shown that growing plants need certain mineral elements in order to maintain their metabolic functions, and that these mineral elements can be obtained, under normal conditions, from the soil. All subsequent investigation has confirmed these statements and they can now be accepted as facts with as much assurance as any known law of nature.

The determination and formulation of these two fundamental facts came at a time when analytical chemistry was being rapidly developed and was finding wide and useful applications in numerous fields of activity. It was natural, therefore, that analytical chemistry should be enlisted in this new field of work, obviously of the first importance to the welfare of mankind. It was early found, however, that the chemical analysis of a soil fails to explain its relative productivity. In other words the content of a soil with respect to potash, phosphoric acid, or other mineral plant-food constituent, bears no necessary relation to its crop-producing power. Many cases were found where one soil “analyzed well” but did not produce as large a crop as another soil which “analyzed poor.” To meet this difficulty a subsidiary hypothesis was brought forward, which rapidly gained general acceptance although lacking experimental support.

See also, Die Aufnahme der Nährstoffe aus dem Boden durch die Pflanzen, von J. König und E. Haselhoff, Landw. Jahrb., 23, 1009, 1030, (1894).

This hypothesis supposes that the mineral constituents of the soil are present in two different chemical conditions or distinct kinds of combinations, one of which readily gives up its constituents to growing plants, while the other does not; and the constituents have, therefore, been called respectively “available” and “non-available.” It would appear from his writings that Liebig regarded this distinction as applying to the “absorbed” or “adsorbed” mineral matter; that is, on the one hand the material held in or upon the soil grains by surface forces, and on the other the chemically combined constituents in the minerals themselves. We know that Liebig was much impressed by the absorption experiments of Way, and himself did much work in this field. But the great body of soil investigators has evidently held to the opinion that there are two general classes of minerals in the soil. Some have held that the “available” potassium is held in zeolites or “zeolitic” minerals, an interesting example often cited being glauconite or “green sand marl,” which sometimes contains phosphorus as well as potassium; in minerals which are easily broken down by alkaline solutions, as by sodium carbonate solutions or ammonia; or in minerals which are easily broken down by organic acids supposedly excreted from the roots of growing plants, or formed by the decay of plant tissue.

Way was misled, as we now know, in considering the results of his absorption experiments with soils as merely metathetical reactions; see Absorption by soils, by Harrison E. Patten and William H. Waggaman, Bull. No. =52=, Bureau of Soils, U. S. Dept. Agriculture, 1908.

The formation of zeolites in the soil has often been assumed, but has not yet been proven; see Rocks, rock-weathering and soils, by George P. Merrill, 1906, p. 363.

The classic experiments of Sachs, in producing etchings on marble slabs, and the etchings observed occasionally on rock surfaces are the proofs universally cited. The experiments of Czapek, who substituted slabs of aluminum phosphate and other substances for the marble, and those of Kossowitch, show that the action can be accounted for more satisfactorily and reasonably as due to dissolved carbon dioxide. In fact such etchings can be produced on marble slabs by laying platinum wires upon them and covering with moist soil, or cotton, or mats of filter-paper; see Bull. No. =22=, p. 14, and Bull. No. =30=, p. 41, Bureau of Soils, U. S. Dept. Agriculture.

With the advent of this idea of a distinction between the available and non-available mineral plant-food elements in the soil, came attempts to distinguish them by analytical methods. Of these attempts we now have a bewildering array, most of them frankly empirical. For instance, Hilgard, in his classical investigation of the cotton soils for the Tenth Census, treated his soil samples with an excess of hydrochloric acid, evaporated to dryness, extracted with water, and regarded the extracted mineral constituents as available. In Germany, a method similar to Hilgard’s is now in common use, while in France nitric acid is preferred generally because it is supposed to have peculiar solvent powers on soil phosphates. In the United States the “official method” of the Association of Official Agricultural Chemists is to keep 10 grams of the soil in contact with 100 cc. of a solution of hydrochloric acid (specific gravity 1.115) at the boiling point of water for exactly 10 hours. In England the popular method is that proposed by Dyer, namely, to treat the soil with a 1 per cent. citric acid solution, this strength of solution being supposed at one time to represent the average acidity of root sap. Maxwell, in Hawaii, and afterwards in Australia, claimed good results for the extraction of the soil with a 1 per cent. solution of aspartic acid, this acid being employed on the erroneous ground that the organic acids of the soil are amino acids, and that these are the effective agents in dissolving the soil minerals and rendering their constituents “available.” The Kentucky Agricultural Experiment Station favors an N/5 nitric acid solution, but does not recommend its use for soils of other localities, while in a contiguous state, the Tennessee Station favors the “official” method. Many other methods have been proposed, but the foregoing are typical and sufficient to illustrate the present status of soil analysis.

Soils, by A. M. Peter and S. D. Averitt, Bull. No. 126, p. 66, (1906).

The soils of Tennessee, by Charles A. Mooers, Bull. No. 78, p. 49, (1906).

It is clear that these several methods must give differing results. And it is not clear that any one of them is to be preferred to the others for any reasons than analytical convenience. There is no reason to expect that the proportion of solvent to soil required in these methods bears any relation whatever to the mechanism of absorption by plant roots. And the attempts to simulate the properties of plant sap in some of these solvents are obviously illogical, for the plant sap does not come in contact with the soil grains, except through an accidental destruction of the plant.

Naturally, comparisons were attempted between the amounts of the mineral constituents extracted from a soil by these various solvents and the amounts taken up by crops growing on the soil. It was found, however, that the amount of any given mineral constituent extracted from the soil by a solvent is not, generally, the same as that taken up by the plant. Moreover, the ratio of one constituent to another in the extract bears no definite relation to the ratio of these constituents in the plant. Nevertheless many efforts were made to establish “factors.” For instance, the percentage of potash extracted from the soil of a field by hydrochloric acid is some multiple of the percentage removed by a wheat crop; it was sought to determine this multiple, assuming it to be a definite ratio and a natural constant, and it was designated as the potash factor. But there is a different factor for phosphorus, another for calcium, and still others for each and every constituent. The factors found for a soil from one area generally do not hold for a soil from another area. Again, different factors obviously must be used for different crops. And, finally, the whole scheme becomes hopeless when it is realized that the same crop will yield widely varying ash analyses, depending upon the cultural methods employed, the judicious selection of seed, the amount and distribution of rainfall and sunlight, and possibly other agencies, all of which affect the growth and absorptive functions of the plant to as great an extent as does the particular soil upon which it may be growing.

Moreover, from the purely analytical point of view the situation is no better. For instance, the addition of potassium in the amounts usually employed in ordinary fertilizer practice generally does produce a noticeable effect on the yield of crop. The average application of potash (K₂O) is certainly less than 50 lbs. to the acre. It is customary to consider the surface foot of soil as the region affected by the fertilizer, and an acre foot in good moisture condition weighs about 4,000,000 lbs. To be conservative, let it be assumed that 60 lbs. of potash have been added to 3,000,000 lbs. of soil. The official method of the Association of Official Agricultural Chemists calls for the determination of the potash in 2 grams of soil, which on the basis of the present assumption calls for the estimation of an added amount of 0.00004 gram of potash or 0.002 per cent. Taking as an example the report of the Association of Official Agricultural Chemists for 1895 there are given the following results obtained independently by a number of analysts, on soils which had presumably been sampled by the referee with all possible care:

Proceedings of the Twelfth Annual Convention of the Association of Official Agricultural Chemists, Bull. No. 47, Division of Chemistry, U. S. Dept. Agriculture, p. 36, (1896).

POTASH CALCULATED AS PER CENT. OF THE FINE DRIED EARTH.

============================================================= | 1 | 2 | 3 | 4 Analyst +-----+------+-----+------+-----+-----+-----+-------- | Per | | Per | | Per | | Per | |cent.| Var.|cent.| Var.|cent.| Var.|cent.| Var. --------+-----+------+-----+------+-----+-----+-----+-------- A |0.359| 0.044|0.154|-0.002| — | — | — | — B |0.345| 0.030|0.112|-0.044|0.380|0.051|0.104|-0.050 C |0.354| 0.039|0.235| 0.079|0.396|0.067|0.225| 0.071 D |0.260|-0.055| — | — | — | — | — | — E |0.373| 0.058|0.179| 0.023|0.365|0.036|0.175| 0.021 F |0.210|-0.105|0.130|-0.026|0.220|0.109|0.109|-0.045 G |0.304|-0.011|0.125|-0.031|0.286|0.043|0.158| 0.004 Mean |0.315| — |0.156| — |0.329| — |0.154| — --------+-----+------+-----+------+-----+-----+-----+--------

Not only do the individual determinations show differences far in excess of 0.002 per cent., but the differences between each individual reading and the mean is greater than 0.002 per cent., so that it is evident from these results that the analytical procedure fails to recognize appreciable amounts of the so-called available plant foods. Consequently the “acid digestion” of a soil fails of the purpose for which it was designed, and it is one of the mysteries of chemical history that so much time and energy have been devoted to such a hopeless quest.

This state of affairs is the more surprising when the limitations of the analytical procedure are considered. The data tabulated above indicate that the analyses were made with an exactness that justifies a statement to three decimal places, that is, to three significant figures; and in fact, as was shown, such is necessary if the figures are to have any significance regarding fertilizer applications. It is obvious that the analysis of a finely pulverized definite mineral or rock is less subject to error than a sample of soil sifted through a 2 mm. mesh. Yet the U. S. Geological Survey commonly reports its analytical data to only hundredths of a per cent., that is, to two decimal places. What variation may be expected in duplicate determinations by the same analysts it is difficult to say, for such duplicates are not commonly published. In spite of the widespread view that the chemical analysis of a soil is a statement of great accuracy, it is improbable that as usually determined the potash content is correct to three or even two significant figures; it is also doubtful if the phosphoric acid content is correct to even one significant figure, if the total amount is below 0.1 per cent. of the soil. That these determinations have a higher accuracy than here stated is not shown by an inspection of the literature including the fairly numerous results reported in the annual Proceedings of the Association of Official Agricultural Chemists.

See: On the interpretation of mineral analyses, by S. L. Penfield, Amer. Jour. Sci., (4), 10, 33, (1900); The analysis of silicate and carbonate rocks, by W. F. Hillebrand, Bull. No. 305. U. S. Geol. Surv., 1907; Manual of the chemical analysis of rocks, by H. S. Washington, 1904, p. 24; Über Genauigkeit von Gesteinanalysen, von M. Dittrich, Neues Jahrbuch für Mineralogie und Palaeontologie, 2, 69, (1903).

It was early felt by some investigators that soil analyses were unsatisfactory for studying the relation of the soil to the food requirements of a crop, and a second method was devised, namely, the growing of a crop, and determining the amount of mineral constituents removed from the soil by analyzing the ash of the crop. From the point of view of practical soil management this procedure involves the serious difficulty of being first obliged to get the crop before determining what must be done to best get it. It apparently has the scientific advantage of directness in determining the mineral needs of the plant from the plant itself. If these needs were constant, the advantage would be real, but as already mentioned, one and the same plant may have a very different ash content as the result of different cultural methods, different climatic and seasonal factors, as well as different soils. Generally, a poor crop has a higher percentage of ash content than a good crop, and sometimes the poor crop may remove from the soil more in absolute amounts of some one or other of the ash constituents than does the good crop. The ratio of the ash constituents is by no means constant for any one crop, and of course varies with different crops. Finally, it is now known that the amount of the several mineral nutrients which a soil must furnish to a crop in the earlier stages of growth is greater than the crop contents at maturity, consequently an analysis of the ripe crop would not indicate the plant’s drain upon the soil at all growing periods. So that, while ash analyses have taught some important things concerning plant growth, they have of necessity failed as guides or criteria of the crop-producing power of a soil, its fertilizer requirements, or its content of “available” plant-food.

For a brief but comprehensive discussion of ash analyses see, The ash constituents of plants, etc., by B. Tollens, Expt. Sta. Rec., 13, 207-220, 305-317, (1901-02).

Über die Nährstoffaufnahme der Pflanzen in verschiedenen Zeiten ihres Wachstums, von Wilfarth, Römer und Wimmer. Landw. Vers. Sta., 63, 1-70, (1905); Plant food removed from growing plants by rain or dew, by J. A. Le Clerc and J. F. Breazeale, Year Book, U. S. Dept. Agriculture, 1908, p. 389-402.

A third method of soil investigation, also essentially analytical in character, is the plot or pot test. The difference between a plot or pot experiment is mainly one of size, although it is claimed, and with a certain amount of justice, that the plot experiment more nearly approximates actual practice, and should be given a somewhat different consideration than the more readily controlled pot experiment. Here again it has to be considered that seasonal factors and factors other than the soil play a relatively large part in the production of the crop, so that conclusions regarding the productivity of a soil can not be drawn from one season’s crop. Also, nowadays it is recognized generally that continuous growing of one crop is an incorrect practice, and a rotation should be followed and repeated several times before conclusions regarding the productivity of the soil are justified. If, however, the rotation has been well managed, the cultivation, fertilizing and soil management generally been well done for sixteen, twenty or more years, the soil has materially changed, and there can be no assurance that the treatment then best for it, is that which was best at the beginning of the experiment. Therefore the method throws no certain light on the productive power of the soil, or the availability of its mineral plant-food constituents. Although much has been learned from plot experiments, and especially from the better controlled pot experiments, they are inadequate to meet the fundamental problem of the relation of the chemical characteristics of the soil to its crop-producing powers.