CHAPTER XIX.
THE ANALYSIS OF VIRGIN SOILS BY
EXTRACTION WITH STRONG ACIDS.
As stated already, the analysis of soils by extraction with strong acids is intended to enlighten us, not in regard to their immediate productiveness (the “Düngerzustand” of German agricultural chemists), but as to their permanent value or productive capacity. As has been seen in the preceding chapter, the efforts to unite investigators upon a generally applicable and acceptable method for the testing of immediate productiveness have not been very successful, and the number of methods employed in different countries and by different chemists within the same country are widely at variance, with no immediate prospect of agreement. Moreover, in most cases the effort is to combine both problems—temporary and permanent productive capacity—in one method or operation; which still farther confuses the issue.
Convinced that the only way to unification lies in the direction of falling back upon a method that is based upon a natural limitation about which there can be no difference of opinion, the writer has, in following the lead of Owen and Robert Peter, endeavored to settle definitely the natural limit of the action of a suitable acid upon soils, and the time and strength of acid producing the maximum effect.
Loughridge’s Investigation.—Systematic work on these points was undertaken, at his suggestion, by Dr. R. H. Loughridge in 1871 and 1872. The results of this work were published in the succeeding year in the Amer. Journal of Science, and in the proceedings of the A. A. A. S. for 1873. They seem to be of sufficient general interest to be reproduced here.
The soil selected for this purpose was a very generalized one, representing large areas in the states of Kentucky, Tennessee, Mississippi and Louisiana, bordering on the east the immediate valley of the Mississippi river, and known locally as the “Table lands;” a noted cotton-producing upland region. The brown or yellow, moderately clayey loam is of great uniformity throughout its region of occurrence, and is evidently derived from such widely-spread sources that it represents no special rock or complex of rocks. Its natural growth is a mixture of oaks and hickories, strong and well-developed trees, such as any land-seeker would at once approve for settlement. Its cotton product when fresh was a 400-pound bale of cotton lint per acre. It may therefore well be considered a typical generalized soil of the humid upland of the Mississippi valley. Its physical analysis is given in chapter 6, it being [No. 219 of the table on p. 98].
Strength of Acid used.—Three different strengths of acid were simultaneously employed, viz., chlorhydric of 1.10, 1.115 and 1.160 density. With these the soil was digested at steam heat in porcelain beakers covered with watch glasses for five days each, then evaporated and analyzed as usual. The results were as follows:
ANALYSIS WITH ACID OF
DIFFERENT STRENGTHS.
| Ingredients. | Sp. G. of Acid. | ||
|---|---|---|---|
| 1.10 | 1.115 | 1.160 | |
| Insoluble residue | 71.88 | 70.53 | 74.15 |
| Soluble silica | 11.38 | 12.30 | 9.42 |
| Potash | .60 | .63 | .48 |
| Soda | .13 | .09 | .35 |
| Lime | .27 | .27 | .23 |
| Magnesia | .45 | .45 | .45 |
| Br. ox. Manganese | .06 | .06 | .06 |
| Ferric Oxid | 5.15 | 5.11 | 5.04 |
| Alumina | 6.84 | 8.09 | 6.22 |
| Sulfuric acid | .02 | .02 | .02 |
| Volatile matter | 3.14 | 3.14 | 3.14 |
| Total | 100.02 | 100.69 | 99.29 |
| Amount of soluble matter | 24.00 | 27.02 | 22.27 |
| Amount of soluble bases | 13.50 | 14.70 | 12.83 |
It will be noted that the strongest acid produced the smallest amount of decomposition of the soil silicates, e. g. the silica soluble in carbonate of soda solution being 3% less than in the case of the acid of medium strength; a result possibly due to some difficultly-soluble compound formed on the surface of the soil grains. The weakest acid had a stronger solvent power; but the maximum effect was produced by the acid of 1.115 density. This being also the most readily obtainable, by simple steam distillation of acid of any other strength, the writer adopted it as best suited to the purposes of soil analysis.
To ascertain the time required for the desired action, viz., the solution of the plant-food ingredients to the extent likely to be of any avail to growing plants, digestions of the same soil were made in the same manner for periods of 1, 3, 4, 5 and 10 days, with the acid of 1.115 density. The results were as follows: