COMPOSITION OF LIMESTONE,
AND RESIDUAL SOIL AND SUBSOIL,
FROM BLACK PRAIRIE,
MONROE CO., MISSISSIPPI.
| “Rotten Limestone.” | “Subsoil (YELLOW)” | “Soil (BLACK)” | |
|---|---|---|---|
| Fine Earth. Chemical analysis of fine earth. | Depth 12 ft. | 2-3 ft. | 15 ins. |
| Insoluble matter | 10.90 | 71.54 | 78.29 |
| Soluble silica | |||
| Potash (K₂O) | .25 | .54 | .33 |
| Soda (Na₂O) | .32 | .23 | .08 |
| Lime (CaO) | 45.79 | 1.08 | 1.37 |
| Magnesia (MgO) | .88 | .77 | .36 |
| Br. Ox., of Manganese (Mn₃O₄) | .05 | .14 | |
| Peroxide of Iron (FeO) | 1.42 | 5.42 | 14.22 |
| Alumina (Al₂O₃) | 1.96 | 13.15 | |
| Phosphoric Acid (P₂O₅) | .05 | .10 | |
| Sulfuric Acid (SO₃) | .04 | .03 | |
| Carbonic Acid (CO₂) | 35.73 | ||
| Water and Organic matter | 2.84 | 6.99 | 5.75 |
| Total | 100.09 | 99.86 | 100.67 |
| Humus | 1.93 | ||
| “ Ash | 4.38 | ||
| Hygroscopic moisture | 10.35 | 12.82 | |
| absorbed at °C | 19° | 19° |
It appears from the above table that in the change from the original limestone to the soil mass as found at three feet depth, 81.5% of the lime carbonate has been eliminated by leaching, leaving behind somewhat less than one fifth of the original mass. Taking the average depth of the soil mass at 8 feet, this thickness of material has required about 45 feet of the rotten limestone. Considering that notwithstanding the tenacity of the clay soil, some of it must in the course of time have been washed away, we may safely assume that the original rock surface was from 50 to 60 feet higher than at present.
Sandstone Soils.—The indefiniteness of the nature of “sandstones” as such renders generalizations in regard to the soils formed from them rather difficult, save as to their physical qualities, which in the nature of the case are always “light.” In the Old World and in the humid region generally, sandstone and sandy soils are usually spoken of as being poor, because there the sand almost always consists of quartz grains only, and hence the fine portions alone can be looked to for plant nutrition. Consequently, the more sand is seen in a soil, the poorer it is usually presumed to be. But this presumption would be wholly erroneous in the arid regions. ([See chapt. 6, p. 86]).
Clearly, the nature of the soils produced by the weathering of sandstones depends upon two points: first, the nature of the cement binding the sand grains, and second the character of the latter themselves.
Varieties of Sandstones.—As has been stated above, the cements may be roughly classified into five kinds, and their intermixtures, to wit: quartzose or siliceous, calcareous, ferruginous, aluminous or clayey, and zeolitic. As regards the first, it is obvious that siliceous sandstones will disintegrate with great difficulty, since neither the cement nor the grains are susceptible of material change by weathering. Such sandstones frequently pass insensibly into quartz rock, and the light, unsubstantial soils they produce are of the poorest, containing often mere traces of the plant-food ingredients. This of course, is true, not only of the soils formed by the actual weathering of sandstones, but equally of those consisting of quartz-sand deposited by water or drifted by winds.
Of this character are the pine-forest soils of the coast region of the Gulf of Mexico, particularly the “Sand hammocks” of the immediate Gulf border, from Mississippi Sound to Charlotte Harbor, Florida; the sandy lands of the Grand Traverse region of Michigan, and many other minor areas in the United States, usually characterized by a pine growth, often more or less stunted, according to the nature of the sand grains.
Calcareous sandstones usually form a very much better class of soils, partly for the intrinsic reason given above as regards limestones as soil-formers. The calcareous cement is very rarely pure calcite; in most cases it is very impure, as, most commonly, is also the “sand” itself. This is explained from the fact that such rocks (mostly soft and often quite unconsolidated) are, like limestones themselves, the result of deposition in shallow seas or lakes, receiving deposits from the land drainage, and enriched by the animal and vegetable life of such waters. Not uncommonly they contain, disseminated through them, grains of the mineral glauconite (a hydrous silicate of iron and potash), which readily supplies available potash; while the remnants of animals and plants furnish more or less of available phosphates. Thus the general presumption regarding calcareous sandstones is that the derived soils are of good quality, frequently of the very best. The same, however, does not appear to be true of sandstones cemented by dolomite; the soils derived from magnesian sandstones are in many cases noted for their unproductiveness. ([See chapt. 3, p. 42]).
Ferruginous Sandstones manifestly derive no important soil ingredients from their cement when the latter is measurably pure ferric hydrate; and when in addition the sand itself is purely siliceous, the soils resulting from the disintegration of the rocks are very poor.
Such are, e.g., the soils derived from the ferruginous sandstones of the Lafayette formation in a part of northern Mississippi and adjacent portions of Tennessee and Alabama, characterized by small scrubby oak or dwarfed pine. On the whole, however, such purely ferruginous quartz sandstones are exceptional, and should not detract from the favorable inferences usually to be drawn from the iron-rust tint of soils ([see chapter 15]).