Soils, their formation, properties, composition, and relations to climate and plant growth in the humid and arid regions

by Eugene W. Hilgard

APPENDIX B.

SUMMARY DIRECTIONS FOR SOIL—
EXAMINATION IN THE FIELD OR ON THE FARM.

While the general principles upon which the cultural value and adaptations of lands should be judged, have been given in the text of this volume, it seems advisable to summarize their practical application to land examination here, for convenient reference.

The directions given in [Appendix A] for the sampling of soils having been carried out, the samples so taken may be subjected to farther examination by any intelligent farmer to good purpose, and often with great saving of time and expense.

Spread the samples from the several depths in regular order upon a table or bench, and note the differences in color and texture apparent to the eye or touch, and whether they will or will not crush readily between the fingers, wet and dry. Whatever the fingers can do, can similarly be done by the harrow, cultivator, clod crusher or roller.

The tilling qualities of the surface soil and immediate subsoil are the first and most important matter to be ascertained; including especially their behavior to water. Place some air-dried lumps in a shallow dish with a little water; observe whether they take up the water quickly or slowly, and whether in so doing the lumps fall to pieces or retain their form. Slow penetration, and maintenance of form, will at once indicate a soil somewhat refractory and difficult to till; while if the water is taken up easily and the lump falls to pieces, the land is easily cultivated and will absorb the rainfall and irrigation water readily. The darkening of the tint on wetting will also give an approximate idea of its humus-content.

Then take a wetted lump and work it between the fingers and on the palm of the hand, until its “stickiness” or adhesiveness ceases to increase. This “hand test” is of first importance and in skillful hands will largely supersede the need of elaborate mechanical analysis. It will at once enable the operator to classify the soil as a light or heavy loam, clay loam or clay soil; it will show directly what will be the result of plowing the land when wet, the liability to the formation of a plowsole, and whether a single or a double team will generally be needed to cultivate it properly. Also whether stock can be allowed to pasture the land soon after rain. Comparison with the known land of neighbors will also thus become easy, and in a measure the crops best adapted to the physical qualities of the soil, subsoil and substrata, taking into account their respective depths, will at once be at least approximately determined. The presence of coarse and fine sand in greater or less amounts will also be thus readily ascertained, allowing estimates of the percolative properties; the latter can, of course, be more practically tested in the field, in the manner described in [chapter 13, page 242].

A more definite estimate of the amount and kind of sand present in the soil materials can be obtained by washing the kneaded sample into a tumbler, and allowing a thin stream of water to flow into it from a faucet while gently stirring the turbid water. The clay, together with the finest silts, will thus be carried off over the rim of the glass, and sand of any desired degree of fineness, according to the strength of the stream of water used, will be left behind. The kind and amount of these sandy materials can then be estimated, or definitely ascertained by weighing or measuring.

This will, generally speaking, be as far as the uninstructed farmer can readily go; but these simple operations will give him an insight into the nature of his soil and subsoil that will enable him to avoid a great many costly mistakes.

RECOGNITION AND MEANING OF
THE SEVERAL SOIL MINERALS.[203]

Those somewhat familiar with scientific methods and operations, and supplied with pocket lens or microscope, can profitably go much farther towards the definite ascertainment of the permanent cultural value of the land, by the study of the minerals of which the sand is composed, and which as a rule represent those from which the entire soil has been formed; therefore indicate in a general manner its chemical composition. Such examinations are specially feasible and important when soils are not far removed from their parent rocks, as in most of the arid region, and in the states north of the Ohio. In the Southwestern states, in the coastal plain of the Gulf of Mexico, the original soil minerals have usually been too far decomposed to admit of definite identification. Sand is there as a rule made up of quartz grains of many varieties, with only an occasional tourmaline and pyroxene.

Among the prominent soil minerals, quartz is almost always recognizable by its glassy luster and the irregular fracture—absence of definite planes or facets of cleavage, causing the grains to be abraded or rounded nearly alike in all directions. The feldspars, on the contrary, always show a tendency to cleave into fragments having definite, obviously oblique angles, which are perceptible even when the grains are somewhat worn; because of the difference in the ease with which wear takes place in the several directions. Potash feldspar, moreover, which is the most important to be recognized because it indicates a relatively large supply of potash in the natural soil, is but rarely glassy in luster, but mostly dull white, or reddish-white.—The lime and lime-soda feldspars rarely show as definite forms, because of their tendency to form complex crystalline aggregates (twins): and their definite recognition requires somewhat complex (polarizing) appliances in connection with the microscope. In such cases, however, the accompanying minerals (hornblende, pyroxene, mica and others) often afford valuable indications, because of their known association with soda-lime feldspars in certain rocks.

An abundant occurrence of hornblende fragments, characterized by their flat, tabular form, and bottle-green or black tint, indicate, almost always a fairly good supply of lime in the soil, but leaves the potash-supply in doubt. Pyroxene (distinguished by its smooth, polished surface from the angularly-weathering, usually rusty hornblende fragments) rarely occurs with potash feldspar; and soils strongly charged with it are mostly poor in potash.

Mica occurs in so many rocks and is of so little consequence as a soil-ingredient from the chemical point of view, because of its difficult decomposition, that its presence can mostly only serve to corroborate or contradict conclusions as to the derivation of a soil from some particular rock or region. But mica serves a good purpose in improving the tilling qualities of soils. Its thin scales must not be mistaken for the tabular fragments of hornblende.

Calcite in its several forms is mostly easily recognized both by its form under the microscope, and by the effervescence its granules show when touched with an acid. This effervescence can generally be observed on touching the wetted soil with chlorhydric acid, so soon as the content exceeds two per cent; but something depends upon the size of the grains, as when these are very small, the giving-off of gas is less readily noted. To facilitate it, the wetted soil may be warmed before touching it with the acid. The recognition of the presence of lime carbonate in soils is so important as to justify considerable trouble in rendering it definite. When the aid of a chemist cannot be commanded, fairly definite conclusions may be drawn from the character of the native vegetation; regarding which, detailed information may be found in Parts III and IV of this volume. But where, as in the arid region, this criterion is not available, since the controlling factor there is the moisture supply, a presumption may be gained by the application of a slip of moistened red litmus paper to the wetted soil. Should the red paper be turned blue within one or two minutes it would indicate the presence of carbonate of soda (“black alkali”) as well as of lime carbonates: but if blued only after twenty minutes or more, it would indicate the presence of the carbonates of lime and magnesia. If not changed at all, the conclusion would be that either lime carbonate is in very small supply, or that the soil is in an acid condition. ([See chapter 8, p. 122]).

Saline and Alkali Soils.—The presence of an unusual or injurious amount of soluble salts, as in the case of sea-coast and alkali soils, is commonly easily ascertained in the field; where, if the surface soil is at all seriously contaminated with soluble salts of any kind, these may be seen on the surface during a dry season, forming a whitish efflorescence, which in most cases is definitely crystalline. In doubtful cases a tablespoonful of the surface soil may be leached with water, and the first ten or fifteen drops caught in a clean, bright silver spoon and evaporated. Or the soil may be stirred up with about twice its bulk of water and the mixture be allowed to clear by settling, then evaporating. A slight whitish film will almost always remain in the spoon; but if the amount be somewhat considerable, the presence of soluble salts is very readily recognized by pouring a few drops of clear water on one side of the spot, and then allowing it to flow gently over the spot to another place, where it is again slowly evaporated. Any considerable amount of salts present will be shown both in the diminution of the original spot, and in the soluble residue accumulated where the water was last evaporated. Should common salt be present to any considerable extent, the residue in the silver spoon will, if the last drops be allowed to evaporate slowly, show square or cubical crystals to the naked eye, and certainly to a common pocket lens. The residue may also be tested with red litmus paper for carbonate of soda, which would quickly turn it blue.

More detailed examination requires chemical reagents and experience, but the above tests should be sufficient to prevent the mistaking of mere white spots, whose humus has been destroyed by fermentation caused by bad drainage, with true alkali caused by excess of soluble salts; a mistake not uncommon in both the arid and humid regions.