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

by Eugene W. Hilgard

Other silicate minerals, so far as they contain the same bases, are acted upon similarly to the feldspars.

In the decomposition of the feldspars by carbonated water, the compounds of potash and soda so formed are soluble in water, those of lime and magnesia are insoluble or nearly so. Hence pure clays can be formed only in the decomposition of the potash-and soda-feldspars (orthoclase, albite) while in the case of lime feldspar (labradorite) and the mixed feldspars (plagioclase, anorthite) calcareous clays (marls) are the result. Lime feldspar resists decomposition more tenaciously than do those containing large proportions of the strong bases potash and soda; potash feldspar especially is attacked most readily, and is the main source of the formation of the valuable deposits of porcelain earth or kaolin, which is essentially a mixture of kaolinite with fine silex and more or less of undecomposed feldspar, and is of a chalky texture.

Formation of Clays.—When instead of remaining in place, this kaolin is washed away and triturated in the transportation by water, it is partially changed from its original chalky condition to that plastic and adhesive form which is the characteristic ingredient of all clays. The remarkable properties of this substance and the part it plays in the physical constitution of soils, will be discussed in another chapter. Its lightness and extreme fineness of grain (if grain it can be called) cause it to be carried farther on by the streams than any other portion of the products of rock decomposition save those actually in solution; it can therefore be deposited only in water that is almost or quite still (as in swamps) so long as the latter is fresh. So soon however as brackish or salt water is encountered, clay promptly gathers into floccules (“flocculates”), and thus enveloping the finest-grained silts that may have been carried along with it, it quickly settles down, forming the “mud banks” and heavy clay soils that are so characteristic of the lower deltas of rivers, as well as of swamps formed by the backwater or overflow of the same.

When instead of potash feldspar alone, the lime- or soda-lime feldspars are also concerned in the decomposition process, the resulting clay soils will be more or less calcareous, while the soda, as stated above, is for the greater part leached out permanently.

Hornblende (Amphibole) and Pyroxene (Augite). These are two very widely diffused minerals, differing but little in composition though somewhat differently crystallized, mostly in short columnar forms. The typical and most abundant varieties of these minerals appear black to the eye, though in thin sections they are bottle-green; they form the black ingredient of most rocks.

The color is due to ferroso-ferric (magnetic) oxid of iron; the mineral as a whole may be considered as a silicate of lime, magnesia, alumina and iron, varying greatly in their absolute proportions; alumina and iron being sometimes almost absent. When iron is lacking the mineral may be almost white (tremolite, asbestos), and its weathering is then much retarded, since the oxygen of the air cannot take part in the process of disintegration.

The black variety of hornblende is not only the most abundant as a rock-ingredient, but it also the one most easily decomposed and therefore most commonly concerned in soil formation. The black hornblende owes its easy decomposition under the atmospheric influences to two properties; one, its easy cleavage, whereby cracks are readily formed and extended by the agencies already mentioned ([pp. 1-3]). The other is its large content of ferrous silicate (silicate of iron protoxide), whereby it is liable to attack from atmospheric oxygen; the latter forms ferric hydrate (iron rust) out of the protoxide, thus causing an increase of bulk which tends to split the masses of the mineral in several directions, while the silex is set free. At the same time the carbonic acid of the air converts the silicate of lime and magnesia, which forms the rest of the mineral, into carbonates; and the alumina present forms kaolinite, as in the case of the feldspars. There is thus formed from this mineral, when alone, a strongly rust-colored, more or less calcareous and magnesian clay, constituting the material for rather light-textured “red” soils. In most cases however the hornblende is associated in the rock itself with the several feldspars, (mostly lime- and soda-lime feldspars) as well as with more or less quartz. The rust-colored soils are therefore most commonly the joint result of the weathering of these several minerals. This is well exemplified in the case of the “red” soils formed from the so-called granites and slates of the western slope of the Sierra Nevada of California.

Pyroxene or Augite so nearly resembles hornblende in its chemical composition and crystalline form, that what is said of the latter may be considered as applying to augite also. Owing however to the absence of any prominent tendency to cleavage, the smooth crystals of this mineral are attacked much less readily than is hornblende, so that we often find them as “black gravel” in the soils formed from rocks containing it. Such soils are particularly abundant and important in the region covered by the great sheet of eruptive rocks (basalts, so-called) in the Pacific Northwest, and on the plateau of South Central India (the Deccan), and result likewise from the decomposition of the black lavas of volcanoes; thus in the Hawaiian islands, and in the Andes of Peru and Chile.

Both hornblende and augite being either free from, or deficient in potash, of coarse the soils formed from them are apt to lack an adequate supply of this substance for plant use. This is markedly true of hornblende schist or amphibolite rocks.

Mica, commonly known as isinglass, is so conspicuous wherever it occurs that it is more readily recognized than any other mineral. It occurs in glittering scales in soils and sands, and in rocks it sometimes forms sheets of sufficient size to supply the small panes for the doors of stoves, lamp chimneys, etc., which being flexible are not liable to break, but only gradually scale into very thin films, into which it can also be split by hand. When white, (muscovite, phlogopite) its scales are sometimes mistaken for silver by mine prospectors; when yellow, for gold; but their extreme lightness should soon remove these delusions. The composition of mica is not widely different from that of the two preceding minerals; like these it sometimes contains much iron, and is then dark bottle-green (biotite); this variety in weathering becomes bright yellow, and soon disintegrates.