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

by Eugene W. Hilgard

In some cases the above salts are in part at least derived from the leaching of adjacent or subjacent geological deposits impregnated with them at the time of their formation. Such is the case in portions of Wyoming, Colorado and New Mexico, in the Colorado river delta, and in the Hungarian Plain; and it is in these cases especially that the chlorids of calcium and magnesium also form part of the saline mixture.

Geographical Distribution of Alkali Lands.—In looking over a rainfall map of the globe[163] we see that a very considerable portion of the earth’s surface, forming two belts to poleward of the two tropics, has deficient rainfall; the latter term being commonly meant to imply any annual average less than 20 inches (500 millimeters). The arid region thus defined includes, in North America, most of the country lying west of the one hundredth meridian up to the Cascade Mountains, and northward beyond the line of the United States; southward, it reaches far into Mexico, including especially the Mexican plateau. In South America it includes most of the Pacific Slope (Peru and Chile) south to Araucania; and eastward of the Andes, the greater portion of the plains of western Brazil and Argentina. In Europe only a small portion of the Mediterranean border is included; but the entire African coast-belt opposite, with the Saharan and Libyan deserts, Egypt and Arabia, are included therein, as well as, south of the Equator, a considerable portion of South Africa (Kalahari desert). In Asia, Asia Minor, Syria (with Palestine), Mesopotamia, Persia, and northwestern India up to the Ganges, and northward, the great plains or steppes of central Asia eastward to Mongolia and western China, fall into the same category; as does also a large portion of the Australian continent.

Utilization of World-wide Importance.—Over these vast areas alkali lands occur to a greater or less extent, the exceptions being the mountain regions and adjacent lands on the side exposed to the prevailing winds. It will therefore be seen that the problem of the utilization of alkali lands for agriculture is not of local interest only, but is of world-wide importance. It will also be noted that many of the countries referred to are those in which the most ancient civilizations have existed in the past, but which at present, with few exceptions, are occupied by semi-civilized people only. It is doubtless from this cause that the nature of alkali lands has until lately been so little understood, that even their essential distinctness from the sea-border lands has been but recently recognized in full. Moreover, the great intrinsic fertility of these lands when freed from the noxious salts, has been very little appreciated; their repellent aspect causing them to be generally considered as permanently waste lands.

Repellent aspect.—This aspect is essentially due to their natural vegetation being in most cases confined to plants useless to man, commonly designated as “saline vegetation,”[164] of which but little is usually relished by cattle. Notable exceptions to this rule occur in North and South America, Australia, and Africa, where the “saltbushes” of the former and the “karroo” vegetation of the latter form valuable pasture and browsing grounds. Apart from these, however, all efforts to find culture plants for these lands generally acceptable, or at least profitable, in their natural condition, have not been very successful.

Fig. 60.—Alkali Lands in San Joaquin Valley, California.

[Figure 60] illustrates the usual aspect of alkali lands in the San Joaquin valley of California. It will be noted that the alkali-covered surface is only in spots, with clumps of vegetation between, so that cattle can find both pasture and browsing on a portion of such lands, even though the plants so growing are not usually of the most desirable kind. We find in all arid regions, however, considerable areas either wholly destitute of vegetation, or bearing only such saline growth as is rejected by all kinds of domestic animals.

Effects of Alkali upon culture plants.—In land very strongly impregnated with alkali salts, most culture plants, if their seed germinates at all, will show a sickly growth for a short time, “spindle up” and then die without fruiting. In soils less heavily charged the plants may simply become dwarfed, and fruit scantily. The effect on grown trees around which alkali has come up, is first, scanty leafage and short growth of shoots, themselves but sparsely clothed with leaves. This state of things is well shown in figures 61 and 62, which represent apricot trees growing but a short distance apart, but one coming within range of an expanding alkali spot. The characteristic sparseness of the foliage of the “alkalied” tree as compared with the adjacent one is well shown.

Nature of the injury to plants from Alkali.—When we examine plants that have been injured by alkali, we will mostly find that the visible damage has been done near the base of the trunk, or root crown; rarely at any considerable depth in the soil itself. In the case of green herbaceous stems, the bark is found to have been turned to a brownish tinge for half an inch or more, so as to be soft and easily peeled off. In the case of trees, the rough bark is found to be of a dark, almost black, tint, and the green layer underneath has, as in the case of herbaceous stems, been turned brown to a greater or less extent. In either case the plant has been practically “girdled,” the effect being aggravated by the diseased sap poisoning more or less the whole stem and roots. The plant may not die, but it will be quite certain to become unprofitable to the grower.