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

by Eugene W. Hilgard

Heat and Moisture are the main governing conditions of plant growth. In a preceding chapter the relations of soils and plant growth to water have been considered; in the present one the relations of both moisture and heat to soils and plants will be discussed; and to do this intelligibly to those not making a specialty of such studies, it becomes necessary to introduce, first, a summary consideration of the subject of climate.

Climatic conditions control, and to a great extent determine, the industrial pursuits of every country; all the more so as the rapid communication and transportation by means of modern appliances now brings every part of the globe in competition with every other. The question is not now what it may be intrinsically possible to do under certain climatic and geographical conditions, but whether these things can be done with a reasonable prospect of profit and commercial success, in competition with other countries offering more or less of similar possibilities. While it is true that the cost of labor frequently enters most heavily into such problems, yet favorable or unfavorable climatic or soil-conditions may in many cases turn the scale. Thus the high price of labor and fuel on the Pacific coast of the United States would at first blush seem to render competition with Europe and the East in the production of beet sugar commercially impossible; yet exceptionally favorable climatic and soil-conditions concur to turn the scale in favor of California at least, so as to have placed that state at the head of the sugar-producing states of the Union. A general understanding of the climatic conditions which concern the United States more or less directly, is therefore needful to an appreciation of their agricultural possibilities.

Climatic Conditions.—The factors usually mentioned as constituting climate are temperature, rainfall and winds. Since the latter two factors, however, are themselves merely the result of heat conditions, it is proper to discuss from the outset the origin and mode of action of heat.

TEMPERATURE.

The temperature of stellar space outside of the atmosphere is known to be very low. The increasing cold as we ascend to greater heights, is a fact familiar to all. Langley has calculated upon the basis of observations made at the summit and foot of Mount Whitney in California, that the temperature of space lies near 200° Cent. (360° F.) below the freezing point of water; and this would be the temperature near the Earth’s surface, were it not for the surrounding atmosphere. The latter absorbs but a small amount of the sun’s direct heat rays (which are of high intensity), as they penetrate it to the Earth’s surface. But as the earth’s surface is warmed, the heat rays of low intensity which it emits cannot pass back through the atmosphere to the sun or to outer space; they are “trapped,” as it were, by the dense air resting near the earth’s surface, which is then warmed partly by the radiation from, partly by direct contact with, the soil. It is to the existence of our atmosphere, then, that the possibility of our animal and vegetable life in their present form is due; and a decrease of the trapping effect on the sun’s heat rays makes itself quickly felt when ascending, either in balloons or on high mountains. Moreover, it is well known to mountain climbers that at great elevations the sun’s heat is extremely intense at noon; even though the temperature may fall to the freezing point at night, owing to the failure of the thin air to prevent the radiation back into space of the heat absorbed during the day. On the high plateaus of the Andes and of Asia, therefore, very extreme climates prevail, on account of the great range of temperature between day and night.

Ascertainment and Presentation of Temperature-Conditions.—The proper understanding of the temperature conditions of any locality or region is by no means a simple matter. Shall we study the daily, monthly, or annual changes of temperature, or the means deduced from either or all of them, in order to gain a clear insight into the climatic conditions that control crop production and health conditions?

The Annual Mean Temperature not a Good Criterion.—Since one and the same figure may result equally from the averaging of two widely divergent data, and from such as are close together, it is clear that the mean annual temperature cannot be a proper criterion of the agricultural adaptations of a country. Thus an average temperature of 60° F. might result, equally, from the averaging of 65 and 55 degrees, or from taking the mean of 15 and 105 degrees; yet the respective cultural adaptations would be widely different.

Extremes of Temperature are Most Important.—It is, on the contrary, rather the extremes of temperature, more particularly of cold, but frequently also of heat, together with the total amount of heat available during the growing season, that determines such adaptation so far as temperature is concerned; for no culture plant can be successfully grown where the temperature during winter even occasionally falls for more than a few hours below the point which it can resist; and for each plant there is a certain aggregate requirement of heat to carry it from germination to fruiting. Even different varieties of one and the same plant differ materially in the latter respect, so that it is very important that in the selection of varieties to be grown, this factor should be taken into consideration. It cannot be too strongly urged that the comparison of annual means of temperature, so commonly made by promoters of colonization schemes, must not be taken as a guide either in the estimate of cultures in which the immigrant may desire to engage, or by those in search of a climate adapted to their health-conditions.

Seasonal and Monthly Means.—The statement of the mean temperatures of the conventional four seasons—spring, summer, autumn and winter—afford a much clearer conception of climatic adaptations; provided always that the extreme temperatures be considered at the same time. With the same understanding, the monthly means are still more instructive; but here again, it is most essential that the distribution and amount of rainfall in each be regarded at the same time, since the most desirable temperature is of no avail without the moisture required for vegetation.

In some cases, e. g., that of California, it becomes necessary for practical purposes to regard the “season,” and not the calendar year, as the unit or reference for crop production. There the crops depend upon what rainfall may occur from October to May, there being no summer rains of agricultural significance, and outside of irrigated lands, almost all vegetation save that of trees being in abeyance. In India, there are two distinct growing seasons (“kharif” and “rabi”), corresponding to the two “monsoon” seasons; and no matter how much rain may fall during one, almost total failure may occur in other tropical and arid sections of that country.