Though the substances of which soils chiefly consist are so few in number, yet every practical man knows how very diversified they are in character—how very different in agricultural value. Thus, in some of our southern counties, we have a white soil, consisting apparently of nothing else but chalk; in the centre of England a wide plain of dark red land; in the border counties of Wales, and on many of our coal-fields, tracts of country almost perfectly black; while yellow, white, and brown sands give the prevailing character to the soils of other districts. Such differences as these arise from the different proportions in which the sand, lime, clay, and the oxide of iron which colours the soils, have been mixed together.
But how have they been so mixed—differently in different parts of the country. By what natural agency?—for what end?
Again, the soil on the surface rests on what is usually denominated the subsoil. This, also, is very various in its character and quality. Sometimes it is a porous sand or gravel, through which water readily ascends from beneath or sinks in from above; sometimes it is light and loamy like the soil that rests upon it; sometimes stiff and impervious to water.
The most ignorant farmer knows how much the value of a piece of land depends upon the characters of the surface soil,—the intelligent improver understands best the importance of a favourable subsoil. “When I came to look at this farm,” said an excellent agriculturist to me, “it was spring, and damp growing weather: the grass was beautifully green, the clover shooting up strong and healthy, and the whole farm had the appearance of being very good land. Had I come in June, when the heat had drunk up nearly all the moisture which the sandy subsoil had left in the surface, I should not have offered so much rent for it by ten shillings an acre.” He might have said also, “Had I taken a spade, and dug down 18 inches in various parts of the farm, I should have known what to expect in seasons of drought.”
But how come subsoils thus to differ—one from the other—and from the surface soil that rests upon them? Are there any principles by which such differences can be accounted for—by which they can be foreseen—by the aid of which we can tell what kind of soil may be expected in this or that district—even without visiting the spot—and on what kind of subsoil it is likely to rest?
Geology explains the cause of all such differences, and supplies us with principles by which we can predict the general quality of the soil and subsoil in the several parts of entire kingdoms;—and where the soil is of inferior quality and yet susceptible of improvement, the same principles indicate whether the means of improving it are likely, in any given locality, to be attainable at a reasonable cost.
It will be proper shortly to illustrate these direct relations of geology to agriculture.
CHAPTER VI.
Direct relations of Geology to Agriculture—Origin of Soils—Causes of their Diversity—Relation to the Rocks on which they rest—Constancy in the relative Position and Character of the Stratified Rocks—Relation of this fact to Practical Agriculture—General Character of the Soils upon these Rocks.