The subsoil is usually of a different character from the surface soil, but this difference is more often the result of circumstances than of formation. The surface soil from having been long cultivated has been more opened to the influences of the air than is the case with the subsoil, which has never been disturbed so as to allow the same action. Again the growth of plants has supplied the surface soil with roots, which by decaying have given it organic matter, thus darkening its color, rendering it warmer, and giving greater ability to absorb heat and moisture, and to retain manures. All of these effects render the surface soil of a more fertile character than it was before vegetable growth commenced; and, where frequent cultivation and manures have been applied, a still greater benefit has resulted. In most instances the subsoil may by the same means be gradually improved in condition until it equals the surface soil in fertility. The means of producing this result, also farther accounts of its advantages, will be given under the head of _Cultivation_ (Sect. IV.) IMPROVEMENT. From what has now been said of the character of the soil, it must be evident that, as we know the _causes_ of fertility and barrenness, we may by the proper means improve the character of all soils which are not now in the highest state of fertility. Chemical analysis will tell us the _composition_ of a soil, and an examination, such as any farmer may make, will inform us of its deficiencies in _mechanical_ character, and we may at once resort to the proper means to secure fertility. In some instances the soil may contain every thing that is required, but not in the necessary condition. For instance, in some parts of Massachusetts, there are nearly _barren_ soils which show by analysis precisely the same chemical composition as the soil of the Miami valley of Ohio, one of the most _fertile_ in the world. The cause of this great difference in their agricultural capabilities, is that the Miami soil has its particles finely pulverized; while in the Massachusetts soil the ingredients are combined within particles (such as pebbles, etc.), where they are out of the reach of roots. [Why do soils of the same degree of fineness sometimes differ in fertility? Can soils always be rendered fertile with profit? Can we determine the cost before commencing the work? What must be done before a soil can be cultivated understandingly? What must be done to keep up the quality of the soil?] In other cases, we find two soils, which are equally well pulverized, and which appear to be of the same character, having very different power to support crops. Chemical analysis will show in these instances a difference of composition.