[Illustration: FIG. 69.--_a_, Humus, or stratum of arable earth,--the horizontal line shows the depth reached by the labourer; _b_, subsoil; _c_, subsoil; _d_, arable earth; _e_, humus in an inclined stratum; _f_, humus mixed with subsoil.] Arable land is the most superficial stratum of the cultivable terrestrial crust; it is this which the plough turns up and subdues; it is this which, properly manured, and enriched by the decomposition of organic matter, furnishes to vegetables their principal nourishment. As it varies in thickness, it necessarily presents one or other of the following circumstances:--1st, The labourer, penetrating the entire stratum of arable earth (Fig. 69, _a_), will strike down to the subsoil (Fig. 69, _b_); or, 2d, he will not traverse the entire stratum (Fig. 69, _c_); or, 3d, after having traversed the entire depth of the humus, he will reach a portion of the subsoil (Fig. 69, _d_); or, 4th, after having gone through both humus and subsoil, he will discover another layer of arable earth, which may be either pure humus, in a thick inclined stratum (Fig. 69, _e_), or humus mixed with the débris of the subsoil. As for the subsoil, it may, by its composition, completely modify, stimulate, or delay the action of the vegetable mould, however rich this may be in assimilating principles. Thus, where the subsoil is argillaceous, the pluvial waters are arrested by it as by a bed of impervious cement, and render the ground too damp and cold to yield abundant harvests. In such a case subsoil-drainage is the best remedy. But if the earth be porous, the moisture gradually percolates through its various layers, fertilising and warming, communicating to the plants the needful humidity, and assisting in the production of that most glorious of all the scenes of cultivated nature--a corn-field thickly ripe with golden grain. In the poet's "Palace of Art" no finer picture can be seen than this: "The reapers at their sultry toil. In front they bind the sheaves. Behind Are realms of upland, prodigal in oil, And hoary to the wind." Oh! a sight to thank God for, and rejoice in, is the field all aglow with the splendour of the harvest! [Illustration: FIG. 70.--"Behind are realms of upland."] Without having recourse to chemical analysis, which is within the reach of a very limited number of persons, _clayey_ soils may be distinguished by the vegetable species that most commonly flourish in them: as-- I. _Plants belonging to clayey soils._--The Queen of the Meadows, _Spiræa ulmaria_ (order _Rosaceæ_). Wild Angelica, _Angelica sylvestris_. Common Sorrel, _Rumex acetosa_ (order _Polygonaceæ_), and various kinds of Ranunculaceæ, as _Ranunculus lingua_, _Ranunculus flamma_, and _Ranunculus sceleratus_.