124

SUBSOIL PLOUGHING.

which the subsoil is rich in; and in consequence thesoil becomes considerably improved by mixture, or byspreading over the surface some of the decomposingstones dug from below.

383. The system of subsoil ploughing, so successfullypractised in many parts of the country, is connectedwith this subject, and depends mainly upon the advan-tages derived from admixture of the subsoil with thesurface. In this operation a plough is employed tobreak up and pulverise the soil to a considerable depthbelow the surface, without bringing the soil so disturbedup to the top ; by this process the permeability of thesoil to air is greatly increased, and in consequence, thesubsoil is soon brought into a fit state to be mixedwith the ordinary soil, by the use of a suitable plough.

384. Part of the benefit derived from this operation isdoubtless due to the greater openness conferred upon thesoil, which permits a more perfect access of air andmoisture, and allows the roots of the growing plantsmore easily to penetrate throughout it; but at the sametime, advantage is also derived from the greater supplyof saline and soluble earthy matters which are thusmade available for the use of the plants.

385. We are apt to think that a soil must ofnecessity contain portions of those substances whichenter into the composition of the substratum; but thisis by no means always the case. Thus the surface-soilof a chalk district is not unfrequently nearly whollydestitute of calcareous or chalky matters; and soils