Cultivation of friable soils

1205. Soils, if not kept friable by cultivation, soon become hardened on the surface. Even a free siliceous soil will, if left untouched, become too compact for the proper admission of air, rain, and heat, and for the free growth of the fibres; and strong upland clays, not submitted to the plough or the spade, will, in a few years, be found in the possession of fibrous rooted perennial grasses, which will form a clothing on their surface, or strong tap-rooted trees, as the oak, which have forced their way through the interior of the mass. Annuals and ramose-rooted herbaceous plants cannot penetrate into such soils.