But there are certain substances contained in every soil, whether in pasture or under the plough, which gradually make their way down towards the subsoil. They sink till they reach at last that point beyond which the plough does not usually penetrate. Every farmer knows that lime thus sinks. In peat-soils top-dressed with clay, the clay thus sinks. In sandy soils also which have been clayed, the clay sinks; and in all these cases, I believe, the sinking takes place more rapidly when the land is laid down to grass. Where soils are marled, the marl sinks; and the rains, in like manner, gradually wash out that which gives their fertilizing virtue to the under chalk-soils (see page 88), and render necessary a new application from beneath, to renovate its productive powers. If this be the case with earthy substances such as those now mentioned, which are insoluble in water, it will be readily believed that those saline ingredients of the soil which are readily soluble will be still sooner washed out of the upper and conveyed to the under soil. Thus the subsoil may gradually become rich in those substances of which the surface-soil has been robbed. Bring up a portion of this subsoil by deep-ploughing, and you restore to the land a portion of what it has lost—substances, perhaps, which may render it much more fruitful than before. Such is an outline of the theory of deep-ploughing, and it is entirely unexceptionable. But suppose the land to have originally contained something noxious to vegetation, which in process of time has been washed down into the subsoil, then to bring this again to the surface would be materially to injure the land. This also is true, and a sound discretion must no doubt be employed, in judging when and where such evil effects are likely to follow. Such cases, however, are more rare than many suppose. There are few subsoils which a full and fair exposure to a winter’s frost will not in a great degree deprive of all their noxious qualities, and render fit to ameliorate the general surface of the poorer lands. If the reader doubt this fact, let him visit Yester, and give a calm consideration to the efforts produced by the use of deep-ploughing on the home-farm of the Marquis of Tweeddale. In many cases the farmer fears, as he does in the county of Durham, to bring up a single inch of the yellow clay that lies beneath his soil. In the first inch lodges, among other substances, the iron worn from his plough, which in some soils, and after a lapse of years, amounts to a considerable quantity. Till it is exposed to the air, this iron is hurtful to vegetation, and one of the benefits of a winter’s exposure of such subsoils to the air, is the effect produced upon the iron it contains.