The process of chalking, which has been carried out on a large scale in some parts of England, and which consists in bringing up the chalk from pits, penetrating through the overlying tenacious clay, and mixing it with the soil, operates, to some extent, in a similar manner, though no doubt the lime also exercises a strictly chemical action. It is probable that the mixing of soils might be advantageously extended, and it merits more minute study than it has yet obtained. Its use is obviously limited by the expense, because, of course, where good effects are to be obtained, it is necessary to remove large quantities of soil, in some instances as much as 50 or 100 tons per acre, but the expense might be much diminished if it were carried out methodically, and on a considerable scale. The admixture of highly fertile soils with others of inferior quality is also worthy of attention; indeed, it is understood that this has been done, to some extent, with the rich trap soils of some parts of Scotland, but the extent of the benefit derived from it has not been made public. FOOTNOTES: [Footnote J: Mr. Dudgeon, Spylaw. Transactions of the Highland Society, vol. cxxix., p. 505.] CHAPTER VII. THE GENERAL PRINCIPLES OF MANURING. In their natural condition all soils not absolutely barren are capable of supporting a certain amount of vegetation, and they continue to do so for an unlimited period, because the whole of the substances extracted from them are again restored, either directly by the decay of the plants, or indirectly by the droppings of the wild animals which have browzed upon them. Under these circumstances, a soil yields what may be called its normal produce, which varies within comparatively narrow limits, according to the nature of the season, temperature, and other climatic conditions. But the case is completely altered if the crop, in place of being allowed to decay on the soil, is removed from it, for, though the air will continue to afford an undiminished supply of those elements of the food of plants which may be derived from it, the fixed substances, which can only be obtained from the soil, decrease in quantity, and are at length entirely exhausted. In this way a gradual diminution of the fertility of the soil takes place, until, after the lapse of a period, longer or shorter, according to its natural resources, it will become entirely incapable of maintaining a crop, and fall into absolute infertility unless the substances removed from it are restored from some other source in the form of manure. When this is done, the fertility of the soil may not only be sustained but greatly increased, and, in point of fact, all cultivated soils, by the use of manure, are made to yield a much larger crop than they can do in their natural condition.