to prepare a soil for Potatoes it is necessary to employ a manure strongly charged with salts of potash and phosphates, but it need not be highly charged with soda or lime, for we find but a small proportion of these ingredients in the Potato.  There are soils so naturally rich in all that crops require, that they may be tilled for years without the aid of manures, and will not cease to yield an abundant return.  But such soils are exceptional, and those that need constant manuring are the rule.  One point more, ere we proceed to apply to practice these elementary considerations.  In almost every soil, whether strong clay, mellow loam, poor sand, or even chalk, there are comminglings of all the minerals required by plants, and, indeed, if there were not, we should see no herbage on the downs, and no Ivies climbing, as they do, to the topmost heights of limestone rocks.  But usually a considerable proportion of those mineral constituents on which plants feed are locked up in the staple, and are only dissolved out slowly as the rain, the dew, the ever-moving air, and the sunshine operate upon them and make them available.  As the rock slowly yields up its phosphates, alkalies and silica to the wild vegetation that runs riot upon it, so the cultivated field (which is but rock in a state of decay) yields up its phosphates, alkalies and silica for the service of plants the more quickly because it is the practice of the cultivator to stir the soil and continually expose fresh surfaces to the transforming power of the atmosphere.  It has been said that the air we breathe is a powerful manure.  So it is, but not in the sense that is applicable to stable manure or guano.  The air may and does afford to plants much of their food, but it can only help them to the minerals they require by dissolving these out of pebbles, flints, nodules of chalk, sandstone, and other substances in the soil which contain them in what may be termed a locked-up condition.  Every fresh exposure of the soil to the air, and especially to frost and snow, is as the opening of a new mine of fertilisers for the service of those plants on which man depends for his subsistence.

The application to practice of these considerations is an extremely simple matter in the first instance, but it may become very complicated if followed far enough.  Here we can only touch the surface of the subject, yet we hope to do so usefully.  Suppose, then, that we grow Cabbage, or Cauliflower, or Broccoli, on the same plot of ground, one crop following the other for a long series of years, and never refresh the soil with manure, it must be evident that we shall, some day or other, find the crop fail through the exhaustion of the soil of its available sulphur, phosphates, lime, or potash.  But if this soil were allowed to lie fallow for some time, it would again produce a crop of Cabbage, owing to the liberation of mineral matters which, when the crops were failing, were not released fast enough, but which, during the rest allowed