r 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 t