i88 THE ROTATION OF CROPS 



salts of potash and phosphates, but it need not be highly charged 

 with soda or lime, for \ve find but a small proportion of these 

 elements 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 these 

 soils are exceptional, and those that constantly need 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, as it were, locked up in the staple, and are dissolved 

 out slowly as the rain, the dew, the ever moving air, and the 

 sunshine operate upon them. As the rock slowly yields up its 

 phosphates and alkalies and solutions of silica to the wild vegeta- 

 tion that runs riot upon it, so the cultivated field (which is but 

 rock in a state of decay) yields up its phosphates and alkalies and 

 solutions of silica for the service of plants quickly^ because it is the 

 practice of the cultivator to stir it about 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 them out of pebbles, 

 flints, nodules of chalk, sandstone, and other substances in the soil 

 which contains them in what may be termed a locked-up condition. 

 The importance of frequent and deep stirrings of the soil is seen in 

 this, that every fresh surface exposed 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 compli- 

 cated 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 as rapidly as possible for a long 

 series of years, and take care never to refresh the soil with a scrap of 

 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, 



