The Rotation of Crops 



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 contains 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 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 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 to the soil, accumulated sufficiently 



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