THE ROTATION OF CROPS 189 



soda, phosphates, lime, and 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 from the locked-up state of mineral matters 

 which, when the crops were failing, were not liberated fast enough, 

 but, owing to the rest allowed the soil, have accumulated sufficiently 

 to sustain a crop. Obviously this mode of procedure is unprofitable, 

 to begin with, and tends of necessity to exhaustion of the soil, although 

 we must confess that utter exhaustion of any soil is a thing at present 

 unknown. However, instead of following an exhaustive practice, we 

 enrich the soil with manure, and change the crops on the same plot, 

 so that when one crop has largely taxed it for one class of minerals 

 another crop is put on which will tax it for another class of minerals. 

 Let us take for a moment's consideration one of the necessary con- 

 stituents of a fertile soil, common salt. In the ash of a Cabbage there 

 is about six per cent, of this mineral, in the Turnip about ten per 

 cent., in the Potato two to three per cent., in the Beet eighteen to 

 twenty per cent. On the other hand, the Beet contains very little 

 sulphur, but both Turnip and Beet agree in being strongly charged 

 with potash and soda. It follows that if we crop a piece of ground 

 with Cabbage, and wish to avoid the failure that may occur if we con- 

 tinue to crop with Cabbage, we may expect to do well by giving the 

 ground a dressing of common salt and alkalies, and then crop it 

 with Beet. 



The whole subject is not exhausted by this mode of viewing it, for, 

 in the first place, all the facts are not yet fully understood by the 

 ablest of our chemists and physiologists, and, in the next place, crops 

 differ in their modes of seeking nourishment. We might find two 

 distinct plants nearly agreeing in chemical constitution, and yet one 

 might fail where the other would succeed. Suppose, for instance, we 

 have grown Cabbage and other surface-rooting crops until the soil 

 begins to fail, even then we might obtain from it a good crop of 

 Parsnips or Carrots, for the simple reason that these send their roots 

 down to a stratum that the Cabbage never reached ; and it is most 

 instructive to bear in mind that although the Parsnip will grow on 

 poor land, and pay on land that has been badly tilled for years, yet 

 the ashes of the Parsnip contain thirty-six per cent, of potash, eleven 

 per cent, of lime, eighteen per cent, of phosphoric acid, six per cent, 

 of sulphuric acid, three per cent, of phosphate of iron, and five per 

 cent, of common salt. How does the Parsnip obtain its mineral 

 food in an exhausted soil ? Simply by pushing down for it into a 

 mine that has been but little worked, though the Cabbage might fail 



