ALTERNATION OF CROPS. 33 



ily or species of plants requires a different food 

 from that which another family or species requires ; 

 and it seems to be also a law of nature, that what 

 is not essential to one family or species shall be left 

 in the soil, or returned to it through the excretory 

 organs of the growing crop. Of course, the specific 

 food for any class or species continues to accumu- 

 late in the soil, the general fertility being kept up, 

 till the return again to the field of this particular 

 crop. Thus it is supposed to require ten or a dozt n 

 years for the specific food of flax to accumulate suf- 

 ficiently for a second crop after one has been taken 

 from a field. Even the specific food of clover be- 

 comes exhausted by a too frequent repetition of it in 

 the same field ; it being found necessary, in Norfolk 

 husbandry, to substitute for it, in every other course 

 of crops, other grass seeds, so that this may not be 

 repeated oftener than once in eight years. There 

 are exceptions to the rules of practice which these 

 laws inculcate. Some soils seem natural to wheat, 

 others to oats or grass ; and successive crops of 

 these are taken without apparent diminution of pro- 

 duct. Yet it is better to regulate our practice by 

 general laws than by casual exceptions. In the 

 cases noted as exceptions, there is probably so great 

 an accumulation of the specific food of the particular 

 crop, that it has not been exhausted, though it evident- 

 ly must have been diminished. It is in accordance 

 with the natural laws we have noticed that the grass- 

 es in our meadows change ; that the timber trees of 

 the forest alternate new species springing up as 

 the old ones decay or are cut down ; and it is in ac- 

 cordance with these laws that the alternation of 

 crops has been adopted in all good farming. 



To simplify and render the subject still more plain, 

 the generality of tillage crops have been grouped 

 into two classes, differing essentially in their charac- 

 ter, culture, and exhausting influence upon the soil. 

 These two classes are denominated culmiferous crops 



