154 ALTERNATION OF CROPS. 



profit. The pasture is the only portion of such a farm 

 that is improving ; and, even in this, bushes, brambles, 

 and noxious weeds are too often permitted to intrude, to 

 choke and destroy the better herbage. 



It is equally apparent, that we cannot take two or more 

 arable crops, of the same kind, from a field, in successive 

 seasons, without a manifest falling off" in the product. 

 The reason of this may be found in an immutable law of 

 Nature, which has provided for each species of plant a 

 specific food, suited to its organization and its wants. 

 Thus some soils will not grow wheat, although abounding 

 in the common elements of fertility, and although they 

 will make a profitable return in other farm-crops — in con- 

 sequence of such soils being deficient in the specific food 

 required for the perfection of the wheat. The same re- 

 mark applies to other farm-crops. One family or species 

 of plants requires a different food from that which another 

 family or species requires ; and it seems to be another 

 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 

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

 till the return aga^in to the field of this particular crop. 

 Thus it is supposed to require ten or a dozen years for 

 the specific food of flax sufficiently to accumulate for a 

 second crop, after one has been taken from a field. Even 

 the specific food of clover becomes 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 four years' course of crops, other grass-seeds, 

 so that this may not be repeated oftener than once in 

 eight years. In the analysis of plants, wheat is found to 

 contain lime, the turnip to contain sulphur, &c., and hence 

 we infer that these elementary matters are essential, in 

 the soil, to the growth of these crops. 



There are exceptions to the rules of practice which 

 these laws inculcate. Some soils seem natural to wheat, 

 others to oats, timothy, &c., and successive crops of 

 these are taken without apparent diminution of produce. 



