134 



ishing of the soil annually with manure will keep up its fertil- 

 ity, or permit the continued repetition of the same crop for a 

 considerable length of time, yet in other cases the application 

 of the ordinary manures, in any quaiitity, will avail nothing to 

 the renovation of the productiveness of the land without rest, 

 or without a change of , crop, when, after awhile, the first crop 

 may again be resorted to. 



The occasion of this obviously lies in the soil. The atmos- 

 pheric influences are the same, and no change of seed has been 

 found to remedy the evil. Two causes have been assigned for 

 a fact better attested than understood. In the first case it is 

 supposed that the soil, by certain plants, becomes exhausted of 

 the peculiar element necessary to the perfection of that kind of 

 plants, but retains all that is sufficient for another class ; in 

 the second place, it is supposed that, as with animals, so plants 

 excrete, or throw off from their roots those portions of the food 

 they receive, which they cannot assimilate ; and the soil be- 

 coming, by cultivation, filled with these excretions, the same 

 crop being renewed is, if it consumes them, poisoned by them, 

 or otherwise it fails to find the nourishment which it requires. 

 Nor can the crop be repeated with advantage until these excre- 

 tions become dissolved or decomposed, and again incorporated 

 into the soil. Both these suppositions are compatible with each 

 other, and may operate at the same time. Chemical analysis 

 has determined, as analogy would lead us to believe, that differ- 

 ent plants are composed of different elements, and these too in 

 various jiroportions ; and observation equally shows that that 

 which, in the process of digestion or assimilation, has been ex- 

 creted by an animal or plant cannot, without change, be made 

 again to contribute to the nourishment of the same animal or 

 plant. 



It is obvious, likewise, as suggested by a profound observer, 

 that the" excretions or excrements of plants may be of two 

 kinds. Plants, it is a well-established fact, are liable to take 

 up by their roots whatever is in their vicinity in a soluble state, 

 the process of absorption being, in this case, purely mechanical, 



