172 EUROPEAN AGRICULTURE. 



It would be presumptuous to pronounce it impossible, but the 

 proof does not appear conclusive. Two other theories have 

 been suggested to account for the necessity or expediency of an 

 alternation or change of crop. The first is. that every plant 

 throws out from its roots, as excrement, that which is unfavora 

 ble to the growth of, or poisonous to, any plant of the same kind 

 that may succeed it, but which, on the other hand, may be favor 

 able to the growth, or be the proper food, of a plant of another 

 kind. It is maintained, that by the cultivation of another kind 

 of plant, of different wants from the former, this pernicious excre 

 ment is used up ; or even by the suspension of cultivation, by 

 what is termed a naked fallow, by the stirring or ploughing of 

 the ground without cultivation of any crop, this offensive matter 

 is destroyed, and the former plant may be again successfully 

 cultivated. 



The second opinion is, that different kinds of crops make 

 demands of different elements from the soil ; that the cultivation 

 of a particular crop tends to exhaust the soil of the ingredients 

 or elements which it particularly requires; that this element 

 may be sometimes artificially supplied by manuring ; or that the 

 land being suffered to remain without cropping, or by a succes 

 sion of different crops, nature itself will provide for a renewal of 

 the deficient or exhausted element. The former theory is in a 

 great measure abandoned ; the latter is the prevalent opinion 

 with those who claim a right to speak with authority. 



The latter theory, however, is not without some difficulties or 

 exceptions. Onions, for example, are cultivated successfully, 

 year after year, on the same land, from preference, because it is 

 found that the longer the land has been accustomed to this crop, 

 the more favorable does it become to the growth of it. I have 

 known cases in which Indian corn has been successfully culti 

 vated forty years without interruption on the same land. A 

 distinguished agricultural traveller and observer,* says that he 

 has seen, &quot;in the table lands of the Andes, wheat fields which 

 had yielded excellent crops annually for more than two centu 

 ries ; and that potatoes may come again and again upon the 

 same soil; they are incessantly cultivated at Santa Fe and 

 duito, and nowhere are they of better quality.&quot; 



* Boussingault. 



