34 Change of Crops. [Book VIII. 



this difference of roots. From this circumflance fome 

 vegetables draw their nourifhmcnt from the furface of 

 the earth, while others derive it partly frcm a greater 

 depth. So that by a change of crops, a larger portion 

 of the foil is made to contribute to the nourilhment of 

 plants than could have been effected by the cultivation 

 of any fingle fpecies. One other advantage to be de- 

 rived from a change of crops, is this : fome plants 

 extract almoft the whole of their nourifhmenc from the 

 foil ; and this is particularly the cafe with thofe which 

 are moil valuable, and which contain the greateft quan- 

 tity of folid matter. By the repetition of fuch crops, 

 however, the foil is found to become too much ex- 

 haufted. There are other plants which derive a large 

 proportion of their nourifhment from the air ; by fuch 

 therefore the foil will be much lefs exhaufted, and un- 

 "der a crop of them will be in fome meafure at reft. 

 The good effects of a change of crops may therefore 

 be fufficiently explained, without fuppofing that each 

 particular fpecies of vegetables is nourilhed by a differ- 

 ent kind of food. This opinion is alfo neceffarily 

 attended with two great difficulties : one is, that there 

 exifts in every foil as many diftinct kinds of nourish- 

 ment, as there are fpecies of plants capable of growing, 

 in that foil j the other, that plants are endued with the 

 faculty of difcerning and.feleeting, from all thefe kinds, 

 their own proper nourifhment. The former of thefe 

 fuppofitiohs is too abfurd to merit the leaft attention, 

 and the latter has been difproved by actual experiment, 

 lince plants are not able to prevent their roots from 

 absorbing fuch matters as prove poifonous to them. 

 Other writers, however, have been more moderate, 

 and though they have rejected the idea of fpecific nou- 

 rifhment in general, have neverthelefs imagined that 

 the hypothecs might be well founded with relpect 



to 



