148 THE ART OF CULTURE. 



the most powerful agents in effecting the disinte- 

 gration of rocks. 



Air, water, and the change of temperature 

 prepare the different species of rocks for yielding 

 to plants the alkalies which they contain. A soil 

 which has been exposed for centuries to all the 

 influences which effect the disintegration of rocks, 

 but from which the alkalies have not been removed, 

 will be able to afford the means of nourishment to 

 those vegetables which require alkalies for its 

 growth during many years ; but it must gradually 

 become exhausted, unless those alkalies which have 

 been removed are again replaced ; a period, there- 

 fore, will arrive, when it will be necessary to expose 

 it, from time to time, to a further disintegration, in 

 order to obtain a new supply of soluble alkalies. 

 For small as is the quantity of alkali which plants 

 require, it is nevertheless quite indispensable for 

 their perfect development. But when one or more 

 years have elapsed without any alkalies having been 

 extracted from the soil, a new harvest may be 

 expected. 



The first colonists of Virginia found a country, 

 the soil of which was similar to that mentioned 

 above ; harvests of wheat and tobacco were obtained 

 for a century from one and the same field without 

 the aid of manure, but now whole districts are 

 converted into unfruitful pasture land, which with- 

 out manure produces neither wheat nor tobacco. 



