DISINTEGRATION OF SOILS. 151 



ry, and that nothing has vegetated oh it for thou- 

 sands of years. Now this soil would become a mag- 

 azine of alkalies in a condition favorable for their 

 assimilation by the roots of plants. 



The interesting experiments of Struve have proved 

 that water impregnated with carbonic acid decom- 

 poses rocks which contain alkalies, and then dis- 

 solves ^ part of the alkaline carbonates. It is evi- 

 dent that plants also, by producing carbonic acid 

 during their decay, and by means of the acids which 

 exude from their roots in the living state, contribute 

 no less powerfully to destroy the coherence of rocks. 

 Next to the action of air, water, and change of tem- 

 perature, plants themselves are the most powerful 

 agents in effecting the disintegration 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 

 affect 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 their growth during many years ; 

 but it must gradually become exhausted, unless those 

 alkalies which have been removed are again replaced ; 

 a period, therefore, will arrive when it will be neces- 

 sary to expose it from time to time to a further dis- 

 integration, in order to obtain a new supply of solu- 

 ble alkalies. For small as is the quantity of alkali 

 which plants require, it is nevertheless quite indis- 

 pensable 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 without manure 



