OF THE FERTILITY OF SOILS. 147 



alkaline salts than at any period of their life. 

 (De Saussure). Now we know also from the experi- 

 ments of Bequerel in what manner these alkaline 

 salts enter young plants ; the acetic acid formed 

 during germination is diffused through the wet or 

 moist soil, becomes saturated with lime, magnesia, 

 and alkalies, and is again absorbed by the radicle 

 fibres in the form of neutral salts. After the ces- 

 sation of life, when plants are subjected to decom- 

 position by means of decay and putrefaction, the 

 soil receives again that which had been extracted 

 from it. 



Let us suppose that a soil has been formed by the 

 action of the weather on the component parts of 

 granite, grauwacke, mountain limestone, or por- 

 phyry, and that nothing has vegetated for thousands 

 of years. Now this soil would have become a 

 magazine of alkalies, in a condition favourable for 

 their assimilation by the roots of plants. 



The interesting experiments of Struve have 

 proved that water impregnated with carbonic acid 

 decomposes rocks which contain alkalies, and then 

 dissolves a part of the alkaline carbonates. It is 

 evident 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 cohe- 

 rence of rocks. Next to the action of air, water, 

 and change of temperature, plants themselves are 



L 2 



