112 THE ART OF CULTURE. 



century. A single cubic foot of felspar is sufficient to supply an 

 oak copse, covering a surface of 26,910 square feet, with the 

 potash required for five years. 



Land of the greatest fertility contains argillaceous earths and 

 other 'shitegrated minerals, with chalk and sand in such a pro- 

 portion is to give free access to air and moisture. The land in 

 the vicin! v of Vesuvius may be considered as the type of a fer- 

 tile soil, an., its fertility is greater or less in different parts, ac- 

 cording to iis proportion of clay or sand. 



This soil being derived from the disintegration of lava, cannot 

 possibly, owing to its origin, contain the smallest trace of vege- 

 table matter ; yet every one knows that when lava or volcanic 

 ashes have been exposed for a time to the influence of air and 

 moisture, all kinds of plants grow in them with the utmost luxu- 

 riance. 



This fertility of lava is owing to the alkalies, alkaline earths, 

 and silica, contained in it, which by exposure to the weather are 

 rendered capable of being absorbed by plants. Thousands of 

 years have been necessary to convert stones and rocks into the 

 soil of arable land, and thousands of years more will be requisite 

 for their perfect reduction, that is, for the complete exhaustion of 

 their alkalies. 



We see from the composition of the water in rivers, streamlets, 

 and springs, how little alkali the rain-water is able to extract 

 from a soil, even after a term of years ; this water is generally 

 soft, and the common salt, which even the softest invariably con- 

 tains, proves that the alkaline salts, which are carried to the sea 

 by rivers and streams, are returned again to the land by wind 

 and by rain. 



Let us suppose that a soil has been formed by the action of 

 the weather on the component parts of granite, grauwacke, moun- 

 tain limestone, or porphyry, and that the vegetation upon it has 

 remained the same for thousands of years. Now this soil would 

 become a magazine 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 decomposes rocks containing 

 alkalies, and then dissolves a part of the alkaline carbonates. 



