OF THE FERTILITY OF SOILS. 145 



process from the component parts of the atmo- 

 sphere. This opinion found adherents even after 

 the method of detecting potash in soils was known, 

 and suppositions of the same kind may be found 

 even in the writings of some physiologists of the 

 present day. Such opinions belong properly to 

 the time when flint was conceived to be a product 

 of chalk, and when everything, which appeared in- 

 comprehensible ,on account of not having been in- 

 vestigated, was explained by assumptions far more 

 i n comprehensible. 



A thousandth part of loam mixed with the quartz 

 in new red sandstone, or with the lime in the dif- 

 ferent limestone formations, affords as much potash 

 to a soil only 20 inches in depth as is sufficient to 

 supply a forest of pines growing upon it for a cen- 

 tury. A single cubic foot of felspar is sufficient to 

 supply a wood, covering a surface of 40,000 square 

 feet, with the potash required for five years. 



Land of the greatest fertility contains argilla- 

 ceous earths and other disintegrated minerals with 

 chalk and sand, in such a proportion as to give free 

 access to air and moisture. The land in the vicinity 

 of Vesuvius may be considered as the type of a 

 fertile soil, and its fertility is greater or less in dif- 

 ferent parts, according to the proportion of clay or 

 sand which it contains. 



The soil which is formed by the disintegration of 

 lava cannot possibly, on account of its origin, 

 contain the smallest trace of vegetable matter, and 



L 



