COMPOSITION OF SOILS. 143 



And in the muschelkalk and the calcaire grassier 

 they exist in greater or less quantity. 



It is known, that the aluminous minerals are the 

 most widely diffused on the surface of the earth, 

 and as we have already mentioned, all fertile soils, 

 or soils capable of culture, contain alumina as an 

 invariable constituent. There must, therefore, be 

 something in aluminous earth which enables it to 

 exercise an influence on the life of plants, and to 

 assist in their development. The property on 

 which this depends is that of its invariably con- 

 taining potash and soda. 



Alumina exercises only an indirect influence on 

 vegetation, by its power of attracting and retaining 

 water and ammonia ; it is itself very rarely found 

 in the ashes of plants, but silica is always present, 

 having in most places entered the plants by means 

 of alkalies. In order to form a distinct conception 

 of the quantities of alkalies in aluminous minerals 

 it must be remembered that felspar contains I7f 

 per cent, of potash, albite 11*43 per cent, of soda, 

 and mica 3 5 per cent. ; and that zeolite con- 

 tains 13 16 per cent, of both alkalies taken 

 together. The late analyses of Ch. Gmelin, 

 Lowe, Fricke, Meyer, and Redtenbacher, have 

 also shown, that basalt contains from f to 3 per 

 cent, of potash, and from 5 7 per cent, of soda, 

 that clay-slate contains from 2*75 3*31 per 

 cent, of potash, and loam from 1^ 4 per cent, of 

 potash. 



