HO THE ART OF CULTURE. 



nent parts. Sand, clay, and lime, are the names given to the 

 principal constituents of the different kinds of soil. 



Pure sand and pure limestones, in which there are no other 

 inorganic substances except siliceous earth, carbonate or silicate 

 of lime, form absolutely barren soils. But argillaceous earths 

 form always a part of fertile soils. Now, from whence come 

 the argillaceous earths in arable land, what are their constituents, 

 and what part do they play in favoring vegetation 1 They are 

 produced by the disintegration of aluminous minerals, among 

 which the common potash and soda felspars, Labrador spar, mica, 

 and the zeolites, are those which most commonly undergo this 

 change. These minerals are found mixed with other substances 

 in granite, gneiss, mica-slate, porphyry, clay-slate, grauwacke 

 and the volcanic rocks, basalt, clinkstone, and lava. As mem- 

 bers of the grauwacke series we have pure quartz, clay-slate, 

 and lime ; in the sand-stones, quartz and loam. The transition 

 limestone and the dolomites contain an intermixture of clay, 

 felspar, porphyry, and clay-slate ; and the mountain limestone is 

 remarkable for its quantity of argillaceous earths. Jura lime- 

 stone contains 3—20, that of the Wurtemburg Alps 45 — 50 per 

 cent, of these earths. And in the muschelkalk and in the cal- 

 caire grossier they exist in greater or less quantity. 



It is thus obvious that the aluminous minerals are the most 

 widely diffused on the surface of the earth, and, as we have 

 already mentioned, they are never absent from fertile soils ; 

 and, if they should happen to be absent in soils capable of culti- 

 vation, this only happens when certain of their constituents are 

 supplied by other sources. Argillaceous earth must, therefore, 

 contain something which enables it to exercise an influence on 

 the life of plants, and to assist in their development. The pro- 

 perty on which this depends is that of its invariably containing 

 alkalies and alkaline earths, with sulphates and phosphates. 



Alumina exercises only an indirect influence on vegetation, by 

 its power of attracting and of retaining water and ammonia ; it 

 is itself very rarely found in the ashes of plants,* but silica is 



* Hydrate of alumina, when mixed vrith extract of humus, decolorizes 

 this substance and renders insoluble the coloring matter. ( Wiegmann 

 und Pohtorf ^ 



