FERTILITY OF DIFFERENT SOILS- 147 



lime, are the names given to the principal constitu- 

 ents 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 argil- 

 laceous earths in arable land, w^hat are their con- 

 stituents, and what part do they play in favoring 

 vegetation ? They are produced by the disintegra- 

 tion of aluminous minerals by the action of the 

 weather ; the common potash and soda felspars, 

 Labrador spar, mica, and the zeolites, are the most 

 common aluminous earths, which undergo this change. 

 These minerals are found mixed with other sub- 

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

 slate, grauw^acke, and the volcanic rocks, basalt, clink- 

 stone, and lava. In the grauwacke, we have pure 

 quartz, clay-slate, and lime ; in the sandstones, quartz 

 and loam. The transition limestone and the dolo- 

 mites contain an intermixture of clay, felspar, por- 

 phyry, and clay-slate ; and the mountain limestone 

 is remarkable for the quantity of argillaceous earths 

 which it contains. Jura limestone contains 3 — 20, 

 that of the Wurtemberg Alps 45 — 50 per cent, of 

 these earths. And in the muschelkalk and the cal- 

 caire grossier 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 inva- 

 riable constituent. There must, therefore, be some- 

 thing 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 de- 

 pends is that of its invariably containing 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 



