SOILS. 253 



-would soon undergo decomposition, and the whole of 

 the alkaline salts would then be washed out by the 

 rain (651). 



657. The best soils are those in which the earthy 

 constituents are so proportioned that the light, 

 porous qualities of the one are balanced by the close, 

 retentive properties of the others ; for they are then 

 most uniformly suitable to vegetation. 



658. The silica and alumina in soils are of course 

 almost wholly free and uncombined with any acid, as 

 the former is not a base, and the latter has hardly 

 any affinity for the weaker acids, such as the carbonic. 

 Small quantities of silica are almost always found in 

 soils, combined with either soda or potash, forming 

 those curious compounds before alluded to, in which 

 the silica seems to play the part of an acid (259) ; 

 soils never contain more than a very small quantity 

 of these substances; but it is evident that plants, 

 such as grasses, which contain silica, must obtain it 

 from the soil in a soluble form, by gradually absorb- 

 ing it in combination with alkali, dissolved in water. 



659. Silica also exists in soils in combination with 

 several bases together, such as lime, potash, soda, 

 magnesia, and alumina, constituting the natural 

 rocky silicates just spoken of ; the nature and com- 

 position of these compounds, as well as their tend- 

 ency to decompose, varies considerably in different 

 soils. 



660. Lime and magnesia, both of which have a 

 powerful affinity for acids, are never present in the 



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