284 ON THE CULTIVATION 



cliang'es of lieat and cold, wet and dry, v.'hich are not less 

 capable of accomplishing' the disintegration of the hardest 

 materials of which the g-lolje is composed, and, in course of 

 time, their decomposition also. By marking- the operation 

 of these ag-ents, we learn the jn'ocess by which the alkaline 

 bases are made to form part of the soil, and the various 

 saline combinations into which they are enabled to enter. 

 Rocks of every formation contain some one or other of the 

 alkalies, and often more than one, according- to Liebig-. 

 Feldspar contains not less than 17 per cent, of potash ; albite 

 11 per cent, of soda ; zeolite, 13 per cent, of both alkalies 

 taken tog-ether ; g-ranite, grey wacke, porphyry, basalt, clay- 

 slate, clinkstone, sandstone, lime, lava-loam, contain each 

 their certain proportions ; and the decomposition of any one 

 of these rocks must always restore to the soil one or more of 

 the alkalies. In the course of cultivation, there is always a 

 constant breaking- down of the materials of the soil, to which 

 the tear and wear of the iron implements emplo3'ed bear 

 ample testimony ; and although portions of the stones (frag- 

 ments of the dilferent rocks) of which they consist are in 

 this way pulverized, still they supply no new food for the 

 succeeding- crops until they have been decomposed, and ren- 

 dered soluble — chang-es which will then be more speedily 

 effected, and the alkalies restored in g-reater abundance, than 

 required for the purposes of pasturage. Carbon, azote, and 

 the elements of water, which, in different proportions, unite 

 to form so many of our most valuable animal and vegetable 

 substances, and which are equally essential, are derived from 

 other sources which can be more readily supplied to the soil; 

 but, imtil of late years, when science began to lend her aid 

 to agriculture, the application of the alkalies (with the ex- 

 ce])tion of lime), as necessary constituents of every fruitful 

 soil, formed no part of the practice of agriculture ; and it is 

 only when these substances are exhausted that a soil is re- 

 duced to the sterile condition alluded to by Liebig, when he 

 says, that " no quantit}' of manure could fertiUze it for the 

 production of certain crops ;" and not, as he attributes it, to 

 the injurious nature of the substances excreted from the 

 roots. 1 



' Although Liebig gives countenance to the doctrine of radical ex- 

 cretion, which we have previously tjuoted, there is scarcely a chapter in his 

 Chemistry of Agriculture, in which he does not, in one shape or another, 

 recognise the truth which we have here stated. " It must be admitted," 

 he says, " as a principle of agriculture, that those substances which have 



