OF AGRICULTURE. 19 



As to the incombustible, or, as he designated them, 

 the ' miner aV constituents, Liebig adduced many 

 illustrations in proof of their essentialness. He 

 called attention to the variation in the composition 

 of the ash of plants grown on different soils ; and he 

 assumed a greater degree of mutual replaceability of 

 one base by another, or of one acid by another, than 

 could now be admitted. He traced the difference in 

 the mineral composition of diiferent soils to that of 

 the rocks which had been their source ; and he seems 

 to have been led by the consideration of the gradual 

 action of ' weathering,' in rendering available the 

 otherwise locked-up stores, to attribute the benefits 

 of fallow exclusively to the increased supply of the 

 incombustible or mineral constituents which would, 

 by its agency, be brought into a condition in which 

 they could be taken up by plants. 



It will be seen further on, how very materially 

 subsequently acquired experimental evidence has 

 tended to modify our views as to the explanation of 

 the benefits of fallow. 



The benefits of an alternation of crops, Liebig 

 considered to be in part explained by the influence 

 of the excreted matters from one description of crop 

 upon the growth of another. He did not attach 

 weight to the assumption that such matters would 

 be directly injurious to the same description of crop ; 

 but he supposed rather that the matters excreted by a 

 plant were those which it did not need, and that they 

 would therefore be of no avail to the same description 

 of plant, but would be of use to others. He, how- 

 ever, attributed much of the benefits of a rotation, to 

 different mineral constituents being required from 

 the soil by the different crops. 



Since the enunciation of these views, very much 



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