* 210 DR. DAUBENY ON THE ROTATION OF CROPS, ETC. 



inorganic principles, which form, as it were, the skeleton of each plant, although he 

 attributes the peculiar benefit derived from fallow crops to their power of generating 

 the organic matter which is required by the cereals that are to succeed them. 



In order to determine then in what degree the falling off of the permanent crop 

 arose from the one or the other of these causes, it seemed necessary to obtain an 

 analysis of the plants derived from this and from the shifting crop corresponding, 

 and to compare the composition of both with that of a standard specimen of the same 

 plant determined by the method pursued with respect to the two former ; and it 

 would have been also satisfactory, not only to ascertain, whether the soil itself origi- 

 nally contained such a store of all the principles existing in the crop, as might be 

 sufficient to meet the demand made upon it for that purpose during the whole decen- 

 nial period, but also whether its present composition was such, as actually indicated 

 a deficiency in any of the principles which entered into the constitution of the plants 

 grown in it. 



It is evident that the former branch of the inquiry would have been superfluous, 

 if I could have depended on two things : — 



1st. That the analyses given by Sprengel and others, of the plants to which the 

 inquiry related, were trustworthy ; and 



2ndly. That the composition of the same vegetable was at all times uniform both 

 as to the quality and quantity of its ingredients. 



But with respect to the former point, I found, on turning to the analyses given of 

 the ashes of the same plants by different authorities, many marked discrepancies, 

 and that those of Sprengel, which are the most numerous of any we could appeal to, 

 were regarded as inaccurate by other chemists of higher distinction. 



Nor, even if they had represented truly the composition of the plants which were 

 actually examined by that analyst, could we be sure, that they would apply to those 

 of the same species, grown in a different country, and under altered circumstances, 

 more particularly as the recent researches of Liebig, Will, Fresenius and others, 

 appeared to indicate, that certain ingredients admit of being substituted for others, 

 according to laws as yet not fully made out. 



For all these reasons then, it became necessary for my purpose to obtain a correct 

 analysis, both of the crops, and of the soil ; and I was the more reconciled to the ex. 

 penditure of labour involved in this undertaking, when I reflected, that the results 

 obtained were likely not only to lead to an explanation of the cause of the utility of 

 a rotation of crops, but also to throw some incidental light upon certain other 

 points connected with the chemistry of agriculture, which did not appear to be suf- 

 ficiently elucidated ; such for instance, as the degree of variation of which a plant 

 may admit in the quality and quantity of its inorganic ingredients, or in other words, 

 its power of substituting one principle for another, and likewise as to the state of 

 combination, in which the alkalies, phosphates, &c. exist with the other constituents 

 of the soil, when in a condition to be assimilated by a plant. 



