Agricultural C/iemistr?/. 
489 
ing upon a consideration of our own evidence and conclusions on 
this point, it may be well to give a very rapid review, of wliat 
may be called the natural history of the various opinions which 
have of late years been entertained, regarding the two most im- 
portant and closely allied subjects, of mamire and rotation. 
At the time of the appearance of Baron Liebig's first work in 
1840, the prevailing impression, which liad received much con- 
firmation from the investigations of M. Boussingault, was, that 
nitrogen was one of the most important constituents of manure. 
Baron Liebig, in his first edition, also dwelt particularly on this 
point ; but at the same time, he drew much more special attention 
to the importance of the mineral constituents of plants than had 
hitherto been bestowed upon them. After the publication of 
Baron Liebig's first book, M. Boussingault published much more 
fully the results of his various agricultural investigations, and 
the conclusions to which he had arrived in regard to them. 
VVith respect to manures, he (M. Boussingault) concluded that 
their relative value was determinable, more by the amount of 
nitrogen they contained, than by that of any other constituent. 
And in reference to the subject of rotation, after having given 
the chemical statistics of three separate courses, in which roots, 
leguminous crops, and cereals had been alternated with each 
other, and also of one with wheat grown two years in succession, 
after fallow and manure, he thus speaks in regard to the mutual 
relations of different crops : — 
"In the five years' rotation, it may Le observed that there are two crops, the 
hoed crop and the forage crop, which yield substances to the ground that are both 
abundant in quantity and rich in azotised matter, and it is unquestionable that 
these croi)s act favourably on the cereals which succeed them. But data are 
wanting for the appreciation of their specific utility to the general rotation." 
— Rural Econcmiy, p. 488. 
And, again, in the next page, he says : — 
" When the relative value of different systems of rotation are discussed in 
the way we have done, we in fact estimate the value of the elementary matter 
derived from the atmosphere by an aggregate of crops ; but the procedure 
generally followed is silent when the question is to assign to each crop in par- 
ticular the special share which it has had in the total profit."' 
Now, Baron Liebig, after detailing the experiments of M. 
Boussingault — -in the course of which he argues, that inasmuch as 
a larger amount of nitrogen was obtained in some of the later 
crops of the rotation, than in the earlier ones which immediately 
succeeded the application of the manure, it was obvious, that the 
nitrogen of the crops could not be due to the nitrogen of the 
manure which had been applied — says : — 
" Boussingault concludes that leguminous plants alone possess the power of 
appropriating, as food, nitrogen from the air, and that other cultivated plants 
do not at all possess this property. Hence the great importance which Bous- 
