492 
Agricultural Chemistry. 
" But, at the same time, it is of great importance for agriculture, to know 
with certainty that the supply of ammonia is unnecessary for most of our 
cultivated plants, and that it iiw.y he even superfluous, if only the soil c&ntain a 
sufficient supply of the mineral food of plants, when the ammonia required 
for their development will be furnished by the atmosplicre. It is also of 
importance to know, that the rule usually adopted in France and in Germany 
of estiniatimi the value of a manure according to the amount of its nitroyen, 
is quite fallacious, and that its vcdue does not stand in proportion to its 
nitrogen.''' ! 
Such, then, were the reasonings and conclusions of Baron 
Liebig regarding the relative value of the constituents of manure, 
founded, to a great extent, upon a consideration of the experi- 
ments of M. Boussingault on the chemical statistics of rotation, 
and in opposition to the opinions arrived at by the latter in 
regard to those experiments. On t!ie cliemical principles involved 
in rotation itself Baron Liebig thus expresses himself: — 
" In a more confined sense, the time of fallow may be limited to the interval : 
in the cultivation of cereal plajits ; for a magazine of soluble silicates and ot 
alkalies is an essential condition to the existence of such plants. The cultiva- 
tion of potatoes or of turnips during the interval will not impair the fertility 
of the field for the cereals which are to succeed, (supposing the sujjply of 
alkalies to be sufficient for both), because the former plants do not rctj^uiiu any 
of the silica necessary for the latter." — 4</t Edition, j5. 133. 
Again, after many pages of illustration of the importance of 
the mineral, or soil-proper constituents, he says : — • 
" It follows, then, from the ])receding observations, that the advantage of 
the alternate system of husbandry consists in the fact that the cultivated 
plants abstract from the soil imequal quantities of certain nutritious matters. 
" A fertile soil must contain in .sufficient quantity, and in a form adapted 
for assimilation, all the inorganic materials indispensable for the growth of 
plants. 
" A field artificially prepared for culture, contains a certain amount of these 
ingi-edients, and also of ammoniacal salts and decaying vegetable matter. The 
system of rotation adopted on such a field is, that a potash-plant (turnips or 
liotatoes) is succeeded by a silica-plant, and the latter is followed by a lime- 
plant." — Ath Edition, ]}. 169. 
And in his ' Principles ' recently published, he sums up the 
rationale of rotation in his 35th Proposition, thus : — 
" On the imequal quantity and quality (solubility, &c.) of the mineral 
constituents, and on the unequal ])roportious in which they are required for 
the development of the different cultivated crops, depends the rotation, of crops, 
and the varieties of rotation employed in different localities." — P. 29. 
Here, then, we have distinctly enough set forth, even up to 
the present time. Baron Liebig's mineral theory of rotation ; and 
we have already seen, that he has pronounced as "thoroughly 
erroneous," M. Boussingault's conclusions as to the greater 
dependence of the cereals grown in our rotations, upon nitrogen 
sentence without the continuation, to illustrate our unfairness in not fully quoting 
Baron Liebig ! 
