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erate the decomposition of the soil, in order to provide a new 
generation of plants with the necessary elements in a condition 
favourable to their assimilation. It is obvious that the rapidity 
of the decomposition of a solid body must increase with the 
extension of its surface; the more points of contact we offer in 
a given time to the external chemical agent, the more rapid 
will be its action.” (Hence arises the advantage of frequently 
stirring the soil, and exposing fresh portions of it to the action 
of the atmosphere.) 
“All plants, cultivated as food, require for their healthy susten- 
ance the alkalies and alkaline earths, each in a certain pro- 
portion ; and in addition to these, the ceralia, or corn tribe, do 
not succeed in a soil destitute of silica, or sand, in a soluble 
condition. The combinations of this substance, found as 
natural productions, namely, the silicates, differ greatly in the 
degree of facility with which they undergo decomposition, in 
consequence of the unequal resistance opposed by their integral 
parts to the dissolving power of the atmospheric agencies. 
Thus the granite of Corsica degenerates into a powder in a 
time which scarcely suffices to deprive the polished granite of 
Heidelberg of its lustre.” 
“ Some soils abound in silicates so readily decomposable, that 
in every one or two years, as much silicate of potash becomes 
soluble and fitted for assimilation as is required by the leaves 
and straw of a crop of wheat. In Hungary, extensive districts 
are not uncommon where wheat and tobacco have been grown 
alternately upon the same soil for centuries, the land never 
receiving back any of those mineral elements which were with- 
drawn in the grain and straw. On the other hand, there are 
fields in which the necessary amount of soluble silicate of potash, 
for a single crop of wheat, is not separated from the insoluble 
masses in the soil in less than two, three, or even more years.” 
" The term fallow, in agriculture, designates that period in 
which the soil, left to the influence of the atmosphere, becomes 
enriched with those soluble mineral constituents. Fallow, 
however, does not generally imply an entire cessation of culti- 
vation, but only an interval in the growth of the ceralia. That 
store of silicates and alkalies, which is the principal condition 
of their success is obtained, if potatoes or turnips are grown 
upon the same fields in the intermediate periods, since these 
