496 
Agricultural Cliemistry. 
Now it is obvious, that if the benefits of rotation depended on 
tlie "unequal quantity and quality (solubility, &c.) of the mineral 
constituents, and on the unequal proportions in which they are 
required for the development of the different cultivated crops," 
and if the benefit of the clover here introduced between the 
cereal barley and the cereal wheat, were the less exhaustion of the 
minerals by the leguminous crop, we should surely find, that in 
the cases here given, the latter had taken out of the land less of 
the important mineral constituents tlian either the barley grown 
before it or the wheat which succeeded it. But what are the facts 
of the case ? In ei:ery case there is more — and in most cases very 
viucli more — of j^hosplioric acid, potash, lime, and maynesia, taken 
off the land in. the clover crop, than in either the. harley or the icheat. 
And it should be remarked, that the produce of wheat obtained 
after the interposition and heavy mineral exhaustion of the clover, 
was in every case a very full one ; in fact, it was such in amount 
that we have every reason for concluding, that it was nearly double 
what would have been obtained had it been grown immediately 
succeeding the barley. The fact is, therefore, that we have a 
very much larger produce of wheat after this great drain from the 
land of phosphoric acid, potash, lime, and magnesia, than we 
should have had without the intervention of the crop which ex- 
tracted them. But there is one mineral constituent, namely, silica, 
which was taken out in very small quantity by the clover, though 
in very large quantity both by the preceding barley and the suc- 
ceeding wheat. 
If, tlierefore, the benefit of the intervention of the clover, de- 
pended upon its exhausting less of certain mineral constituents 
of the soil than the wheat which was to succeed it, it could only 
be in reference to silica that it had tliis beneficial action. But Baron 
Liebig has told us, that, provided there be a sufficiency of avail- 
aljle alkali in the soil, there will never be a deficiency of available 
silica. And if this be so, since we find that in every case, the clover 
found in the land nearly three times as much potash as was required 
by the succeeding heavy crop of wheat, we can hardly attribute 
the benefit of tlie clover upon the wheat crop to its conservation 
of soluble silica in the soil for the latter. But further, since we 
know from other experiments, that immediately after the bar- 
ley, we could have obtained a crop of wheat nearly, if not quite, 
equal to that which was obtained after the intervention of the 
clover, by means of ammoniacal salts, or nitrate of soda alone, it 
is clear that, unless the action of the latter manures was to render 
silica soluble and available for the crop, the beneficial action of 
the clover can have nothing whatever to do with the increase in 
the soil for the wheat crop of any of the important mineral con- 
stituents enumerated. 
