salts of ammonia, and that the liberated alkali
combined with the organic matter. It must be admitted, however, that it
is very doubtful whether the ammonia and other substances are fixed in
the soil by a true chemical combination. They are certainly retained by
a very feeble attraction, for it appears from Brustlein's experiments
that ammonia may be, to a considerable extent, removed by washing with
abundance of water, and that if the soil which has absorbed ammonia be
allowed to become dry in the air, it loses half its ammonia, and after
four times moistening and drying, three-fourths have disappeared. These
facts are certainly not incompatible with the presence of a true
chemical compound, for the humate of ammonia is not absolutely
insoluble, and many cases occur of actions taking place in the presence
of water, which are entirely reversed when that fluid is removed; and it
is quite possible that when humate of ammonia is dried in contact with
carbonate of lime, it may be decomposed, and carbonate of ammonia
escape. There are other circumstances, however, which render it, on the
whole, most probable that the combination is not wholly chemical, but
rather of a physical character, among which may be more especially
mentioned the fact, that the quantity of the substances retained by the
soil is dependent on the degree of dilution of the fluid from which they
are taken; and that the quantity absorbed never exceeds a very small
fraction of the weight of the soil.

The practical inferences to be drawn from these facts regarding the
value of soils are of the highest importance. It is obvious that two
soils having exactly the same chemical composition may differ widely in
absorptive power, and that which possesses it most largely must have the
highest agricultural value. The examination of different soils, in this
point of view, is a subject of much importance, and deserves the best
attention of both farmers and chemists, although little has as yet been
done in regard to it, and the results which have been obtained are not
of a very satisfactory character. Liebig states, that in his
experiments, all the arable soils examined possessed the same absorptive
power, whether they contained a large or a small proportion of lime or
alumina. It can scarcely be expected, however, that this should be true
in all cases, and there are many facts which seem to indicate that
differences must exist. It is well known that there are some soi