The contemporary appearance of the large amount of magnesia
and that of alkalies in D agrees neither with the relative propor-
tion of the substances in A nor with that in B. It seems, how-
ever, proper to say that the substances in D were washed out with-
in both periods of degradation, as represented by A and B. Ac-
cording to this acceptation, the silicate D had to show an analo-
gous character with the silicate C, which finds indeed its confirm-
ation by the proportion of the oxygen of the bases to that of silicic
acid, which is in both an equal one, namely, 1-2; a proportion
which is in the middle of both above mentioned, viz: 1-1.7 and
1-2.8.

We see by the above mentioned facts that the volcanic rocks, es-
pecially those which are rich in alkalies and clay, give away by
the process of degradation a large quantity of potash and soda,
combined with silicic acid, salts, which, by their solubility, are
easily washed out by rain water, and then capable of penetrating
the soils: whilst the remaining products of degradation become
more and more of a clayey character, (the origin of all clayey
compounds,) and poorer in alkaline substances and soluble silicic
acid; that, however, the degradation never can advance to such
a degree that the latter substances will be entirely washed out and
consequently absent. All soils contain therefore soluble silicic
acid and alkalies, on account of the clay present in them, which is
one of their fundamental ingredients. Humus, as we have pre-
viously stated, possesses the property to retain alkaline substances
and soluble silicic acid in a manner similar to clay. This may,
to a certain extent, explain the reason of the fact, that the produce
of the soil, other things being equal, is in direct ratio to its humus
and clay.

From all, then, that has been said, from the known and as-
certained condition of soils of known fertility; from the com-
position of crops that are grown for food; from the effects of
the application of manures, it is to be concluded that the pro-
ductiveness of a soil to the extent of the production of the plant
is due:

1st. To the presence, in exact ratio, of the mineral constit-
uents named above.

2d. The condition in which these substances are found as
to their solubility.

3d. The capacity of the soil, as to its physical texture, to
supply the growing plant with organic food from the atmos-
phere.

The proportion of the several mineral constituents best
adapted to produce fertility and the requisite physical struc-
ture can be found, or at least approximated to, by a large

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