however, very considerably in
their composition. The following is an analysis of a clay soil by Dr
Anderson:--

Silica 60.03
Alumina 14.91
Peroxide of iron 8.94
Lime 2.08
Magnesia 4.22
Potash 3.87
Soda 0.06
Water and carbonic acid 5.67
-----
99.72
-----

NOTE VII. (p. 91).

FORMS IN WHICH PLANT-FOODS ARE PRESENT IN SOIL.

The forms in which the bases necessary for plant-food are present in the
soil, are chiefly as _hydrated silicates_, and in combination with
organic acids, forming humates, &c., as well as in the form of sulphates
and chlorides.

Phosphoric acid is present in combination with iron, alumina, or lime,
or possibly also as magnesium-ammonium-phosphate. Sulphuric acid is
generally present in a more or less insoluble condition, in combination
with iron and lime; whereas chlorine is combined with the alkali bases
in an easily soluble form. An important point is as to the form in which
the plant absorbs these food constituents. In this connection reference
may be made to a theory put forward by a very distinguished French
agricultural chemist, Professor Grandeau. His theory is that the
necessary ingredients of plant-food are absorbed into the plant as
humates, or, at any rate, that the medium of this transference is humic
acid, and organic acids of a similar nature. This theory, however, while
ingenious, has not yet been supported by sufficient evidence to make its
acceptance advisable. It is probable that it is only in the form of
soluble salts that the plant can absorb its food. It is quite probable,
however, at the same time, that the exact form in which the different
food substances enter the plant may be largely determined by
circumstances. According to Nobbe, chloride of potassium is the most
suitable form of potassium salts, although the plant may absorb its
potassium as sulphate, phosphate, or even silicate.

FOOTNOTES:

[61] Composition of the earth's solid crust in 100 parts by weight:--

Oxygen 44.0 to 48.7 | Calcium 6.6 to 0.9
Silicon 22.8 to 36.2 | Magnesium 2.7 to 0.1
Aluminium 9.9 to 6.1 | Sodium 2.4 to 2.5
Iron 9.9 to 2.4 | Potassium 1.7 to 3.1

(Roscoe's 'Les