cing the
chemical changes in the soil, and the particular value of humus lies in
its affording a supply of that substance exactly when it is wanted; but
the carbonic acid of the atmosphere also takes part in these changes,
although with different degrees of rapidity according to the character
of the soil, acting rapidly in light, and slowly in stiff, clay soils.
The solvent action of the carbonic acid is, no doubt, principally
exerted on the substances soluble in acids, but not entirely, for it is
known that the insoluble part is gradually being disintegrated and made
soluble; and hence it is that the composition of that part of the soil
which resists the action of acids, and which at first sight might appear
of no moment, is really important. It is obvious that this circumstance
must at once confer on the soil of the Carse of Gowrie a great
superiority over those of Mid-Lothian and most other districts; for it
contains in its insoluble part a quantity of alkalies which must
necessarily form a source of continued fertility. Accordingly,
experience has all along shown the great superiority of that soil, and
of alluvial soils generally, which are all more or less similar to it.
The facility with which these matters are attackable by carbonic acid is
also an important element of the fertility of a soil, and it is to the
existence of compounds which are readily decomposed by it that we
attribute the high fertility of the trap soils.

By a further examination of the analyses of fertile soils, it is at once
apparent that the most essential constituents of plants are by no means
very abundant in them. In fact, phosphoric and sulphuric acids, lime,
magnesia, and the alkalies, which in most instances make up nine-tenths
of the ash of plants, form but a small portion of even the most fertile
soils; while silica, which, except in the grasses, occurs in small
quantity, oxide of iron which is a limited, and alumina a rare,
constituent of the ash, constitute by far their larger part. Thus the
total amount of potash, soda, lime, magnesia, phosphoric and sulphuric
acids and chlorine, contained in the Mid-Lothian wheat soil amounts only
to 3.5888 per cent, and in the Perthshire to 6.4385, the entire
remainder being substances which enter into the plant for the most part
in much smaller quantity. And, as these small quantities of the more
important substances are capable of supplying the wants of the plant, it
must be obvious that a very