austs the subject, for numerous instances are known of
soils containing all the essential elements of plants in abundance, but
on which they nevertheless refuse to grow. In these instances the defect
is due either to the presence of some substance injurious to the plant,
or to the state of combination of those it requires being such as to
prevent their absorption. Reference has been already made to the bad
effects of protoxide of iron, and it would appear that organic matter is
sometimes injurious. Even water, by excluding air, and so preventing
those decompositions which play so important a part in liberating the
essential elements from their more permanent compounds, although it
cannot render a soil absolutely barren, not unfrequently materially
diminishes its fertility.

The state of combination of the soil constituents unquestionably
exercise a most important influence on its fertility. That this must be
the case is an inference which may be easily drawn from the statements
already made regarding the different minerals from which it is directly
or indirectly produced. If, for instance, a soil consist to a large
extent of mica, it would be found on analysis to contain abundance of
potash and some other matters, and yet our knowledge of the difficulty
with which that mineral is decomposed, would enable us to pronounce
unfavourably of the soil; and practical experience here fully confirms
the scientific inference.

The forms of combination most favourable to fertility is a subject on
which our information is at present comparatively limited. It was at one
time believed that solubility in water was an indispensable requisite,
but recent investigations appear to lead to a directly contrary
conclusion. The analyses of soils already given, show that the part
directly soluble in water embraces only a certain number of the
constituents of the plant, and of those dissolved the quantity is very
small. This becomes still more apparent if we estimate from the analyses
the actual quantities of those substances contained in an acre of soil.
It is generally assumed that the soil on an imperial acre of land 10
inches deep weighs in round numbers about 1000 tons; and calculating
from this, we find that the quantity of potash soluble in water in the
Mid-Lothian wheat soil, amounts to no more than 70 lb. per acre. But a
crop of hay carries off from the soil about 38 lb. of potash, and one of
turnips, including tops, not less than 200