| 3.38 |
+----+-------+--------+

Although those differences are very large, especially in the straw, and
must be attributed to the soil, it has hitherto been found impossible to
ascertain the nature of the relation subsisting between it and the crops
it yields; indeed, it must obviously be dependent on very complicated
questions, which cannot at present be solved, for it may be observed
that the increase in the grain does not occur simultaneously with that
in the straw, and in several cases a large proportion of ash in the
former is associated with an unusually small amount in the latter. _A
priori_, it might be expected that those soils which are especially rich
in the more important constituents of the ash should yield a produce
containing more than the average quantity, but this is very far from
being an invariable occurrence, and not unfrequently the very reverse
is the case. In some instances the variations may be traced to the soil,
as in the following analyses of the fruit of the horse-chesnut, grown on
an ordinary forest soil, and on a rich soil, produced by the
disintegration of porphyritic rock, in which the latter yields a much
larger quantity of ash:--

Kernel of seed. Green husk. Brown husk.
Forest soil 2.26 4.53 1.70
Porphyry soil 3.36 7.29 2.20

In the majority of instances we fail to establish any connection between
the nature of the soil and the plants it yields, chiefly because we are
still very deficient in analyses of those grown on uncultivated soils;
and on cultivated land it is impossible to draw conclusions, because the
nature of the manure exerts an influence quite as great, if not greater,
than that of the soil itself.

The relative proportion in which the different mineral matters enter
into the composition of the ash varies within very wide limits, as will
be apparent from the following table, containing a selection of the best
analyses of our common cultivated and a few uncultivated plants.

_Table of the Composition of the Ash of different Plants in 100 Parts._

_Note._--Alumina and oxide of manganese occur so rarely, that separate
columns have not been introduced for them, but their quantity is stated
in notes at the end of the table.

+---------------------+--------+------+-----------+---------+------+----------+
| | Potash.| Soda.| Chloride | Chloride| Lime.| Magnesia.|
|