ture; and the presence of certain of the necessary
mineral plant constituents, and the presence of carbonate of lime.

The light which these discoveries throw upon the extremely complicated
question of the fertility of the soil is considerable, as it follows
that no soil can be regarded as really a fertile one in which the
process of nitrification does not freely take place. They furthermore
explain many facts, hitherto observed but not well understood, with
regard to the action of different nitrogenous manures.

_Ash Constituents of the Plant._

We now come to consider the present state of our knowledge on the
essentialness of the ash or mineral portion of the plant. While a
portion of the plant's substance which, up to Liebig's time, had
obtained little notice, it has, since the publication of his famous
"mineral" theory, obtained an ever-increasing amount of investigation.

Up till 1800 practically nothing was known of the function of the ash
constituents. In 1802 de Saussure wrote that it was unknown whether the
constituents of many plants were due to the soils on which they grew, or
whether they were the products of vegetable growth. Some two years
later, however, he was enabled to carry out a number of experiments
which really placed the subject on a firm scientific basis. The
essentialness of the ash constituents was only, however, placed beyond
all doubt by Wiegmann and Polstorff's researches, carried out in 1840.

Reference has already been made to the great stimulus given to research
by the promulgation of Liebig's mineral theory.

_Methods of Research._

In epitomising the vast amount of work carried on since 1840, with the
view of ascertaining the essentialness of the various substances found
in the ash of plants, two methods of experimentation have been
followed.

_Artificial Soils._

The first of these two methods was that adopted in the famous
experiments, carried out by Prince Salm-Horstmar, which have done so
much to further our knowledge on this question. It consisted in growing
plants on an artificial soil--formed out of sugar-charcoal, pulverised
quartz or purified sand--to which were added the different food
constituents.

_Water-culture._

While the results obtained by Prince Salm-Horstmar by this method were
of a most valuable nature, subsequent experimenters have abandoned his
method for the other method--viz., "water-culture." The medium used in
this process is pure water;