in promoting plant-growth, by acting as direct food of the
plant. As we have already seen in the Introductory Chapter,[50] the
substances which have been found in the ash of plants are the following:
_potash_, _lime_, _magnesia_, _oxide of iron_, _phosphoric acid_,
_sulphuric acid_, soda, silica, chlorine, oxide of manganese, lithia,
rubidia, alumina, oxide of copper, bromine, and iodine. The general
presence of some of these substances is doubtful; the presence of
others, again, probably purely accidental; while some are only found in
plants of a special nature, as, for instance, iodine and bromine, which
are only found in the ash of marine plants.

Of these ash constituents, only the first six substances--those marked
in italics--are absolutely necessary to plant-growth. In addition to
these six ash constituents, the plant also derives its _nitrogen_, which
is a necessary plant-food, chiefly from the soil.[51]

_Importance of Nitrogen, Phosphoric Acid, and Potash._

But of these seven constituents of the soil which are necessary to
plant-growth, some have come to be regarded by the agriculturist with
very much greater interest than others. This is due to the fact that
they are normally present in the soil in very much smaller quantities
than is the case with the other equally necessary food ingredients;
that, in short, they are nearly invariably present in the soil, in a
readily available form, in lesser quantities than the plant is able to
avail itself of, and often, as in impoverished or barren soils, in
quantities too small for even normal growth. These ingredients are
_nitrogen_, _phosphoric acid_, and _potash_.[52]

The importance of seeing that all the necessary plant ingredients are
present in a soil in proper quantities will be at once properly
estimated when it is stated that the absence or insufficiency in amount
of one single ingredient is capable of preventing the growth of the
plant, although the other necessary ingredients may be even abundantly
present.

With lime, magnesia, iron, and sulphuric acid, most soils are abundantly
supplied. The substances with which the farmer has to concern himself,
then, are nitrogen, phosphates, and potash. It is these substances
therefore, that, as a rule, are alone added as manures.

_Chemical Condition of Fertilising Ingredients in Soil._

But in considering the chemical properties of a soil, a simple
consideration of the quantity of the different ingredien