n or not. But in the case of those
elements which are not taken out of the sap to be used by the plant cells
in their activities, the total amount taken from the soil is much less than
is that of the elements which are used in the synthetic processes of the
plant. Hence, much larger proportions of some elements than of others are
taken from the soil by plants. The proportions of the different elements
which are used by plants as raw materials for the manufacture of the
products needed for their growth varies with the different species; but a
certain amount of each of the so-called "essential elements" (see below) is
necessary to every plant, because each such element has a definite role
which it performs in the plant's growth. A plant cannot grow to maturity
unless a sufficient supply of each essential element comes to it from the
soil.

From the standpoint of their relative value as raw materials for plant
food, the elements which are present in the soil may be divided into three
classes; namely, the _non-essential_, the _essential and abundant_, and the
_critical_ elements.

The first class includes silicon, aluminium, sodium, manganese, and certain
other rarer elements which sometimes are found in soils of some special
type, or unusual origin. These elements seem to have no role to play in
the nutrition of plants; although silicon is always present in plant ash
and sodium salts are found in small quantities in all parts of practically
all plants. Nearly all species of plants can be grown to full maturity in
the entire absence of these elements from their culture medium. Occasional
exceptions to this statement in the case of special types of plants are
known, and are of interest in special studies of plant adaptations, but
need not be considered here.

The second group includes iron, calcium, magnesium, and, generally, sulfur.
All of these elements are essential for plant growth, but are usually
present in the soil in ample quantities to insure a sufficient supply in
available form for all plant needs. Recent investigations have shown,
however, that there are many soils in which sulfur is present in such
limited quantities that many agricultural crops, when grown on these soils,
respond favorably to the application of sulfur-containing fertilizers. In
such cases, sulfur is a "critical" element.

The "critical" elements are those which are essential to the growth of all
plants and which are present in most soils in