ns of any of
the essential elements. Only soluble _ferric_ compounds seem to serve as a
suitable source of supply of the element; _ferrous_ compounds being usually
highly toxic to plants.

=Sulfur= is an essential element of plant food. The amounts required by
plants were supposed, until recently, to be relatively small. This was due
to the fact that earlier studies took account only of the sulfur which, on
analysis, appeared as sulfates in the ash. Improved methods of analysis,
which insure that the sulfur which is present in the plant tissue in
organic combinations is oxidized under such conditions that it is not lost
by volatilization during the combustion of the material, have shown that
the total sulfur which is present in many plants approaches the quantity of
phosphorus which is present in the same tissue. Furthermore, recent field
and pot experiments have shown that at least a considerable part of the
beneficial effects of many fertilizers, which has previously been
attributed to the calcium, potassium, or phosphorus which they contain, is
actually due to the sulfur present as sulfates in the fertilizers used.

Sulfur occurs in the organic compounds of plants, associated with
phosphorus. It seems probable that its physiological uses are similar to
those of the latter element; but there is as yet no experimental evidence
to establish its exact role in the economy of plant growth. It appears to
be needed in largest proportion by plants which contain high percentages of
nitrogen in their foliage, such as the legumes. There is some evidence that
sulfur has a particular role in promoting the growth of bacteria, and it
may be that the percentages of total sulfur which are found in the tissues
of legumes are due to the presence of the symbiotic nitrogen-gathering
bacteria in the nodules on the roots of these plants. This point has not
yet been investigated, however.

=Sodium= is probably not essential to plant growth, although it is present
in small proportions in the ash from practically all plants. In cases of
insufficient supply of potassium, sodium can apparently perform at least a
part of the role of the former element; but this seems not to be a normal
relationship or use.

=Chlorine= is found in small amounts in the sap and in the ash of nearly
all plants. However, it does not appear to be essential to the growth of a
plant, except possibly in the case of certain species, such as asparagus,
buckwheat, and, pe