relatively small proportions
and any one may, therefore, be the limiting factor in plant growth so far
as plant food is concerned. These are nitrogen, phosphorus, potassium, and
(possibly) sulfur.

ROLE OF PLANT FOOD ELEMENTS IN PLANT GROWTH

The use which a plant makes of the elements which come to it from the soil
has been studied with great persistency and care by many plant
physiologists and chemists. Many of the reactions which take place in a
plant cell are extremely complicated, and the relation of the different
chemical elements to these is not easily ascertained. It is probable that
the same element may play a somewhat different role in different species of
plants, in different organs of the same plant, or at different stages of
the plant's development. But the usual and most important offices of each
element are now fairly well understood, and are briefly summarized in the
following paragraphs. It should be understood that a thorough and detailed
discussion of these matters, such as would be included in an advanced study
of plant nutrition, would reveal other functions than those which are
presented here and would require a more careful and more exact method of
statement than is suitable here. However, the general principles of the
utilization of soil elements by plants for their nutrition and growth may
be fairly well understood from the following statements.

=Nitrogen= is a constituent of all proteins (see Chapter XIII). Proteins
are apparently the active chemical components of protoplasm. Since it is in
the protoplasm of the green portions, usually foliage, of plants that the
photosynthesis of carbohydrates and the synthesis of most, or all, of the
other tissue-building materials and reserve food substances of the plant
takes place, the importance of nitrogen as a plant food can hardly be
over-emphasized. Nitrogen starvation produces marked changes in the growth
of a plant. Leaves are stunted in growth and a marked yellowing of the
entire foliage takes place; in fact, the whole plant takes on a stunted or
starved appearance. Abundance of nitrogen, on the other hand, produces a
rank growth of foliage of a deep rich color and a luxuriant development of
tissue, and retards the ripening process. In the early stages of growth,
the nitrogen is present most largely in the leaves; but when the seeds
develop, rapid translocation of protein material into the seeds takes
place, until finally a larg