of these relationships between mineral soil constituents and the living
plant is that the life phenomena depend upon a balanced adjustment between
the compounds of these different mineral elements with the proteins
(producing the so-called "metal proteids") which constitute the active
material of the cell protoplasm. According to this theory, any excess or
deficiency of any one or more of these elements in the plant juices which
surround a given cell will, of course, cause an interchange with the
mineral components of the supposed "metal proteids" which upsets the
assumed essential balance between them, with disastrous results. A more
recent, and much more satisfactory, explanation of the "antagonism" between
mineral elements in their toxic effects upon plants, which has both
theoretical and experimental confirmation, is that single salts disturb the
colloidal condition (see Chapter XV) of the protoplasm of the plant cells
in such a way as to destroy its permeability to nutrient substances, while
mixtures of salts restore the proper state of colloidal dispersion and
permit the normal functioning of the protoplasm.

It is apparent from the above brief discussions that the role of the
different soil elements as plant food, and their relations to the complex
processes which constitute plant growth, afford an interesting and
promising field for further study.

References

BRENCHLEY, WINIFRED E.--"Inorganic Plant Poisons and Stimulants," 106
pages, 18 figs., Cambridge, 1914.

HALL, A. D.--"Fertilizers and Manures," 384 pages, 7 plates, London, 1909.

HALL, A. D.--"The Book of the Rothamsted Experiments," 294 pages, 49 figs.,
8 plates, London, 1905.

HOPKINS, C. G.--"Soil Fertility and Permanent Agriculture," 653 pages,
Chicago, 1910.

HILGARD, E. W.--"Soils," 593 pages, 89 figs., New York, 1906.

LOEW, O.--"The Physiological Role of Mineral Nutrients," U. S. Department
of Agriculture, Bureau of Plant Industry, _Bulletin_ No. 45, 70 pages,
Washington, D. C., 1903.

RUSSELL, E. J.--"Soil Conditions and Plant Growth," 243 pages, 13 figs.,
_Monographs_ on Biochemistry, London, 1917. (3d ed.)

WHITNEY, M.--"A Study of Crop Yields and Soil Composition in Relation to
Soil Productivity," U. S. Department of Agriculture, Bureau of Soils,
_Bulletin_ No. 57, 127 pages, 24 figs., Washington, D. C., 1909.

CHAPTER II

THE ORGANIC