With reference to the toxic effect of the third type of substances, i.e., the common soluble salts, it is known that single salts of potassium, magnesium, sodium, or calcium, in certain concentrations, are toxic to plants, while mixtures of the same salts in the same concentrations are not. Thus, solutions of sodium chloride, magnesium sulfate, potassium chloride, and calcium chloride which, when used singly, killed plants whose roots were immersed in them for only a few minutes, formed when mixed together a nutrient solution in which the same plants grew normally. The remarkable remedial effect of calcium salts in overcoming the injurious effects of other soluble salts has already been mentioned. One explanation 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 rôle 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 Rôle 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.)