14 



tion " of individual plants for these soluble salts. The tolerance 

 shown by the different species of plants toward these soluble salts 

 (the so-called " alkali " in soils) varies widely; indeed, there 

 seems to be considerable variation in the resistance of different 

 individual plants of the same species to injury from this cause. 



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 concen- 

 trations, 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 im- 

 mersed 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 men- 

 tioned. 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 theo- 

 retical 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 r61e 

 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. 



