[Vor. 7 
2 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
For several years! the writer has given attention, as occasion 
permitted, to the reaction of the medium as affecting growth in 
soil solutions, also its effect upon toxic action; but it is only 
within the past two years that opportunity has been taken from 
time to time to examine this factor critically and so far as prac- 
ticable in respect to physiologically balanced nutrient solutions. 
The illuminating contributions of Clark and Lubs (17) on 
the colorimetric determination of the hydrogen ion concentra- 
tion of culture media for mieroórganisms, and of other biological 
fluids, effected so great an improvement in the simpler technique 
involving active acidity and alkalinity that there is now avail- 
able a convenient, rapid, and sufficiently accurate method of 
investigating the hydrogen ion relations in connection with the 
salt requirements of higher plants. On the other hand, the 
extensive contributions of Schreiner and Skinner (710, *10a, *11, 
'12), Tottingham (714), Shive (15), McCall (16), and many 
others, on salt requirements and the constitution of the mineral 
nutrient solution have made it possible to select, within certain 
limits, well-balanced solutions as points of departure. In the 
work here reported, therefore, the writer has not concerned him- 
self to any great extent with an investigation of the effects of 
variations in the proportions of the different salts involved in 
the nutrient solution. In this last-mentioned direction the 
recent literature represents a rapid advance both in technique 
and in result, and while, as will be pointed out later, the problem 
may not be finally solved, it has certainly been placed on a 
rational basis. 
The triangle-diagram scheme was first rendered of biological 
significance by Schreiner and Skinner, referred to above, and it 
was effectively employed by them in studies of the relations of 
plants to certain toxic agents which might exist in the soil and 
in studies of the ameliorating action of the nutrient ions, K, 
NO;, and PO,, upon such deleterious compounds. They like- 
wise investigated by this means the growth of wheat as affected 
by different ratios of phosphate, nitrate, and potassium. The 
same general diagram scheme has been perfected and advan- 
1 During the progress of these investigations the writer has had much assistance in 
the details of the culture work from various members of the graduate laboratory, to 
all of whom he is indebted, especially to Mrs. Emily Schroeder, Research Assistant, 
1918-19, and Mr. R. W. Webb, Research Assistant, 1919-20. 
