on Plant Growth in Nutrient Solutions, &c. 367 
graphs show clearly the marked increase in the average pH values of the 
unmodified Tottingham solutions of series E and F, and a considerable 
decrease in these values for all but one of the solutions containing ammonium 
sulphate (series G), thus indicating that the effect of the growing plants upon 
reaction change is directly opposite in the two types of solutions here used. 
The graphs of series E and F further show that the greatest reaction change 
occurred in the solutions with low proportions of the mono-potassium phos¬ 
phate, and that the reaction change was correspondingly less as the concen- 
T-11111111113333335557 
R-13135573111311353111 
C - 5 3 3 1 1 3 1 5 1 73315111131 
Fig. 3. Graphs of pH values of culture solutions after contact with plant roots during the growth 
intervals between solution renewals; averages of all tests made during the growth period. 
tration of this salt increased. On the other hand, no such relation is shown 
for the solutions containing ammonium sulphate (series G). 
It has been found by a number of investigators that one of the effects 
of plants upon a nutrient medium containing an ammonium compound is to 
increase the acidity of the medium. As far back as 1860 this was recognized 
by Knop(24) in solution cultures, and shortly afterwards was mentioned by 
Rautenberg and Kuhn (34). Among many other workers who have con¬ 
firmed this observation may be mentioned Maze (28), Nathansohn (30), 
Ehrenberg (5), Prianischnikow (33), Nikitinsky (31), Jones and Shive ( 20 ), 
and others. Pantanelli (32) in a series of studies with single salt solutions 
has shown how the ratio of the anion to the kation changes as one ion 
is removed by the plant more rapidly than the other. In a similar way 
Breazeale and Le Clerc (1) showed that this differential ion absorption by 
plant roots occurs in solutions of potassium chloride and potassium sulphate. 
B b 2 
