Jan. 26,1924 
Changes in Hydrogen-Ion Concentration 
209 
Table I. —Changes produced by wheat seedlings in hydrogen-ion concentration of a 
nutrient solution as affected by its concentration and its initial reaction « 
Concentration of solution. 
Reaction. 
Initial. 
After 1 day. 
Afteradays. 
Undiluted. 
Ph. 
5 - 5 
5 - 1 
5-5 
5 -1 
Ph. 
5-4 
4.8 
5-2 
3-9 
Ph. 
5-2 
4. 6 
4. 1 
3*6 
Do. 
Diluted ? times. 
Do. . 
0 The acidity data are reported in this and the other tables in the P H system, not because this system 
is believed to be the best for the purpose, but because other workers are using it and its use facilitates 
comparison of these results with theirs. 
The nutrient solution used had the following composition: 
Gm. 
Calcium nitrate.. 2. 7 
Di-potassium phosphate.... 1.5 
Magnesium sulphate.6 
Potassium chlorid..75 
Ferrous sulphate..... ..01 
Twenty wheat seedlings were used for each culture. They were about 
4 days old when transferred from tap water to the solution. The nutrient 
solution was used in two concentrations—full strength and diluted five 
times. The initial reaction of the full-strength solution, expressed in 
the P H system, was 5.5. The other initial reactions were produced by 
the addition of hydrochloric acid. Variations in the changes of reaction 
were obtained while using the same plant and solutions of the same 
composition. The changes were clearly affected by the initial concen¬ 
tration of the nutrient solution as well as by its initial reactions. The 
general tendency toward increasing acidity, it appears, was due to the 
use of very young seedlings, most of the investigators cited having 
worked with seedlings of more advanced age than those used here. In 
its present phase, however, this investigation deals primarily with the 
causes of which the changes under discussion are the result. 
Very little work has been done to determine the causes of the changes in 
reaction which occur in nutrient solutions under the influence of growing 
seedlings. Arrhenius (r, 2) assumed that these changes are due to root 
excretions which are regulatory and adapt the reaction of the medium to 
the needs of the plant. He did not, however, bring any evidence to 
support this assumption. His finding that the reaction of the soil in 
immediate contact with roots is different from its average reaction in the 
same vicinity does not prove that the change is due to neutralization. 
Selective absorption, which is the alternative, would explain the observa¬ 
tions just as well. 
Breazeale and Le Clerc (3), and Hoagland (6) also, concluded on the 
basis of chemical analyses that the changes of reaction produced by 
growing seedlings in the medium of growth are due to selective absorption. 
There is, however, some question as to the reliability of direct chemical 
analysis in dealing with the minute quantities involved, for instance, in a 
change from P H 5.6 to P H 6.8, as was the case in Hoagland's experiment. 
