ACIDITY AND ADSORPTION IN SOILS AS MEASURED 
BY THE HYDROGEN ELECTRODE 
By L. T. Sharp and D. R. Hoagland, 
Assistant Chemists, Agricultural Experiment Station of the University of California 1 
INTRODUCTION 
The problem of soil reaction has come to occupy an increasingly 
important position in the realm of soil-fertility studies. This is evident 
from the numerous papers recently appearing upon the subject of soil 
acidity. The great diversity of opinion concerning the nature and 
methods of measuring soil acidity has served thus far only to confuse the 
matter. By adopting modern methods capable of measuring specifically 
the hydrogen ion, 2 data have been obtained by the writers which seem 
to offer a clearer conception of the question. 
Before considering the question of soil acidity, it seems desirable to 
recall the fundamental significance of the terms “neutrality,” “acidity,” 
and “alkalinity.” The dissociation of pure water produces H ions and 
OH ions in equal concentration, denoting neutrality. The product of 
the concentrations of these ions in any solution is a constant, approxi¬ 
mately i X io~ 14 . When the H ions are present in a concentration greater 
than the OH ions—that is, in a concentration greater than 1X10" 7 , 
then the resulting solution is acid. Conversely, the presence of OH ions 
in greater concentration than i X io -7 gives an alkaline solution. The 
term “acidity” is often construed to mean the total quantity of H ions 
which may be produced when the equilibrium is continually shifted by 
the introduction of OH-ions. This total acidity is referred to as potential 
acidity, while the H-ion concentration at any given moment determines 
the intensity of acidity, to use Gillespie’s (13) 8 expression. In all 
probability excessive concentrations of H or OH ions in soil solutions 
exercise pronounced effects on plant growth and on the activities of 
soil bacteria. 
Potential acidity or alkalinity may be due to undissolved substances 
or to soluble compounds only partly hydrolyzed or dissociated. An 
obvious illustration of this is afforded by the titration of a suspension of 
calcium carbonate in water. So long as solid calcium carbonate remains, 
1 From the Divisions of Soil Chemistry and Bacteriology and Agricultural Chemistry in equal cooperation. 
Acknowledgment is made to Mr. C. L. A. Schmidt for valuable suggestion during the progress of this work. 
2 To be more exact, according to recent physical-chemical views, it is the activity of the H ion, rather 
than the concentration, which is measured here. However, for the present purpose this distinction seems 
unimportant. 
8 Reference is made by number to “ Literature cited/’ p. 143-145. 
(123) 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D, C. 
fq 
Vol. VII, No. 3 
Oct. 16,1915 
Cal.—5 
