Oct. 16, 1916 
Acidity and Adsorption in Soils 
139 
tions of soil acidity and adsorption on a new basis. The evidence has 
been confirmatory of the view that soil acidity is fundamentally de¬ 
pendent upon the equilibria of reactions yielding an excess of H ions, 
and is not necessarily related to the various phenomena grouped under 
the terms “absorption” and “ adsorption.” Although the literature con¬ 
cerning soils constantly refers to u absorption ” and “ adsorption/ 7 yet no 
very concise meaning has been attached to these terms. In fact, there 
is quite as much evidence in favor of a chemical, as opposed to a physical, 
interpretation of such phenomena. 3 ?or these reasons ,a more critical 
examination of this field would be a welcome addition to agricultural 
science. 
DESCRIPTION OF EXPERIMENTAL APPARATUS 
The apparatus used in determining the H-ion concentration is similar 
to that described by Hildebrand (17), with such modifications as are 
necessary or convenient for purposes of soil investigations (fig. 1). 
Precise methods for determining small differences of potential have fre¬ 
quently been described, but the use of an elaborate potentiometer system 
is neither practicable nor necessary in work with soils or nutrient solutions. 
Obviously measurements of physical-chemical exactitude are useless 
unless all the factors involved are capable of equally exact control, which 
is not possible with most substances of agricultural interest. For prac¬ 
tical application, therefore, the voltmeter method of Hildebrand is 
entirely adequate in point of accuracy and at the same time rapid and 
convenient. 
For information in regard to the physical-chemical principles under¬ 
lying the method, the reader is referred to Hildebrand (17), Michaelis (21), 
Itano (18), or to textbooks on electrochemistry. Very convenient tables 
for the transformation of voltmeter readings into H- and OH-ion con¬ 
centrations have been prepared by Schmidt (30). It is deemed desirable, 
however, to present here certain details of the apparatus and method of 
procedure, since these are not easily available to the general worker in 
agricultural laboratories. Moreover, measurements with soil suspen¬ 
sions require special precautions, to avoid otherwise very misleading 
results. , 
The arrangement of the apparatus and method of wiring are shown in 
figure 1. The entire system includes the following pieces of apparatus: 
Dry cell, two rheostats of 40 ohms resistance, with sliding contacts; 
Weston voltmeter, o to 1.2 volts; Leeds and Northrup portable galva¬ 
nometer, sensitivity of 1 megohm; contact key; calomel cell, hydrogen- 
electrode vessel; hydrogen generator, with purifying tube, wash bottles, 
and rheostat or lamp board connected with direct current. 
A convenient Cottrell hydrogen electrode, designed by the Department 
of Chemistry, University of California, consists of a glass tube of about 
1 cm. in diameter and 15 cm. in length, in the end of which is sealed a 
