86 
Journal of Agricultural Research 
Vol. 26, No. 3 
desired figure. Furthermore, the nearer the value approaches the neu¬ 
tral point the longer time is required to attain equilibrium. This is 
particularly true with clay and silt soils. 
The fact that on soils a P H value may be obtained which indicates 
a larger H-ion concentration than hydroxyl-ion concentration means 
that the acid-producing substance is soluble and ionized, even if the 
degree of such solubility and ionization is small. Regardless of whether 
it is large or small, the gradual introduction of OH ions causes a change 
such that more hydrogen ions are produced. The slowness of this change 
is the fundamental difficulty in using the hydrogen electrode as the 
basis of a quantitative method for soil. Sometimes the complicated and 
expensive apparatus necessary is cited as an objection, but this is no 
more valid for soil than for the large number of other substances on which 
electrometric measurements are made accurately and rapidly. 
THE PROBLEM INVESTIGATED 
The experiments presented in this paper were prompted by a desire to 
overcome some of the difficulties encountered in the use of the hydrogen 
electrode for the quantitative measurements involved in the adjustment 
of the hydrogen-ion concentration of a soil suspension to any other 
desired concentration. At the same time, it was desired to study some of 
the phenomena associated with the adsorption of calcium when added to 
the soil in the form of a Ca(OH) 2 solution. The time of the investigator 
is an important factor. To eliminate the time factor as an objection 
to the use of the electrometric method for soil, it is necessary to devise 
apparatus and methods which require the minimum amount of con- 
tinous attention. 
For work on soil acidity neutral salts have been much used. A neutral 
salt, such as KC 1 , also forms a part of the electrical connection between 
the electrode vessel and calomel cell. For these reasons this salt was 
included in the work here presented. The relative solubility of the acid- 
producing substance was studied by making measurements on both 
suspensions and extracts of soils. Calcium was for the most part added 
in the form of a Ca(OH) 2 solution. In some experiments precipitated 
CaC 0 3 was used in addition to the Ca(OH) 2 . 
METHODS OF EXPERIMENTATION 
SPECIAL, APPARATUS 
The potentiometer system, essentially as outlined by Hildebrand 
(21) was used in this investigation. The apparatus consists of the 
following principal pieces: One Weston direct-reading laboratory standard 
voltmeter, No. 5; one Kohlrausch slide-wire bridge, one Leeds and North- 
rup No. 2500 type R galvanometer with lamp and scale, Edison storage 
batteries, and other necessary accessories. The saturated potassium 
chlorid-calomel cell was chosen as best suited for soil work. Fales and 
Mudge ( 12) have shown that this type is the most reliable. 
An apparatus was devised by which it was possible to have six elec¬ 
trodes m operation at the same time, so arranged as to require very 
little continuous attention. The position of these electrodes with reference 
to one another and the wiring arrangement are shown in figure 1. A 
side view of one of the combinations of hydrogen electrode and calomel 
