I 4 2 
Journal of Agricultural Research 
Vol. VII, No. 3 
prolonged shaking is required after each addition of the titrating solu¬ 
tion, in order to obtain constant readings. This is due to the slow rate 
of solubility possessed by the acid constituents of the soil. The use 
of a mechanical shaking device would doubtless greatly facilitate the 
operation. 
Duplicate determinations on soil suspensions usually agree within 
o.oi to 0.02 volt. In any one determination constancy of readings 
may be obtained to within 0.002 volt. The larger sources of error 
result from the change in the decomposition of the solution due to 
reduction of nitrates or the breaking down of the HCO s ions. These 
points have already been thoroughly considered. Robertson (28) and 
later Desha and Acree (11) have shown the possible interference of cer¬ 
tain types of organic matter. Saturation of the electrode with hydrogen 
before dipping seems to obviate the error, and no difficulty from this 
source was experienced in the present work. 
The use of the modification of Hildebrand's apparatus described in 
this paper and due observance of the special precautions noted will, it 
is believed, enable the investigator of soils and plants to obtain valuable 
information in regard to H-ion concentrations without undue loss of 
time. In many fields of biochemistry 1 similar methods have been 
extensively employed during the last few years. 
SUMMARY 
(1) Soil acidity is due to the presence of an excess of hydrogen ions in 
the soil solution. 
(2) Direct evidence of this fact is given by hydrogen-electrode meas¬ 
urements. 
(3) The hydrogen-ion concentration of different soil suspensions was 
found to vary within wide limits, from a conditipn of high acidity to one 
of high alkalinity. 
(4) Soils containing calcium in equilibrium with HCOJ and C0 2 have 
a very slightly alkaline reaction. 
(5) The effect of heating, grinding, and of varying the ratio of soil to 
water on the hydrogen-ion concentration was studied. 
(6) An electrometric method for the determination of the lime require¬ 
ment of soils is suggested. 
(7) The addition of sodium chlorid, potassium chlorid, and barium 
chlorid to certain soil suspensions was found to increase the hydrogen-ion 
concentration. 
1 For a review of the literature concerning the application of the hydrogen electrode to biochemistry 
the reader is referred to the following: 
Schmidt, C. L. A. Changes in the H+ and OH - concentration which take place in the formation 
of certain protein compounds. In Jour. Biol. Chem., v. 25, no. 1, p. 63-79, 9 fig* 1916. Bibliographical 
footnotes. « 
Sorensen, S. F. I*. Uber die Messung und Bedeutung der Wasserstoffionenkonzentration bei biologis- 
chen Prozessen. In Ergeb. Physiol., Jahrg. 12, p. 393-532,12 fig* 1912. Eiteratur, p. 394-398. 
