GILLESPIE: HYDROGEN-ION CONCENTRATION IN SOILS 9 



a hydrogen-ion concentration of about 10 ~ 6 or lower (and which 

 were therefore feebly acid, neutral, or alkaline), constant poten- 

 tials could not be obtained. It is objectionable to prolong the 

 measurement unduly, as a platinized platinum electrode may be- 

 come "sick," a phenomenon frequently observed after continued 

 use. Hasselbalch and Gammeltoft 5 state that for this reason 

 measurements on blood must not take more than one hour. 



As a step toward the satisfactory determination of hydrogen- 

 ion concentration in soil the writer has attempted to apply the 

 hydrogen electrode to mixtures of soil and water and to apply 

 indicators to extracts prepared by centrifuging such mixtures. 



GENERAL PROCEDURE IN THE PRESENT INVESTIGATION 



Since it seemed to be impossible to apply either the hydrogen elec- 

 trode or indicators to soil in the condition in which we are most inter- 

 ested, namely, at optimum moisture content, it was decided to add 

 enough water, in this series of experiments, to facilitate the determina- 

 tions. For the hydrogen electrode work a suitable quantity was found 

 to be 2 cc. per gram of dry soil. The same quantity was used for the 

 colorimetric, or indicator, work, though it was much more than is neces- 

 sary. For expediency the soil was air dried, though in some cases the 

 drying was interrupted when the soil was still somewhat damp. With- 

 out forcing or grinding, the soils were put through a coarse sieve to 

 remove sticks and stones, and in four cases through a one-millimeter 

 sieve in the same way in order to remove fine woody material which 

 interfered with the pipetting of the fluid after the use of the centrifuge. 



Twenty-two soils were taken for investigation. They included 

 samples of muck, sandy loam, loam, silty loam, silt loam, clay loam, 

 and clay. Some reacted strongly acid to litmus paper, some neutral, 

 and some alkaline. 



The hydrogen ion concentration has been expressed, in the customary 

 manner, as the hydrogen-ion exponent of Sorensen. 6 



ELECTROMETRIC DETERMINATIONS 



In figure 1 is shown the electrode vessel together with the electrode 

 in position, the soil and water mixture and the junction between the 



h Biochem. Ztschr., 68: 206. 1915. 

 6 Biochem. Ztschr., 21: 131. 1909. 



