STUDIES IN SOIL REACTION AS INDICATED BY THE 

 HYDROGEN ELECTRODE 



By J. K. Plummer, 

 Soil Chemist, Division of Agronomy, North Carolina Agricultural Experiment Station 



INTRODUCTION 



There has been developed in the past few years a rather voluminous 

 literature dealing with the subject of soil reaction. One has only to scan 

 this literature in order to find wide variations of opinion between inves- 

 tigators as to the cause and nature of soil acidity. 



Until recently the various qualitative and quantitative methods in 

 vogue for indicating soil acidity or lime requirement have not been suffi- 

 ciently delicate to draw definite conclusions as to the "true reaction'' of 

 soils. The lack of uniformity and accuracy of methods has undoubtedly 

 caused such confusion on this subject. By adopting modem methods 

 for measuring soil reaction^many of the contentions should be obliterated. 



Though the hydrogen electrode has been used for some time in indi- 

 cating changes in reaction, Gillespie (7) ^ was the first to use it on an 

 extended scale as an indicator of reaction in soils. Sharp and Hoag- 

 land (12) have since measured the H-ion concentration of a number of 

 oils in suspension, and have extended this method to studying other 

 soil phenomena. 



The significance of the terms "true acidity," "true alkalinity," and 

 "true neutrality" need not be defined here, except in so far as an expla- 

 nation of the method adopted in reporting results obtained. Pure water 

 dissociates into H and OH ions in equal concentration. The product of 

 the concentration of these ions in a solution is a constant, approximately 

 I X 10— 14. When the H ions are present in a concentration greater 

 than I X 10—7, the solution is acid; the presence of OH ions in greater 

 concentrations than 1X10—7 results in an alkaline solution. For a 

 more detailed discussion of this subject, the reader is referred to texts 

 on electrochemistry. 



The investigations herein reported were begun about the time of the 

 appearance of Sharp and Hoagland's paper, to ascertain if appreciable 

 differences occurred in the H-ion concentration of soils of humid regions, 

 especially from those of the Southern States. It would appear that with 

 the excessive rainfall of this region an accumulation of soluble acids in 

 the soil would be almost impossible. 



1 Reference is made by number (italic) to "Literature cited," p. 30-31. 



Journal of Agricultural Research, Vol. XII, No. i 



Washington, D. C. Jan. 7, 1918 



lo - Key No. N. C— 7 



(19) 



