RELATION OF CARBON DIOXID TO SOIL REACTION 

 AS MEASURED BY THE HYDROGEN ELECTRODE ^ 



By D. R. HoAGtAND and L. T. Sharp,' 

 Assistant Chemists, Agricultural Experiment Station of the University of California 



INTRODUCTION 



In a previous article (ii) ' the authors have presented data con- 

 cerning the question of soil reaction as determined by the hydrogen 

 electrode. Since this work did not include direct measurements of the 

 effect of carbon dioxid on the reaction of soils, it was thought desirable 

 to carry out further experiments on this point. Before discussing the 

 data obtained in these additional experiments it will be well to emphasize 

 again the fundamental principles upon which the conclusions of our 

 first paper were based. 



The present tendency to advance involved explanations of the nature 

 of soil acidity seems to be unnecessary, for the simple conception of the 

 relations of H- and OH-ion concentrations are in accord with the facts 

 so far ascertained and are warranted by the accepted teachings of chemis- 

 try. The lack of agreement in the literature appears to be due to the 

 attempt to use interchangeably the terms "lime requirement" and 

 "soil acidity." In the methods of determining the lime requirement it 

 is proposed to measure the amount of lime required to bring the soil to an 

 end point dependent upon arbitrarily selected conditions. These methods 

 are in themselves so varied and the final measurement of reaction so dif- 

 ficult, that the discordant results which appear in the literature are wholly 

 to be expected. 



On the other hand, the term "soil acidity" has a definite and precise 

 meaning — ^namely, that condition of the soil in which its aqueous solu- 

 tion contains H ions in excess of OH ions. In our opinion it would be 

 preferable to refer to soil acidity, soil neutrality, and soil alkalinity as 

 those phases of soil reaction in which the H-ion concentration is respec- 

 tively greater than, equal to, or less than the OH-ion concentration. 

 These H- ion concentrations may be definitely determined by measure- 

 ments with the hydrogen electrode. 



The lime requirement, in so far as it is related to soil acidity, would 

 consist of that amount of lime necessary to bring an acid soil to the neutral 

 point as ascertained by the above-mentioned procedure. Such a lime 

 requirement implies that the dissolved and total undissolved soil acids 

 have been neutralized. To put this procedure into practice may involve 



1 From the Divisions of Agricultural Chemistry and Soil Chemistry and Bacteriology in equal cooper- 

 ation. 



2 Reference is made by number (italic) to "Literature cited," p. 147-148. 



Journal of Agricultural Research, Vol. XII, No. 3 



Washington, D. C. Jan. 21, 1918 



Iq Key No. Cal.— 13 



(139) 



