196 



THE MEASUEEMENT OF THE HYDEOGEX ION CON- 

 CENTRATION IN SOUTH AFEICAN SOILS IN RELATION 

 TO PLANT DISTRIBUTION AND OTHER ECOLOOICAL 



PROBLEMS, 



BY 



J. W. Bews, M.A., D.Sc, AXD R. D. Aitkex, M.Sc, 



Departnient of Botany, Natal University College. 



Read July 11, 1922. 



Introduction. 



The modern develoiDment of the ionic tiieory of solutions has 

 given a much more definite meaning to the formerly somewhat 

 vague terms " acid," " neutral " and " alkaline." The acidity 

 of a solution is no longer defined according to a qualitative 

 colour reaction with litmus or litmus paper, but is capable of 

 exact quantitative expression in terms of the hydrogen ion 

 concentration. The application of this definite measurement of 

 the degree of acidity or alkalinity to the study of soil reaction, 

 particularly in relation to the distribution of individual native 

 plants and plant associations and to ecological problems generally, 

 has recently been developed with very fruitful results by O. 

 Arrhenius** in Sweden and by E. T. Wherry^- ^ in America, 

 apparently independently. Salisbury^ in England has also applied 

 it to problems of plant succession particularly in woodlands. The 

 present writers have only recently commenced a study of the 

 acidity of South African soils from the modern standpoint, and' 

 the present paper is intended to illustrate some of the preliminary 

 results obtained, and to indicate the particular problems on which 

 such a study may be expected to throw considerable light. Since, 

 however, the method has not, so far as we are aware, been 

 previously described in South Africa, it seems advisable to discuss 

 in greater detail the meaning of the terms used, and the method 

 of their determination. The account here given is largely based 

 upon the papers and books mentioned in the bibliography,'- -•^' "^ 

 and makes no claim to be in any way original. Quotations from 

 these papers have been freely used and are acknowledged here, 

 though not directly indicated in the text. 



The Meaning of Acidity. 



According to the modern ionic theory, many chemical com- 

 pounds when in solution imdergo an electrolytic dissociation into 

 positively and negatively charged particles, known as ions. Eor 

 example, when hydrochloric acid is dissolved in water, only a 

 small proportion of it is present in solution as actual molecules 

 of HCl, the greater portion being almost completely dissociated 



* These numbers refer to the papers listed in the References at the end. 



