HYDROGEN IO.\ CONCEXTRATION IN S.A. SOILS. 201 



soil acidity is by the determination of the hydrion concentration 

 or pH of the soil solution. The hydrogen ions of the soil solution 

 are probably not derived from any single substance or class of 

 substances, but from a great variety of substances such as strong, 

 highly-- ionised inorganic acids, such as nitric, hydrochloric, and 

 organic acids, such as oxalic acid, weak slightly-ionised 

 inorganic acids, such as carbonic acid, ana organic acids, sucli 

 as acetic acid, salts of weak bases with strong acids, for example, 

 ammonium oxalate, amino-acids, and the controversial " humie 

 acids." There is still room for a great deal of work on the 

 substances which produce soil acidity, but it shovild be clearly 

 remembered that no matter what these substances are, their 

 acidic effects are produced through the agency of hydrogen ions, 

 and that the only satisfactory method of measuring the acidity 

 is by the determination of the hydrion concentration. 



One further point requires mention here. All soil solutions 

 exhibit strong buffer action, probably owing to the presence of 

 weak acids and of proteins and other such substances. As a 

 consequence of this, the soil solution can be considerably diluted 

 without producing a marked change in the pH value. 



Method of Dktermixixg pH. 



The most accurate method of measuring the hydrion concen- 

 tration of a solution is an electrometric one depending on the 

 use of the hydrogen electrode. This, however, involves the use 

 of costly apparatus and is too complicated to be of real value 

 in ecological studies ; in any case it gives a much higher degree 

 of accuracy than is really needed in such work. The second and 

 more practicable method is a colorimetric one depending on the 

 use of various indicators. These indicators exhibit characteristic 

 colour changes corresponding with changes in the hydrion concen- 

 tration of a solution. For instance, phenol red is yellow for all 

 values of pH less than 6'o. For higher values it gradually changes 

 through various shades of orange until at pH8 it becomes pink. 

 Other indicators exhibit similar colour changes at different pH 

 values, and it is possible to obtain a set of indicators showing 

 definite colours over a whole series of pH values. Standard solu- 

 tions of known pH can be prepared from substances which exhibit 

 strong buffer action, and by choosing the standard solution 

 which gives exactly the same shade of colour as the unknown 

 solution with the same amount of indicator we can determine the 

 pH with sufficient accuracy. Our first supply of the necessary 

 indicators was obtained from the Veterinary Research Depart- 

 ment, Onderstepoort, by courtesy of the Director, Sir Arnold 

 Theiler, to whom and to Dr. H. H. Green our thanks are due. 



The actual method of determining the soil acidity will now 

 be described in greater detail. Stock solutions of standard acidity 

 are prepared from sodium and potassium phosphates. A N/15 

 solution of disodium hydrogen phosphate, Na2HP04 is obtained 

 by dissolving 11 '876 gms. of the salt in one litre of distilled 

 water. This solution has a pH^G'lS. A N/15 solution of 

 potassium dihydrogen phosphate (pH = 4'49) is obtained by 

 dissolving 9 "078 g-ms. of the salt in one litre of distilled water. 



