370 Scientific Proceedings, Eoyal Dublin Society. 



Fisher (1921). Whether the acidity is produced by humic or other organic 

 acids, by the selective absorption of bases, by the interaction of salts with 

 insoluble alumino-silicates, by the presence of alumino-silicic acids, or by the 

 presence of sulphur acids derived from pyrites, there is only one method of 

 stating precisely the effective acidity — namely, by recording the coucentia- 

 tion of hydrogen ions in the soil solution or soil extract. This may be done 

 by giving the hydrogen ion concentration in grams per litre, symbolised as 

 C„ or [H*], or by using the expi'ession pH, which is defined as the logarithm 

 of the reciprocal of the hydrogen ion concentration expressed in grams per 

 litre. For a discussion of the advantages of this method of stating acidity, 

 reference may be made to Clark (1920). But to render what follows more 

 intelligible to those biologists who are familiar neither with the methods by 

 which hydrogen ion concentrations are determined nor with the terminology 

 adopted, a brief account of the subject is given here. 



(a) Hydrogen ion concentration and acidity.— The acidity of soils has been 

 measured in various ways, of which an account is given by Clark, but it is 

 found that the behaviour of the soil solution or soil extract towards plants is 

 not correctly shown by titration, which gives a measure of the total replaceable 

 hydrogen, not of the concentration of the hydrogen ion effective at any 

 instant. It is by the latter that the effect on the plant is governed, as well 

 as the effect on enzyme action and the solubility of various salts. For 

 example, equal volumes of N/100 hydrochloric and acetic acids require 

 identical amounts of alkali for their neutralization, but the former solution 

 has a far greater concentration of hydrogen ions than has the latter, since in 

 dilute solution hydrochloric acid is almost completely ionized, whereas acetic 

 is not ; it is accordingly said to be a weaker acid. If it is assumed that the 

 ionization of hydrochloric acid is complete at this dilution, its concentration 

 in terms of hydrogen ions is Ch or [H+] =1x10"^ grams per litre. This 

 may be written 



[H^] = 1,, or pH = log ^^ = - log [H^] = 2. 



It may at fii'st sight appear to be both cumbersome and unnatural to use 

 such an expressson as pH, viz., - log [H^j, to denote hydrogen ion concentra- 

 tion, but in practice it is extremely convenient, and gives simple numerical 

 values which are easily remembered. Moreover, since an increase of unity 

 in a pH value denotes a decrease to one-tenth in the hydrogen ion concentra- 

 tion, it is obvious that for any graphical presentation of results difficulties 

 arise when the changes are of the order of 10"' or 10"^, yet these are quite 

 common in researches on this subject. For certain purposes, however, it is at 



