114 SOIL CONDITIONS AND PLANT GROWTH 



The titration value measures the quantity of the acid but 

 it makes no distinction between a strong acid, such as sulphuric 

 acid, and a weak acid, like acetic acid : the great difference 

 in action on plant life exhibited by the two acids is missed 

 altogether. Moreover, the titration value is not an absolute 

 constant ; polybasic acids, which are by far the most numer- 

 ous, have several titration values, according to whether one, 

 two or more of the hydrogen atoms are affected. Different 

 indicators, therefore, give different numerical results. So long 

 as the constitution of the acid is known this does not matter, 

 but it causes complications in dealing with a mixture of un- 

 known acids. 



Many of the methods suggested fo^ measuring soil acidity, 

 including all the " lime requirement " methods, afford a more 

 or less rough measure of the titration value ^ : the differences in 

 results are partly due to absorptions and partly to differences 

 in the number of hydrogen atoms concerned. 



The determination of the hydrogen ion concentration, i.e. 



of the ions actually present, as distinct from those that would 



finally be liberated on neutralisation, is an attempt to measure 



the intensity of the acid as distinct from its quantity. The 



principle of the method is simple in dealing with solutions 



of pure acids : it is based on the ordinary dissociation law : 



H* X X' 



— TJY — ^ -^ where H' and X' are the ions produced on 



the dissociation of the acid HX, and K is a constant. The 

 number or the concentration of the ions is measured by the 

 electrical conductivity of the solution, but once this has been 

 determined for a given acid it can become a standard against 

 which the hydrogen ion concentrations of other acids can be 

 rapidly compared by means of a set of indicators.^ 



The problem becomes more complex when salts are 

 present, because the additional ions thus introduced affect the 



^ Probably all the results are too high, as they include the absorptions. See 

 H. R. Christensen (676), also 105c. 



^ A useful account of the measurement of the hydrogen ion concentration is 

 given by J. F. McClendon in Physical Chemistry of Vital Phenomena (1917), 

 which was written for biological students. 



