THE COMPOSITION OF THE SOIL 115 



amount of dissociation : in particular they make it difficult 

 for the hydrogen ion concentration to increase when another 

 acid is added. Colloidal substances act in the same way. 

 This effect, known as " buffer action," is of great importance 

 in vital phenomena and is well marked in the case of soils. 

 Thus if an acid is added to soil the increase in the hydrogen 

 ion concentration is not nearly so great as if the same quantity 

 of acid were added to water. 



The numerical values of the hydrogen ion concentrations 

 are rather unmanageable and a convention has therefore been 

 adopted in dealing with them. 



In pure water or in a neutral solution 



H* = OH' = I X lo"^*'''^ gram-ions per litre. 



Acidity means a hydrogen ion concentration in excess of 

 that of water, i.e. the index instead of being - 7*0 is greater: 

 as the quantity is negative this means that the number itself 

 is less : it may fall to zero. Simple trial will show that it is 

 very difficult to plot on the same curve numbers ranging from 

 I to I X 10"^. A further complication began to arise but 

 fortunately was checked : investigators expressed their results 

 in terms not of i, but of some other number, multiplied by 

 10 to some negative power, e.g. one solution might be repre- 

 sented by the value 4-4 x 10"", and another by 6-8 x 10"'^, 

 and it was rather difficult to make a comparison. 



To overcome the first of these difficulties Sorensen ^ adopted 

 the familiar device of plotting the logarithms of the numbers 

 instead of the numbers themselves; and to overcome the 

 second he reduced all to terms of i x some power of 10 : 

 thus the two cases just quoted become I x lO"^'^' and 

 I X lO"^'^^ respectively. He calls the numbers 5-36 and 

 6-17 the Pjj values of the solutions: in reality they are the 

 values for the - log (H"). On this notation the Ph value for 

 neutrality is 7*07 ; that for acidity is anything less — down to 

 nothing — that for alkalinity is anything greater — up to 14 "i 4. 



^S. P. L. Sorensen, Biochem. Zeitsch., igog, 21, 130, and 22, 352; see also 

 Ergebnisse d. Physiologic (Asher and Spire), igi2, 12, 3g3. 



8* 



