THE COMPOSITION OF THE SOIL 113 



and found to resemble that of the acid soil extract. Mirasol ^ 

 has confirmed and extended these observations. 



Ramann also adopts this physical hypothesis and gives 

 up the expression "acid soils," using instead " absorptiv 

 ungesattigte Boden". Kappen ^ confirms the observations 

 without entirely accepting the explanation. 



Recent work shows that both views have a foundation of 

 truth ; there are a,t least two causes at work : lack of bases in 

 the soil brings about the absorption phenomena discussed 

 above, but there are also true acids present in soil under 

 certain conditions. Rindall of Helsingfors (239), Sven Od6n 

 of Upsala (218a), Tacke (278), and Ehrenberg and Bahr (93^0 

 have each argued in favour of definite humic acids in peat (see 

 p. 139). Truog (285) finds, in the case of mineral soils, that 

 equivalent amounts of different bases are required to neutralise 

 the acid properties of the soil — which if generally true would 

 be easier to explain by assuming an acid than an adsorption. 



The view that soil acidity is caused by actual acids has 

 gained support from recent work ^ on the hydrogen ion con- 

 centration in soils. 



Chemists study acids in two ways : — 



1. By measuring the hydrogen ion concentration, a value 

 based on the assumption that an acid on solution in water 

 dissociates into two parts, called ions — one being hydrogen, 

 and the other the rest of the molecule. 



2. By determining the titration value, i.e. the number of 

 c.c. of standard alkali solution which a given volume of the 

 acid solution will neutralise. In the language of the dis- 

 sociation hypothesis this value measures the total quantity 

 of hydrogen ions producible under the conditions of the 

 experiment, supposing them to be neutralised or linked up 

 with - OH ions as quickly as they are liberated. 



^ Soil Set., 1920, 10, 153. 



2 H. Kappen, Studicn an saurem Mineralboden aus der Nahe von Jena 

 (Landw. Versuchs-Stat., 1916, 88, 13-104). 



3 For a critical summary of recent work see E. A. Fisher, y. Ag. Set., 1920, 

 II, 19. 



8 



