HYDROGEN ION CONCENTRATION ii 



series of standard solutions of known p^ value made up with 

 the same indicator. The vakies of these standard solutions 

 have been accurately determined by electrometric methods. 

 The indicator method is not so accurate, but, with the 

 necessary precautions, it gives satisfactory results for soil 

 work and is both cheap and convenient. Determinations 

 are made in water extracts of the soil ; the p^ values of 

 such extracts differ little from those of the solution obtained 

 by pressure from the soil, although of course the extracts 

 are much more dilute (Olsen, 1923). This is due to 

 strong buffer action of the soil solution. For details of 

 methods the student should consult Clark (1920), and 

 Atkins (1922). A general account of the biological relations 

 is given by Bayliss (1920). 



Only soils which contain much carbonate — calcium 

 carbonate is much the most important — tend to have an 

 alkaline reaction, and a /)" value of over 7*0. The natural 

 processes in the soil always produce acids, and in absence 

 of abundant neutralising base the soil has an acid reaction. 



Soil acidity may be due to the presence of mineral acids. 

 If ammonium salts are present, the base is absorbed more 

 readily by plant roots and a balance of mineral acid is left 

 in the soil. Atkins (1922) has shown that in certain soils 

 which contain iron pyrites, sulphuric acid is present. Of 

 more general importance is the acidity produced by carbon 

 dioxide in solution, v/hich acts as a weak acid and is always 

 present. Olsen (1923) has shown that the dissolved carbon 

 dioxide increases the acidity of nearly neutral soils by 

 ^" = 0*5, the effect being less marked in more acid soils. 

 Much the most effective cause of acidity is, however, the 

 presence of the organic constituent, the humus. Humus 

 contains organic acids which may act directly. It also 

 interacts with salts — silicates, nitrates, etc. — in solution, 

 removing the base and liberating the acid, either as free acid 

 or in combination with aluminium as acid salts of this metal. 

 The mechanism of the reaction is obscure ; it may be a 

 chemical reaction with the humus acids, or it may be the 

 result of colloidal adsorption (Skene (1915), Russell). In 



