Classification and Determination of Soil Acidity

Hydroxy aluminium and hydroxy iron polymers also can adsorb anions with concurrent release of hydroxyl ions. The pH increase due to this anion exchange can be masked, however, by the simultaneous hydrolysis of desorbed aluminium ions (Bq. 10.2). Adsorption of multicharged anions can also decrease the net positive charge on hydroxy aluminium or hydroxy iron polymers, and thus increase the net negative charge of the soil-polymer mixture. The anion adsorption capacity of soils decreases with increasing pH and becomes virtually zero for all anions except phosphate and arsenate at pH values greater Ilian 5.5 or 6. 

Various approaches have been used to classify the components of soil acidity. As a carryover from the titration curves used to characterize soil acidity in earlier studies, a common category is titratable acidity or total acidity. This is the quantity of a strong base (NaOH or Ca(OH)2) required to raise soil pH to a predetermined level. Time, method of stirring, and period between additions of base must be specified because the neutralization of soil acidity is highly dependent on reaction conditions. The values are also meaningless unless the initial and final pH values are specified, because more base is consumed if the reaction is carried out ova a wider pH range. 

For example, titration curve 3 in Fig. 10.1 required approximately 200 mmol hydroxyl kg-1 of clay to raise the pH from 4 to 5, and another 200 mmol hydroxyl kg-1 of clay to raise the pH from 5 to 6. 

Common endpoints of such titrations are pH 7 or pH 8.2, although soils in the field are rarely limed above pH 6 or 6.5. The value 8.2 was chosen historically because it approximates the pH of soil containing free CaCC 3 in equilibrium with the normal CO2 content (0.0003 mol fraction) of the atmosphere. This pH also corresponds closely with the pH of complete neutralization of soil hydroxy aluminium compounds. The pH 8.2 is conveniently maintained by Mehlich s BaCl2-triethanolamine extraction technique. 

The titration process, if carried out so slowly that the reaction is fairly complete following each addition of base, does not distinguish between exchangeable and virtually nonexchangeable components. Hence, titratable acidity is only a measure of the total acidity neutralized during the experimental technique employed. The titratable or total acidity is nonetheless useful for determining the lime requirement of acid soils. 

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