Nature of Soil Acidity in Some Acidic Soils of USSR

The cause of exchangeable acidity, extracted by treatment with normal neutral KCl solution of acidic soils, is primarily due to Al ions. One of the possibilities of their liberation in the extract appears to be the dissolution of surface of the acidic clay minerals of the soil. On the other hand, the main cause of hydrolytic acidity (sodium acetate extractable) appears to be hydrogen ions. Organic matter acts as protector of aluminium ions, oxidation of which leads to considerable increase in their concentration. Humic acid fraction in the soil predominates over fulvic acid and the p^K values for humic acid ranges between 4.23 and 5.98 lowest being in podsols.