Acid‐ and base titration behaviour of finnish mineral soils

Abstract

AbstractThe acid‐ and base‐buffering properties of 84 non‐calcareous surface soil samples were studied by batch titration with HCI or KOH at a constant ionic strength of I = 0.1. The soil samples were classified according to their pH of zero point of titration (ZPT). Differential buffer values, dB(H) or dB(OH) (H+ or OH− as meq kg−1 needed to reduce or increase the soil pH sequentially by 0.5 units, respectively), were introduced to describe the course of titration curve and the intensity of buffer action.In all soils, the first acid‐buffer value, dB(H)0→0.5, varied from 8 to 78 meq kg−1 and the second one, dB(H)0.5→1, from 10 to 138 meq kg−1. The corresponding base‐buffer values, dB(OH)0→0.5 and dB(OH)0.5→1, ranged from 10 to 48 and from 14 to 44 meq kg−1, respectively. The most acid soils were most strongly buffered against acid, and the soils with the highest initial pH against base. The results reveal the acid‐buffering by exchange reactions to be very important. In the soils with ZPT≦5.4, the first acid‐buffer value was dependent on the content of organic matter and oxalate‐soluble Al, whereas in the more acid soils the role of clay became significant. Thus, it was concluded that at higher pHs the foremost inactivation of H+ is attributable to soil components of pH‐dependent charges, and the significance of constituents of permanent charges to increase with proceeding acidification. In strongly acid soils (ZPT≦4.8) the very effective buffering seemed to be primarily due to the dissolution of Al‐hydroxides and, thus, to exert detrimental effects on the edaphic environment. The general rank of soil factors explaining the variation in the base‐buffer values was in accord with the neutralization sequence, i.e. the strongest acid in the soil being neutralized first. In the strongly acid soils (ZPT≦4.8) the base‐buffer values seemed to depend on the clay as well as KCl‐ and NH4OAc‐extractable Al, whereas in the soils with higher initial pH mostly on organic C.