CHEMICAL STUDIES OF PODZOLIC ILLUVIAL HORIZONS

Abstract

Summary Titration curves of humus suspensions (prepared from some representative podzol B horizons by acetylacetone extraction) in o.1 N KCL are Presented. The shape of each curve (in absence of added metal ions) and the apparent P K value (P K e ) derived from it are affected by the amount of Al contained in the suspension. One soil extract (initial PH 3.6) had a C/Al ratio of 4.9 and gave P K e = 6.21, Whereas the amberlite‐treated extract (initial PH 2.6) had a C/Al ratio of 152 and P K e = 4.12. Extrapolation of P K e values to zero concentration of Al gave limiting values of 4.o and 3.8 for the humus extracted from a Queensland and a New Zealand podzol respectively. These figures are lower than those normally quoted for humic acid, but they may be characteristic of the translocated humus of podzols. Titration curves of fresh humus extracts with added Al +++ , Fe +++ or divalent transition metal ions did not suggest the presence of complex sites although the results for Fe +++ are uncertain owing to the complexity of the equilibria involved Similar titrations with added Al +++ and Cu ++ using amberlite‐treated extracts did not indicate potential complex sites and, as before, the Fe +++ data were inconclusive. By analogy with Cu ++ it is unlikely that Fe complexes ate present in these systems. It is concluded that humus is immobilized in these horizons by elexctrostatic bonding with Al +++ (since free Fe +++ would not exist at the soil PH), and the residual Al contained in the organic micelles in suspension is probably also bound by Coulomb forces. It is pointed out that the approximate methods used in this study for practically metal‐free preparations may not reveal the presence of the type of complexes found by other workers for some synthetic Polyelectrolytes.