Podzols: aspects of their chemistry and development
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Work for this thesis focussed on the complexing of iron (II) and iron (III) in aqueous solution with five polyphenolic ligands, gallic acid, pyrogallol, protocatechuic acid and tiron. These ligands were chosen as model compounds for the larger polymeric species known to be present in the soil system and thought to be important in the translocation of iron in podzol formation. The iron (III)/polyphenol system was characterised by the formation of very stable 1:1 and 2:1 complexes in the pH range relevant to podzol formation (i.e. 3.0- 7.0) and 3:1 complexes at higher pH. Ligands for which the standard reduction potential of the corresponding o-quinone was < 0.9 V underwent a redox reaction with Fe³⁺ at low pH (< 3.0). The formation of complexes with Fe³⁺ at higher pHs was critically dependant on dissolved oxygen concentration. No complex formation, between Fe(II) and the ligands studied, was observed below pH 7. Stability constants have been determined for complexes between Fe²⁺ and the ligands gallic acid and catechol. Protonation constants for gallate and catecholate anions have also been determined. The calculation of stability and protonation constants required accurately known hydrogen ion concentrations. For this reason a calibration relationship was established between pH, measured by the glass/calomel electrode assembly, and hydrogen ion concentration, calculated from buffer solutions of o-phthalic acid/potassium hydrogen phthalate. The response of the glass/calomel electrode pair to various standard buffers over the pH range 1.7- 12.5 has been characterised in terms of a “non-Nernstian” response throughout this pH range, with deviations attributed to the effect of the liquid-junction superimposed in the high and low pH regions (i.e. 3.5 >pH> 9.2). The status of an undeveloped podzolic soil on the West Coast, South Island, New Zealand, is discussed as background to the metal-ligand study. Thus, results are presented for various chemical and physical properties of the soil including important features of the clay mineralogy. The development of this soil as an agricultural unit has also been studied with data collected for important plant and animal nutrients over a period of 5 years.