Genesis of podzols on coastal dunes in southern Queensland .II. Geochemistry and forms of elements as deduced from various soil extraction procedures

Abstract

Distribution of total elemental C, Al, Si, Fe, Mn, P, Ti and Zr in a chronosequence of podzols spanning some 7x105 years is presented. Considerable variation in elemental composition occurred down each profile due to translocation and nonuniform banding of heavy mineral concentrations. The mobility and fate of these elements and Cr were determined by extractions with sodium pyrophosphate, ammonium oxalate (pH 3.0), dithionitelcitrate and dilute HCl. In some cases, pyrophosphate extractions gave higher values than expected when considering the ammonium oxalate values, confirming that peptization can occur with the former. Extraction data demonstrate that all elements with the exception of Mn and Zr have been mobilized from the A horizon and accumulated in the B horizon. Mn is lost from the A horizons but there is no evidence for accumulation elsewhere in the profiles, while there is no evidence for the movement or accumulation of Zr. As the profiles develop and the thickness of the A horizons increases, Al accumulates as Al-organic matter complexes in the Bh and Bhs horizons and as proto-imogolite allophane, determined by 29Si n.m.r. spectroscopy, in the Bhs and Bs horizons. Selective dissolution by using oxalate and dithionitelcitrate demonstrates accumulation of proto-imogolite in the Bhs and Bs horizons with age, and also suggests that its nature may vary within profiles. In most profiles, only a small proportion of P is extracted by oxalate or dithionitelcitrate and appears to be in the form of monazite and gorceixite. Across the chronosequence, P accumulates in the lower B horizons and is associated with Al-organic matter complexes and proto-imogolite. Iron accumulates in B horizons and, as the later stages of podzol development, such as the giant pipey podzols, are reached, many of the lower horizons contain >90% of Fe as secondary Fe oxyhydroxides. Most of the secondary Fe in the Bh and Bhs horizons is present as Al substituted goethite while, in the Bs horizons and below, ferrihydrite is the dominant mineral. There is little evidence for significant concentrations of Fe-organic matter complexes. Fe maxima occur above the Al maxima in the younger profiles and below it in the older ones. Fe is progressively lost during profile development and is depleted from humus podzols which are often the final stage of podzol genesis. Titanium is the most abundant element after Si. Extraction data strongly suggest that Ti is largely present in primary minerals such as ilmenite, rutilelanatase and pseudorutile. Evidence is presented for the presence of Ti-organic matter complexes in A horizons and Ti mobilized in this form appears to result in the formation of pedogenic anatase or rutile in the upper B horizons. The selective dissolution data for Ti and Fe suggest possible Ti-substitution for iron in goethites which appears to be common in B horizons of the freely drained profiles, although little Ti is associated with ferrihydrite.