Assessing parent material uniformity of a red and black soil complex in the landscapes

Abstract

The distribution of red and black soil (Xeralfs–Xerolls) associations in the Monarto area (South Australia) is complex and their genesis either being derived from a uniform parent material or a lithologic discontinuity is not known. The objectives of this study were (i) to assess Zr- and Ti-bearing grains as minerals resistant to chemical weathering prior to employing Zr and Ti in determining parent material uniformity, and (ii) to confirm whether pedological processes or a lithologic discontinuity may be responsible for the textural contrast within the red and black soil profiles. Scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) were used to study chemical weathering and elemental composition of surfaces of Zr- and Ti-bearing grains. X-ray fluorescence (XRF) analysis was used to determine elemental concentrations in various soil fractions. Results showed that Zr-bearing grains consisted of only zircon having smooth clean surfaces, which are characteristic of a mineral resistant to chemical weathering. EDAX spectra confirmed that Zr was a specific element to represent zircon, suggesting its reliability to be used in assessing parent material uniformity. On the other hand, Ti derived not only from minerals (rutile/anatase) resistant but also from minerals (ilmenite/pseudorutile, biotite) susceptible to chemical weathering. This limited the usefulness of Ti as an index mineral for soil development studies. Except for Zr:Ti ratio, all other indicators of parent material uniformity, i.e., depth distribution curves of Zr and Y in various fractions and Y:Zr ratio showed no considerable inflection and variation with depth in a Xeralf (red soil). This indicated the Xeralf derived from a uniform parent material (mica schists), thereby texture contrast within a profile is due to pedological processes. On the other hand, depth distribution curves showed clear inflection and variation in Xerolls (black soils), indicating soils developed from lithologic discontinuities, so different trends in particle size fractions between 0–48 cm for MA5 and 0–59 cm for MA3 profiles compared to the underlying layers are due to parent material differences and not pedogenesis. The presence of complex red and black soil association in the landscape is attributed to the difference in parent materials, where the red soil developed from mica schist and the black soils from calcareous deposits.