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Hardsetting Soils in Southeast Australia: Landscape and Profile Processes

Agronomy Dep., Purdue Univ., West Lafayette, IN 47907

C. J. Chartres

CSIRO Division of Soils, GPO Box 639, Canberra, ACT 2601, Australia

J. T. Wood

CSIRO Biometrics Unit, Canberra, ACT, Australia

*Corresponding author (dfranzmeier{at}dept.agry.purdue.edu).

ABSTRACT

Hardsetting soil horizons are relatively soft when moist but become unusually hard when dry. Soils with these horizons are common in Australia and elsewhere. They have been studied mainly in exposures such as road cuts where their development is most marked. The main objective of this study was to elucidate the relationships between landscape position and hardsetting processes. Soils were sampled along two transects that start in bedrock hills, cross broad eolian-fluvial plains, and terminate in a local flood plain or lacustrine basin. They are mainly Dystrochrepts, Palexeralfs, and Natrixeralfs, and many soils are aquic intergrades. Samples were analyzed using standard characterization methods, and extracted with water, oxalate solution, and citrate-bicarbonate-dithionite solution; Fe, Al, and Si contents were determined from the extracts. Soil strength was characterized by the modulus of rupture test. Strength was partly related to clay content, but several E and BE horizons were very strong relative to their clay content, thus hardsetting. They were in the zone of the pedon in which exchangeable Na and pH increased sharply with depth. This sodicity was also cyclic in the landscape. Strength was correlated positively with extractable Si and negatively with extractable Al. In hardsetting soils, which tend to have impeded drainage, Fe-oxide minerals periodically precipitate and dissolve. We postulate that silica is dissolved and dispersed in upper horizons with a large pH gradient, moves down the profile, and bonds with Fe-oxide minerals on clay surfaces. The bonds strengthen on drying to form hardsetting horizons.

Purdue Univ. Agric. Res. Programs Journal no. 14448.

Received for publication February 10, 1995.

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