Interpretation of morphological features in a salt-affected duplex soil toposequence with an altered soil water regime in western Victoria

This paper is the first of two describing how soil macromorphological and chemical data can be combined with a minimum of hydrological data to distinguish between, and to quantify, past and present hydrological processes. These processes are relevant to both waterlogging and dryland salinity. The purpose of this first paper is to establish a methodological framework. It also describes the initial interpretation of the macromorphological features of the toposequence studied at Gatum on the Dundas Tablelands in western Victoria. A modified version of the soil feature-system-domain grouping method was used. Macromorphological data combined with only limited piezometric data showed that: (1) The soil feature-system-domain grouping method makes it possible to distinguish between the effects of past and present hydrological processes on soil macromorphology at Gatum. (2) Waterlogging of the surface horizons at Gatum is often caused by perching of soil water within the B-horizon (as opposed to on top of the B-horizon). Changes in soil structure and in colour of cutans and mottles can be an indicator of this first restricting layer. (3) It is likely that interpedal cracks and old tree root holes act as preferred paths for water to flow through this first restricting layer. (4) A second fresh perched water table can occur on top of the pallid zone. Where the pallid zone reaches close to the surface the two perched water tables may merge and cause a local increase in waterlogging, as indicated by local soil morphology. When this occurs, hillside seeps can occur quite high up on the slopes, even when there is no apparent irregularity in surface topography. (5) The permanent saline water table occurs on top of the bedrock and causes salting problems where it comes too close to the soil surface. Salting problems at the bottom of a slope are more severe where fresh perched water tables increase waterlogging On the basis of these findings the suitability of various management options to reduce waterlogging and salinisation is discussed. Further findings regarding restricting layers, flow paths through the soils, and relations between duration of saturation and soil morphological features, are discussed in a companion paper (by J. Brouwer and R. W. Fitzpatrick, pp. 927-946 in this issue).