Regolith 2004 In: Roach I.C. ed. 2004. Regolith 2004. CRC LEME, pp. 355-361. 355 REGIONAL PREDICTION OF SALT-AFFECTED SOILS IN AN AREA OF COMPLEX SOIL PATTERNS IN SOUTH AUSTRALIA Mark Thomas 1 , Rob W. Fitzpatrick 2 & Graham S. Heinson 3 1 CRC LEME, University of Adelaide and South Australian Department for Water, Land and Biodiversity Conservation, PMB 2, Glen Osmond, SA, 5064 2 CRC LEME, CSIRO Land and Water, PMB 2, Glen Osmond, SA, 5064 3 CRC LEME, School of Earth and Environmental Sciences, University of Adelaide, SA, 5005 INTRODUCTION Shallow Non-groundwater Associated Salinity (shallow NAS), which is described in Fitzpatrick et al. (in prep.), is found in upland parts of landscapes that have no direct contact with saline groundwater watertables, unlike Groundwater Associated Salinity (GAS). GAS is characterised in terms of catchment-scale hydrological processes, and is managed accordingly. Conversely, shallow NAS is characterised by localised soil patterns, which are governed by soil-landscape processes at various scales. Shallow NAS soils feature: (i) high exchangeable sodium percentage (ESP) (i.e., are "sodic" and feature excessive Na + ions on the exchange complex); and, (ii) high soluble salt concentrations (i.e., are "saline", generally featuring Na + and Cl - ions, and measured by EC se ), in the solum (i.e., A and B-horizons, typically < 1.2 m deep). In Australia, soils with ESP ≥ 5 are generally considered as being sodic (Rengasamy & Churchman 1999). These soils show signs of a decline in soil structure due to clay dispersion, which in turn creates waterlogging, hard-setting physical barriers to root growth, and poor gas transfer rates. Elevated EC se values (i.e. ≥ 2 dS/m) give rise to droughting and toxic conditions in soils, which affect crop growth (Soil Survey Division Staff 1993). When sodicity and salinity combine through shallow NAS, the harmful effects on crops are magnified. Shallow NAS is strongly associated with texture contrast soils, which feature sandy/loamy A-horizons over sodic clay B-horizons. These are very important agricultural soils in southern Australia. According to Rengasamy (2002), approximately A$1,330 million of farm income is lost annually through shallow NAS in Australia. More locally, in the Northern Agricultural District (NAD) (302,000 ha) of South Australia, subsoil (i.e., 0.3-1.2 m) salts are a widespread problem. According to 1:100,000 scale State-wide soil mapping (Soil and Land Information 2002), > 15% of the NAD soils are affected by salinity (EC se ≥ 2 dS/m) and > 60% by sodicity (ESP ≥ 6). Most of the saline areas spatially overlap with the sodic areas in the mapping, indicating that a significant proportion of the NAD is affected by shallow NAS. However, shallow NAS soils are difficult to map by conventional field-based soil-landscape survey methods (e.g., Mcdonald et al. 1998) because they form complex patterns with no apparent visual surface clues (e.g., colour, texture) (Thomas et al. 2003). For this reason, shallow NAS has not been mapped in South Australia at scales suitable for farm management planning (1:5,000 or larger). Our aim is describe a GIS-based regional digital soil mapping methodology to predict shallow NAS for a small regional study area in the NAD. REGIONAL STUDY AREA The small regional study area (2,300 ha) is in an upland farming zone of the NAD (Figure 1). The average annual rainfall is 450 mm, of which approximately 75 % falls during the winter. Winters are cool and summers are hot, giving rise to a temperate, Mediterranean-type cli-mate. The predominant agricultural land use in the area involves wheat, barley, canola and sheep grazing rotations. Figure 1: Map showing location of regional study area in Australia, featuring South Australia's Northern Agricultural District and rainfall zones.