Sustainability in any farming system is dependent upon a number of interacting factors which include climate, soil quality, plant nutrition, management, weed and disease incidence, and economic factors (Greenland and Szabolcs, 1994). Measures of soil quality in agricultural land include soil tilth (described by porosity, aggregation and other structural measures) as an index of soil physical quality, and pH, N, exchangeable cations, salinity, toxic chemicals and soil organic carbon as indicators of soil chemical quality (Karlen et al , 1992; Walker and Reuter, 1996). Among these, soil organic carbon has been proposed as a primary indicator of soil quality (Lal, 1997; Reeves, 1997). The frequency and amounts of carbon and N inputs needed to replenish soil carbon and N reserves have been suggested as good indicators of long-term sustainability of many cropping systems, and has been incorporated into predictive models of sustainability (Fig. I) (Lal, 1997; Reeves, 1997; Freebairn et al, 1998). Predictive models derived for dryland clay soils suggest that the first indicators of a system run-down under commercial cropping are increased requirements of fertilizer N (and other nutrients such as P and S) and water to maintain yields. In the longer-term yield and profitability losses also occur (Freebairn et al, 1998).