SOIL SOLUTION COMPOSITION AND AGGREGATE STABILITY CHANGES CAUSED BY LONG-TERM FARMING AT 4 CONTRASTING SITES IN SOUTH AUSTRALIA

The effect of long-term farming on the cation exchange capacity (CEC), organic carbon content, soil solution composition, and aggregate stab ility was investigated using contrasting soils from 4 sites in the Mid North of South Australia. Undisturbed and farmed profiles were charac terised at each site. Farming led to a 10-50% decrease, approximately, in organic matter and CEC in the surface horizon. Scanning electron m icroscopic study of the surface and selected subsurface soils revealed poor aggregation, compaction, reduced porosity, and a decrease in agg regate particle size in the farmed surface soils. Intra-aggregate bind ing in the undisturbed soils appeared to be largely due to fungal hyph a, with the roots largely contributing to inter-aggregate binding of s oil particles. Electrical conductivity (EC) of soil solutions was gene rally 2-3 times higher in the undisturbed soils than farmed soils, sug gesting increased leaching of ions associated with loss of tree cover. This was also supported by a decrease in the concentrations of mobile ions such as Cl- and Na+ in the farmed soils. The concentrations of N a+ and K+ decreased with farming leading to a decrease in the Gapon se lectivity constant for Na-Ca and K-Ca exchange. The changes in soil so lution composition together with the decline in organic matter concent rations resulted in increased sensitivity of soils to dispersion.