Journal zyxwvutsrqp of zyxwvutsrq Soil Science. zyxwvuts 1982.33.623-631 zyxwvu Changes in soil aggregate water stability induced by wetting and drying cycles in non-saturated soil zy W. H. UTOMO zyxwv & A. R. DEXTER Waite Agricultural Research Institute. The University ?/Adelaide, Glen Osmond, South Australia 5064 Summary Wetting and drying of remoulded soil resulted in water stable aggregation. The greatest proportions of water stable aggregates arose from wetting and drying in the - 1 to - 100 kPa range of matric water potential. The effect occurred with sterile and non-sterile soil. but the proportion ofwater stable aggregates was less with sterile soil. The application of wetting and drying cycles in the laboratory to non-tilled soil resulted in a steady decrease in the proportion of water stable aggregates. With tilled soil, the proportion of water stable aggregates first increased to a maximum and then decreased steadily with further wetting and drying cycles. However. with sterilized, tilled soil, only a steady decrease in the proportion of water stable aggregates was observed. Natural water content fluctuations in the field after tillage gave an increase in water stability to a maximum after a few days followed by a steady decrease. The similarity of this result to that obtained in the laboratory for tilled. non-sterilized soil indicates that micro-organisms were probably contributing to the observed short- term changes in the water stability ofaggregates in the field. Introduction In nature, soil continually undergoes wetting by rainfall, diffusive Row, condensation, etc., and drying by solar radiation, diffusive flow, wind, etc. These cycles of wetting and drying have been related to the formation of soil aggregates in non-aggregated soils. Because of this, some workers (e.g. Telfair et al.. 1957; Richardson, 1976) suggested that cycles of wetting and drying can be used to restore some structurally damaged soils. Wetting and drying has been found to affect the water stability of aggregated soils although the results obtained have been variable. Soulides and Allison (196 I) found that wetting and drying cycles decreased the proportion of water stable aggregates >0.5 mm as measured by wet sieving. A decrease in aggregate water stability, as shown by an increase in per cent slaking has also been shown by Tisdall er al. (1 978). Hofman (1976) obtained a contrary result. He found that the aggregate instability, which was defined as the difference in mm between the average diameters of the aggregates before and after wet sieving, of aggregates sieved immediately after sampling was greater than that obtained when the soil was first air-dried and then rewetted to its original water content or to field capacity. 0022-4588/82/1200-0623 $02.00 0 1982 Blackwell Scientific Publications 623