Changes in soil aggregate water stability induced by wetting and drying cycles in non-saturated soil

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 of water 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 of aggregates in the field.