MICROAGGREGATE STABILITY OF KAOLINITIC AND ILLITIC SOILS DETERMINED BY ULTRASONIC ENERGY

Seal formation, runoff, and soil erosion depend closely on aggregate s tability. The objective of this study was to identify stable and unsta ble soils and the effect of exchangeable sodium percentage (ESP) there on, using the microaggregate stability test. Six predominantly kaolini tic and illitic soils from South Africa, with ESP adjusted to 1, 7, 15 , or 25, were used in this study. Two-gram samples of microaggregates (< 105 mum) were subjected to a range of ultrasonic energy levels up t o 600 J mL-1. After sonification, particle-size distribution was deter mined by a light-scattering technique using a laser particle sizer. Un dispersed clay (UDC) percentage, which is the fraction of the clay tha t is left undispersed after sonification, was used to characterize mic roaggregate stability. According to the distribution of their UDC perc entages with respect to energy level, the six soils were divided into three groups: unstable, semistable, and stable soils. In the unstable soils, UDC percentage dropped exponentially with energy level. In the stable soils, UDC percentage decreased linearly with an increase in en ergy level. In both groups ESP had an insignificant effect on microagg regate stability. In the semistable soils, ESP had a significant effec t on microaggregate stability. Samples with ESP = 1 behaved like the s table soils and samples with ESP > 1 behaved like unstable soils. Micr oaggregate stability depended strongly on the soil clay mineralogy. Th e microaggregate stability determination proved to be a sensitive indi cation for soil susceptibility to seal formation, runoff, and erosion.