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Microaggregate Stability of Kaolinitic and Illitic Soils Determined by Ultrasonic Energy

Institute of Soils and Water, ARO, P.O. Box 6, Bet Dagan 50250, Israel

M. Agassi

Soil Erosion Research Station, Ruppin Institute, Emeq Hefer 60960, Israel

H. J. C. Smith

Institute of Soil, Climate and Water, Private Bag X79, Pretoria 0001, South Africa

*Corresponding author.

ABSTRACT

Seal formation, runoff, and soil erosion depend closely on aggregate stability. The objective of this study was to identify stable and unstable soils and the effect of exchangeable sodium percentage (ESP) thereon, using the microaggregate stability test. Six predominantly kaolinitic 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 µm) were subjected to a range of ultrasonic energy levels up to 600 J mL^–1. After sonification, particle-size distribution was determined by a light-scattering technique using a laser particle sizer. Undispersed clay (UDC) percentage, which is the fraction of the clay that is left undispersed after sonification, was used to characterize microaggregate stability. According to the distribution of their UDC percentages 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 energy level. In both groups ESP had an insignificant effect on microaggregate stability. In the semistable soils, ESP had a significant effect on microaggregate stability. Samples with ESP = 1 behaved like the stable soils and samples with ESP > 1 behaved like unstable soils. Microaggregate stability depended strongly on the soil clay mineralogy. The microaggregate stability determination proved to be a sensitive indication for soil susceptibility to seal formation, runoff, and erosion.

Contribution from the Institute for Soil, Climate and Water, Pretoria, South Africa.

Received for publication May 8, 1992.

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