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Measurement of soil aggregate stability using low intensity ultrasonic vibration

The objective of this study was to analyse the influence of tillage on soil aggregate stability using the ultrasonic dispersion method with different levels of energy. Experiments are performed with self-developed equipment at low vibration amplitudes of 0.5 μm, 2 μm and 5 μm. Ultrasonic power is 0.7 W, 8.9 W and 22.3 W for the three amplitudes. Samples of aggregates 2 000 - 250 μm of a degraded loamy silt Chernozem, a loamy clay Cambisol and a loamy sand Cambisol from eastern Austria were collected under different tillage regimes: conventional tillage (CT) with mechanical weed control and no tillage (NT) with inter cropping in winter. Measuring Soil Aggregate Stability (SAS) according to DIN norm showed that the stability decreased in the sequence loamy clay Cambisol > loamy sand Cambisol > loamy silt Chernozem. Influences of tillage could be observed with SAS for the last two samples mentioned. Ultrasonic dispersion tests at vibration amplitude 0.5 μm and 2 μm show higher stability of the Cambisol soils. Significant influences of soil tillage on aggregate stability for the loamy clay Cambisol and the degraded Chernozem were observed after short ultrasonic dispersion times and absorbed energies of 1 - 1.7 J ml-1. The measured aggregate stability decreased in the following order: Cambisol NT > Cambisol CT > Chernozem NT > Chernozem CT. Differences in loamy sand Cambisol could not be detected with the ultrasonic method due to a low aggregation level of the macro aggregates. This study highlights the importance of quantifying the energy output of the ultrasonic equipment when analysing changes in soil aggregate stability, for the evaluation of tillage treatments. A more comprehensive analysis of aggregate stability can be obtained when using both, the wet-sieving SAS method and ultrasonic dispersion at low energy levels.