An Improved Method for Measuring Aggregate Stability of a Weakly Aggregated Loessial Soil
- Frederick B. Pierson and
- David J. Mulla
The original high energy moisture characteristic (HEMC) technique for measuring aggregate stability is shown to have limitations for use with weakly aggregated soils. Improvements were made to expand the applicability of the technique to weakly aggregated soils and reduce its subjectivity to operator bias. The destructive force used to break down the aggregates was controlled by wetting aggregates 1.0 to 0.5 mm in diameter under suction on a sintered glass plate. The moisture characteristic curve for the aggregates was fit using a modified van Genuchten S-shaped equation, and the specific water capacity curve was analytically calculated. The modal suction and the area under the specific water capacity curve were then computed and used to calculate a structural index value for aggregate stability at a fast and a slow wetting rate. Surface samples of weakly aggregated loessial soil were collected from adjacent long-term organic and conventional farms in the Palouse region of Washington State. The coefficient of variation for replicate determination of structural indices from a slow wetting treatment using the original HEMC method on samples from both farms was 15%, whereas it was 7% using the improved method. Using the original HEMC technique, there was no difference in the stability ratio of either farm due to complete breakdown of aggregates dropped into standing water. With the improved HEMC method, the aggregate stability ratio and structural index for fast wetting were both significantly higher for soil on the traditional farm than on the organic farm. Thus, the improved method facilitates the measurement of differences in stability of weakly aggregated soils subject to long-term differences in cropping and management history. In contrast, the original method was inappropriate for these weakly aggregated soils due to improper wetting rates and high variability resulting from operator subjectivity.
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