Temporal changes in wet aggregate stability

Aggregate stability, a property that influences a soil's erodibility and hydraulic characteristics, has been shown in previous investigations (e.g., Bullock et al., 1988) to vary over time for some northwestern U.S. soils. The objectives of this study were to evaluate three procedures for measuring aggregate stability and quantify variation in aggregate stability over time (that is, within a growing season) for selected soils across the United States. In 1988 and 1989, soils from 11 states were sampled monthly from April to July and in September. The stability of 1- to 4-mm aggregates from each sample was measured by 1) wetting air-dry (A-D) aggregates in a vaporizer followed by wet-sieving; 2) immersing A-D aggregates immediately prior to wet sieving; and 3) vapor-wetting field-moist aggregates followed by wet sieving. The sieving of vapor-wetted field-moist aggregates best revealed temporal variation. In general, the aggregate stability of northern soils varied more over time than did the aggregate stability of southern soils, probably due to differences in freezing and thawing. Biological activity likely accounted in part for temporal changes in the stability of soils from the upper Midwest. Trends in stability change over time in soils from one region of the United States to another were seldom similar