Many physicochemical and biological phenomena in soils relate directly to the soil aggregates and their stability. After several years of massive sludge additions to a soil, the changes in particle-size distribution and aggregate stability were examined. With water dispersion, the treated soil showed a greater component in the 0.05− to 2.0-mm size fraction and less in the fine fractions. Upon high-energy ultrasonic dispersion, the sand (0.050 to 2.0 mm) and silt (0.002 to 0.050 mm) fractions decreased and the clays increased, more pronouncedly on the treated soil. The water stability of aggregates was 85% on the treated soil, compared with 45% on the check. It appears that aggregates originating from the sludge itself and newly formed soil aggregates both contribute to the increased stability. Soil water repellency becomes a serious problem as a result of the massive applications of Oakland municipal sludge. This problem remained even after sludge additions had been terminated for several years and possibly was accentuated by low soil pH. Scanning electron micrographs of surface features of check and treated soil aggregates show the porous nature of the latter, which, together with their stability, was reflected by a dramatic decrease in soil bulk density on the treated plots.
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