Soil Water Matric Potential Effects on Aggregate Stability

The effect of soil water matric potential (ψ m ) is not considered with most techniques utilized to evaluate aggregate stability. Rather, aggregate stability usually is measured at ψ m of 0 Pa or at an unknown ψ m . The objective of this study was to determine ψ m effects, in the range of 0 to −3,000 Pa, on stability of natural soil aggregates obtained from a fencerow and from plots having a range of management histories. Aggregate stability, considered inversely related to the mass of soil detached by impact of falling water drops, increase markedly as ψ m decreased from 0 to −500 Pa. Similar ψ m changes at ψ m < −500 Pa had smaller effects on aggregate stability. Significant differences in stability of aggregates obtained from plots with different cropping and/or manuring histories, were sometimes undetectable at ψ m ≤ −500 Pa but were more consistently detected at ψ m = 0 Pa. Data from this study indicate that aggregate stability is very sensitive to ψ m , particularly at ψ m near zero. Techniques for comparing aggregate stability between soils or soil treatments should ensure equal values of ψ m exist for all aggregates. Measurements made over a range of ψ m should yield better comparisons of aggregate stability between soils than measurements made at one ψ m .