2.6 Aggregate Stability and Size Distribution

A soil aggregate is “a group of primary soil particles that cohere to each other more strongly than to other surrounding particles” (Soil Science Society of America, 1997). Soil aggregates can be formed by both aggregation and fragmentation processes. There are several ways of quantifying aggregate size and cohesive strength. The relative abundance of aggregates at each possible size, after breaking the soil into individual aggregates in a prescribed way, is the usual representation of the characteristic size distribution. Similarly, an abundance index may indicate stability, as the fraction of soil material that remains aggregated after a specified disruptive procedure. Other stability indices relate more directly to the force required to break an aggregate apart, usually in terms of the mechanical energy per soil mass that must be applied to achieve a certain result, such as aggregate rupture. The analysis of soil aggregation is important in a variety of applications. Aggregate stability and size information may be used to evaluate or predict the effects of various agricultural techniques, such as tillage and organic-matter additions, and erosion by wind and water. Aggregate analysis is often used in experiments where various tillage methods are applied and then evaluated by examining the resulting stable aggregates. Because of their direct relation to cohesive forces, aggregate size and stability are important to understanding soil erosion and surface sealing. Analysis of dry aggregates may be used to estimate possible wind-erosion effects, while wet analysis may be more appropriate to evaluate or predict erosion due to rainfall impact and runoff. The stability of wet aggregates can be related to surface-seal development and field infiltration, as water-stable fractions may restrict water entry and form surface seals (Loch, 1994). Through these erosion and sealing effects, as well as the relation between aggregation and structural features such as macropores, aggregate analysis may help us understand most aspects of soil water behavior, including runoff, infiltration, and redistribution, as well as soil aeration and root growth. Increasingly, aggregate properties are being used in models that predict soil hydraulic properties, including water retention and unsaturated hydraulic conductivity (e.g., Rieu & Sposito, 1991a, b; Nimmo, 1997; Kosugi & Hopmans, 1998). The strength of interparticle cohesion depends on a variety of soil physical, chemical, and biological influences, some of the most important being air–water