FREEZING EFFECTS ON AGGREGATE STABILITY AFFECTED BY TEXTURE, MINERALOGY, AND ORGANIC-MATTER

Aggregate stability, an important property influencing a soil's response to erosive forces, is affected by freezing. The objectives of this laboratory study were to determine how constrainment, number of freeze-thaw cycles, and water content at freezing affect the aggregate stability of six continental USA soils differing in texture, mineralogy, and organic-matter content. Moist aggregates, after being frozen and thawed either zero, one, three, or five times, were vapor wetted to 0.30 kg kg-1 and analyzed by wet sieving. Soils with clay contents of 17% or more and organic-matter contents > 3% were the most stable after freezing. Aggregate stability for fine- and medium-textured soils generally decreased linearly with increasing water content at freezing. This linear decrease in stability was more rapid for constrained samples than for unconstrained samples. The stability of field-moist aggregates generally increased from zero to one or three freeze-thaw cycles. For at least one low-organic-matter soil, stability increased from one to three freeze-thaw cycles, but then decreased at five cycles. After thawing, aggregates at water contents of 0.15 kg kg-1 or more that were constrained when frozen were always significantly less stable than aggregates that were unconstrained when frozen.