Physical Breakdown in Relation to Soil Development

Summary Mechanical analyses as well as quartz distribution data for four soils, ranging from chernozemic Black to podzolic Grey Wooded, indicate that a redistribution in particle size has occurred in the solum horizons relative to the parent materials. This is attributed to the physical breakdown, in large part the result of frost action, of the larger rock fragments and minerals. While mechanical disintegration is greatest in the surface horizons, it is least pronounced in the Orthic Black profile and reaches a maximum in the Ae horizon of the Grey Wooded soil. Periodic moisture saturation in layers underlain by slowly permeable textural B horizons is suggested as favouring breakdown by frost action. Implications of such breakdown relative to studies of soil genesis are discussed.