Increase in the bulk density of a Grey Clay subsoil by infilling of cracksby topsoil

Increases in soil bulk density beyond the optimum reduce land productivity and, where soil is affected, may be difficult to remedy. Elucidating the me chanisms causing compaction is a prerequisite to sustainable management of fragile soils. We examined a dense grey soil in Western Australia in which the dominant physical feature of the subsoil was coarse prismatic structure . The prisms were approximately hexagonal in horizontal section with an ave rage side length of 0.66 m. The top of the prisms reached to within approxi mately 0.07 m of the soil surface, their sides becoming indistinguishable b elow approximately 0.9 m. The vertical faces of the prisms were coated by m aterial similar in composition to the topsoil and separated from it by a tr ansition material of intermediate composition. Soil within the prisms had a bulk density at maximum swelling which reached a maximum of 1.86 g cm(-3) in the upper subsoil. We investigate the hypothesis that such a high bulk d ensity could have developed as a result of a simple three-stage process: (i ) soil shrinkage as the profile dries over summer leading to widening of cr acks between prismatic peds, (ii) infilling of cracks by detached topsoil w hich adds to coating thickness, and (iii) swelling during the winter, now p artially restricted by coating material, leading to compression of the pris matic peds. We present a model which accounts quantitatively for this proce ss and explain how soil physical characteristics might facilitate it. The d ense upper subsoil (7-60 cm) limits root penetration and prolongs the perio d of transient waterlogging of the topsoil during winter, adversely affecti ng subsequent crop performance. Our work suggests that stabilizing surface soil to minimize soil detachment could be a relevant management objective o n these structurally unstable soils in order to prevent subsoil compaction.