Effect of compaction on the porosity of a silty soil: influence on unsaturated hydraulic properties

Tillage and traffic modify soil porosity and pore size distribution, leadin g to changes in the unsaturated hydraulic properties of the tilled layer. T hese changes are still difficult to characterize. We have investigated the effect of compaction on the change in the soil porosity and its consequence s for water retention and hydraulic conductivity. A freshly tilled layer an d a soil layer compacted by wheel tracks were created in a silty soil to ob tain contrasting bulk densities (1.17 and 1.63 g cm(-3), respectively). Soi l porosity was analysed by mercury porosimetry, and scanning electron micro scopy was used to distinguish between the textural pore space and the struc tural pore space. The laboratory method of Wind (direct evaporation) was us ed to measure the hydraulic properties in the tensiometric range. For water potentials <-20 kPa, the compacted layer retained more water than did the uncompacted layer, but the relation between the hydraulic conductivity and the water ratio (the volume of water per unit volume of solid phase) was no t affected by the change in bulk density. Compaction did not affect the tex tural porosity (i.e. matrix porosity), but it created relict structural por es accessible only through the micropores of the matrix. These relict struc tural pores could be the reason for the change in the hydraulic properties due to compaction. They can be used as an indicator of the consequences of compaction on unsaturated hydraulic properties. The modification of the por e geometry during compaction results not only from a decrease in the volume of structural pores but also from a change in the relation between the tex tural pores and the remaining structural pores.