Colluvial processes and soil variation at field boundaries in County Down

Abstract

In fields on slopes, colluvium can accumulate against a field bank so that the surface of the eroded field is at a higher level than the field immediately downslope. This creates a drop down from the upper field to the lower. These features, step-like in cross section, form during cultivation and survive in land that has reverted to pasture without the normal restoration of soil upslope being undertaken. The presence of such a deposit distorts the downslope pattern of moisture movement in that a component of this flow is now towards the field bank from which it often seeps. To ascertain if the formation and survival of these colluvium deposits can affect soil property variation, a study was carried out in three fields at three different sites in mid-County Down. Data on organic carbon, pH. pyrophosphate extractable iron, dithionite extractable iron and mass specific magnetic susceptibility were obtained from detailed sampling often profiles located at approximately ten metre intervals downslope in each field. These data were examined graphically. Results show that downslope changes in soil properties are influenced by surface wash, as well as by the development of a saturated wedge and longer soil moisture residence times, following the alteration of pathways of moisture movement, in the vicinity of the colluvial deposits.