Agricultural restoration following gravel extraction

Abstract

The extraction of gravel from river floodplains and terraces for use as high quality aggregate, is a valuable landuse in New Zealand. However, it is increasingly in conflict with agriculture. This is due to the widespread occurrence of recent soils from alluvium in these areas. Such soils are of high value for food production. Gravel extraction may permanently degrade or destroy high quality soils. The extent to which high quality agricultural land is affected by gravel extraction is reviewed with special reference to the Waimea Plains near Nelson City, and the Canterbury Plains adjacent to Christchurch City. In response to pressure for the wise use of a limited high quality soil resource, aggregate producers on the Waimea Plains must restore extraction sites to agriculture. The performance of agricultural restoration in this area is assessed. Several surface soil (0 - 20cm) physical properties were measured on restored areas and nearby undisturbed areas at four gravel extraction sites. The results from these measurements showed a deterioration in soil physical condition had occurred. These changes were of such a magnitude that plant growth was most likely detrimentally affected. The deterioration in soil properties is attributed to compaction and loss of soil structure associated with soil disturbance. The methods of soil handling employed, in particular, trafficking by heavy wheeled vehicles under unsuitable moisture conditions are considered largely responsible. A possible relationship between the inherently weak structure of recent soils and their physical deterioration whenever they are moved remains untested. The over-riding cause of damage to soils following gravel extraction is considered human, rather than technological. This study highlights the need for careful consideration of the management aspects in agricultural restoration. Guidelines are suggested for resource and operation management in the study areas, which aim to minimise damage to recent soils affected by gravel extraction. Such principles should apply equally to the restoration of other shallow mineral workings.