The salinity of motorway soils. II. Distance from the carriageway and other sources of local variation in salinity.
The variation of soil sodium concentration with distance from the carriageway on central reserves and verges was studied on a number of motorway sites. At further sites the interaction of effects of distance with other aspects of motorway construction and management was investigated. newline˜On the standard 4 m-wide central reserve, soil sodium concentration at the centre was usually 50-70% of the value 0.5 m from the carriageway, and curvature of the motorway had no detectable effect on this pattern. On central reserves exceeding 7 m in width, there was a zone at least 2 m wide in which sodium concentration was less than 70% of the value 0.5 m from the carriageway. In some wide (9-10 m) reserves concentration in the central zone was only about 10% of that at the margins. The establishment of shrubs on central reserves increased the soil sodium concentration by about 50%. Provided there was a hard shoulder between verge and carriageway, the sodium concentration at 2 m from the margin of the verge was between 30% and 40% of that at the centre of the central reserve. Beyond this distance sodium concentrations on the verge were very low. There were effects on soil sodium levels related to the direction of the traffic or the orientation of the road which cannot be simply explained.