Sodium accumulation in calcareous roadside soils

Sodium chloride (roadsalt) is routinely applied to roadways during the winter in many parts of the world to reduce ice formation and improve winter driving conditions. While increases in chloride (Cl⁻) levels in waterways influenced by roads and Cl⁻ impacts on roadside vegetation have been widely reported, there has been relatively little research on the fate of sodium (Na⁺) and its impact on soils. Soils in the most heavily populated area of southern Ontario, Canada, are typically rich in calcium (Ca²⁺) and we predicted that high native Ca²⁺ levels in this region would limit Na⁺ accumulation in roadside soils. In order to investigate the extent of Na⁺ accumulation in calcareous soils, we compared soil Na⁺ levels adjacent to roads of three different classes but having the same parent material, including a Freeway, three High Capacity Arterial roads and three Low Capacity Collector roads, as road salt loadings are highly dependent on traffic volume. Soil Na⁺ levels were significantly higher adjacent to the two-lane (mean 0.22–0.50 meq/100 g) compared with the single-lane minor roads (0.013–0.10 meq/100 g), and exchangeable soil Na⁺ was negatively correlated with Ca²⁺, Mg²⁺ and K⁺, suggesting that not only does Na⁺ accumulate in calcareous soils, but it also displaces all three nutrient base cations. Nevertheless, relatively low soil Na⁺ levels adjacent to the Low Capacity Collector roads, which are similar to levels reported for control soils in other studies, suggest that high base saturation in these soils may limit Na⁺ accumulation, and that road salt application rates at the minor roads are not high enough to cause Na⁺ accumulation.