Sodium accumulation in soils and plants along Massachusetts roadsides

The most common de-icing material applied by the Massachusetts state highway department is sodium chloride (NaCl). In Massachusetts, the rate of application of de-icing agents is about 240 lb (110 kg) of sand and 12 lb (5.5 kg) of NaCl per lane mile (1.6 km). The objective of this research was to examine injury to plants along roadsides and to assess relationships of damage to the amount of Na detected in plants and soils. The damage on most plant species was manifested as burning or browning of the leaves or needles. Coniferous species, especially pines (Pinus spp.), were sensitive to NaCl injury. In coniferous species, the damage appeared as browning on the ends of the needles, but new growth was not affected. Most of the damage occurred on the needles on the tree side that faced the road and where salt spray from cars or plows could have been a factor in the degree of damage. Widespread damage was also seen on spruce (Picea spp.), sumac (Rhus typhina), and mountain laurel (Kalmia latifolia) along roadsides. With sumac, injured plants had only 10% of the foliage as uninjured plants. Some salt-tolerant species, apparently undamaged by NaCl, in the same vicinity as the damaged plants, were various oaks (Quercus spp.), maples (Acer spp.), grasses (mixed species), ferns (mixed species), and yarrow (Achillea millefolium). The Na concentrations in the leaves of pines, sumacs, grasses, and oaks decreased as the distance from the road increased. The Na concentrations in pine needles were 3356 mg kg(-1) at 10 feet, 1978 at 15 feet, and 1513 mg kg(-1) at 20 feet. The Na concentrations in maple leaves decreased with the Na concentrations being 249 mg kg(-1) at 10 feet and falling to 150 mg kg(-1) at 30 feet. The concentrations of Na in roadside soil ranged from 101 mg kg(-1) at 5 feet to 16 mg kg(-1) at 30 feet from the roadside, with a marked decrease in the Na concentration in the soil after 15 feet. The pH decreased as the distance from the road increased ranging from 7.60 at 5 feet to 5.78 at 30 feet. The electrical conductivity values decreased as the distance from the road increased and ranged from 0.16 dS m(-1) at 5 feet to 0.12 dS m(-1) at 30 feet. This study suggests a relationship between Na accumulation, in leaves and in soil, and injury to roadside plants.