In many parts of the West, magnesium and calcium chloride are commonly used as dust suppressant agents on unpaved roads during the dry months and for deicing on paved roads during the winter months. Like sodium chloride (table salt), both magnesium and calcium chlorides are salts.

Magnesium or calcium chlorides are often used as dust control agents since they are hygroscopic and deliquescent – having the ability to absorb moisture from the humid air. This absorbed moisture helps limit dust by keeping the surface of the road damp enough to hold the dust particles down. Furthermore, magnesium and calcium chlorides work as deicers by lowering the freezing point of water.

One of the side effects of using magnesium and calcium chloride for dust control and de-icing is that it may have detrimental effects on roadside vegetation. The effect of salts on roadside soils and vegetation are well documented. In fact, the potential impacts on roadside soils and vegetation may be much greater when magnesium or calcium chloride is used as dust control agents rather than for anti-icing. The reason for this is that anti-icing applications are mainly done during the winter months on paved roads, while dust control is done during the spring and summer months on unpaved roads. The timing of the dust control applications will impact the roadside vegetation differently since they are actively growing and transpiring. Also, the absence of snowmelt during the dust control season greatly affects the concentration and dilution of the soil salts when compared to deicers. This reduced dilution help keeps the magnesium chloride from washing downstream with meltwater but enhances downward leaching into the soil, therefore increasing the impacts on nearby vegetation.

Once the chloride gets into the soil it can be taken up by plant roots. When the chloride is absorbed by the plant roots, it moves up the water-conducting system and accumulates at the margins of leaves or needles. This is where die-back first occurs. At high chloride concentrations damaged trees may be evident after two years of treatment.

The notable effect that chloride has on vegetation include leaf scorching, marginal necrosis, and needle tip burn. High concentrations of chloride in soil can affect plant growth and survival both indirectly and directly. At lower chloride concentrations, a reduction in plant growth may be due to osmotic effects that disrupts normal water and nutrient uptake. At high concentrations, chloride can accumulate at the margins of transpiring leaves or the tips of needles which can cause foliar necrosis and leaf abscission through dehydration and/or specific metabolic disruptions. These could lead to branch and tree die-back. Usual symptoms appear as browning of the leaves beginning at the tip or margin of the leaf and advancement towards the base. The higher the chloride content the greater the length of the leaves are injured. In addition, during dry conditions, water stress and dehydration may aggravate chloride toxicity and cause even more extensive damage.

Some plants may be affected more by chloride than others. For example, some plants are considered to be halophytes, which are plants tolerant to salinity. These plants, for example may be found in or near salt marshes and saline arid environments. By far most plants are not considered to be salt tolerant. For example, it has been estimated that only about 2-percent of the plant species are halophytes.

Keep in mind that no plants are entirely immune to salt injury. Even the most halophytic plants do not prosper under highly saline conditions, and, at best, plants may tolerate and cope with salinity but they usually grow better under conditions of low salinity.

The following is a list of native and ornamental plants that have been documented that are considered to have low-to-moderate tolerance to chloride ions. Please note that this list is not exhaustive or complete. These include:

While the perceived cost of magnesium chloride for dust control may be attractive, it may be important to first evaluate the possibility of its effects on native or ornamental roadside vegetation before using. It is important to note that alternative products for dust control exist. For example, Earthbind® 100 for dust control can significantly reduce dust from unpaved roads while being environmentally friendly. Earthbind, once cured, is not water-soluble and stays in the roadbed and can improve the road base with each application. For more information on how you can control dust without killing roadside vegetation, visit www.enviroad.flywheelsites.com.

References
B.A. Goodrich and W. R. Jacobi, Magnesium Chloride Toxicity in Trees No. 7.425
Colorado State University Extension (07/08)
Wayne A. Sinclair, Howard H. Lyon, and Warren T. Johnson (1987) Diseases of trees and Shrubs
Comstock Publishing
Michael J. Dirr (November 1976) Tolerance to Salt Injury
Journal of Arboriculture
Edward P. Glenn, J. Jed Brown & Eduardo Blumwald (1999) Salt Tolerance and Crop Potential of Halophytes, Critical Reviews in Plant Sciences, 18:2, 227-255, DOI: 10.1080/07352689991309207