There are several situations where salt can damage plants: some areas have saline soils, there may be occasional seawater flooding, or salt can accumulate in the soil from slightly saline irrigation water. Coastal areas can have salt spray that affects the local plant communities. In all those situations, problems can arise, particularly in plants out of their native range or when unusual conditions spread salt out of its usual area. For instance, hurricanes can cause plant damage many kilometers inland, not just by breakage but by salt injury to foliage.
But the most common and widespread situation where salt damages trees is when it is applied to roads. Salt is applied for two distinct purposes:
- It may be applied to paved roads and lots in winter for the purpose of deicing (lowering the freezing point of water on the road. Both NaCl and MgCl2 are used for this purpose .
- It may also be applied to unpaved roads in summer for dust abatement. MgCl2 and CaCl2 are used for this purpose because they are hygroscopic and tend to bind soil particles together, reducing dust.
Salt affects roadside plants via two routes :
- through the soil, where it is taken up by roots, and
- as salt spray and dust that is deposited on aerial plant parts.
Salt uptake via soil
Areas most affected are below the road grade, usually quite close to the road, especially where tree roots are all in a low area with limited drainage. Symptoms include leaf scorch, premature defoliation, dieback, and death. This is commonly seen, including in pines in the Adirondacks and many species in the Rocky Mountains. Acer saccharum is susceptible.
The featured image at top shows Populus tremuloides (trembling aspen) acutely affected by MgCl2 along an unpaved road where the salt was used for dust abatement.
Deposition of salt spray and dust
Trees may be affected up to 50 meters from a highway, especially downwind. Exposed plants collect the most droplets so get the most damage. On conifers, needles brown at the tips (tip burn), drop early. On conifers, buds may die and twigs die back, often leading to witches’ brooms as numerous buds develop to replace the dead tips.
Mechanisms of Injury
Excess sodium causes injury to plants through several mechanisms.
- Salts in general decrease the osmotic potential of soil water, making it more difficult for the plant to take up water.
- Sodium causes soil colloids to disperse, disrupting soil structure and water conductivity .
- Sodium salts increase the pH of the soil and can lead to what is called alkali injury . Acid-soluble nutrients, such as iron, become less available and may become deficient.
- Na+ tends to displace Ca++ on cation-exchange sites on soil particles and in uptake by roots, and is preferentially mobilized in plants. These factors induce a Ca deficiency in the plant [1, 2].
- Salt in plants may also decrease cold hardiness .
Although MgCl2 is less damaging to plants than NaCl, it is really the chloride ion that is the most toxic. In plants affected by NaCl, damage is correlated better with Cl– concentration than with Na+ [4, 5]. Cl– is readily mobile in plants and accumulates in leaf margins and tips via transpiration to toxic concentrations, leading to marginal scorch (hardwoods; see featured image above) or tip burn (conifers).
In general, species that are relatively tolerant of salt limit its uptake rather than tolerate high internal concentrations .
In natural and quasi-natural forests, symptoms of nutrient deficiency are not normally seen, although growth can be increased by fertilization. Deficiency symptoms can be seen on occasion in ornamentals and off-site plantations.
Symptoms vary with the tree species, and can be complicated by combinations of nutrients. Worse, the symptoms are rarely diagnostic, meaning the symptoms are non-specific and, considered alone, do not usually allow a reliable diagnosis. Without vast experience with particular cultivars in a particular area, extensive analyses may be necessary to confirm a diagnosis.
Still, there are some generalities that are useful and practical to keep in mind. Here are some facts about two nutrients as examples:
- Nitrogen deficiency usually causes chlorosis, interveinal in hardwoods. Also slow growth, stunted leaves. Nitrogen is highly mobile in the plant, and new growth gets first priority, so chlorosis may be most in older growth. Nitrogen is frequently limiting, and symptoms of deficiency can sometimes be seen in nature.
- Iron deficiency also causes chlorosis, but iron is immobile once put to use in the plant, so symptoms show up more in the newest foliage. In the soil, iron is most soluble at low pH. Trees adapted to low pH often experience iron deficiency when grown in soil that is too alkaline.
More general information is available on abiotic diseases and injury.
- 1.Agrios GN. 1997. Plant Pathology. San Diego: Academic Press. 635 pp. 4th ed.
- 2.Cramer GR, Läuchli A, Lüttge U. 2004. Sodium-calcium interactions under salinity stress. In: Salinity: Environment – Plants – Molecules, pp. 205–227. Dordrecht: Kluwer Academic Publishers <https://pdfs.semanticscholar.org/4e79/80be3c4a28e74ecd875878d062e321d5c1c2.pdf>.
- 3.Goodrich BA, Jacobi WR. 2012. Foliar damage, ion content, and mortality rate of five common roadside tree species treated with soil applications of magnesium chloride. Water Air and Soil Pollution 223(2):847–862 <10.1007/s11270-011-0907-5>.
- 4.Goodrich BA, Koski RD, Jacobi WR. 2009. Condition of soils and vegetation along roads treated with magnesium chloride for dust suppression. Water, Air, & Soil Pollution 198(1–4):165–188 <10.1007/s11270-008-9835-4>.
- 5.Sinclair WA, Lyon HH. 2005. Diseases of Trees and Shrubs. Ithaca, New York, USA: Cornell University Press. 660 pp. 2nd ed.