There are several situations where salt can damage plants: some areas have saline soils, there may be occasional seawater flooding, or slightly saline irrigation water can accumulate in the soil. Coastal areas can have salt spray that affects the local plant communities. In those situations, problems can arise, particularly in plants out of their native range or when unusual conditions spread salt out of it’s usual area. For instance, hurricanes can cause plant damage up to 10 miles 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 may be applied to paved roads and lots for the purpose of deicing. It may also be applied to unpaved roads for dust abatement.
Salt affects roadside plants via two routes: through the soil, where it is taken up by roots, and as salt spray that is deposited on aerial plant parts.
Salt uptake via soil
Areas affected are below road grade, usually quite close to road, especially where tree’s roots are all in low area with limited drainage. Symptoms: leaf scorch, premature defoliation, dieback, can contribute to death. Can see in red pine in the Adirondacks and many species in the Rocky Mountains. Sugar maple is susceptible. Mechanisms: osmotic concentration in soil too high, roots can’t obtain enough water. Chloride accumulates to toxic levels in tissues, especially those with greatest transpiration. Na may replace cations like Ca and K in soil, creating deficiency. Na also raises pH, making iron unavailable. Na also degrades soil texture.
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
Up to 50 meters of highway, especially downwind. Exposed plants collect the most droplets so get the most damage. Symptoms: needles brown at the tips, drop early. Hardwoods- buds die, twigs die back –> brooms. Mechanisms: Cl- toxicity seems to be the big problem, accumulates at tips and margin. Kills tissue, impairs cold hardiness. Tolerant plants limit accumulation.
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.