Most people are only too aware of the damage
and corrosive effects of salt on automobiles. On heavily traveled highways
from 40 to 80 tons of salt per lane mile per year may be applied. Landowners
along these roads also are aware of the damage to plants that such salt
can cause.
Deicing salt is usually refined rock salt
consisting of about 98.5 percent sodium chloride, 1.2 percent calcium sulfate,
0.1 percent magnesium chloride, and 0.2 percent rock. Calcium chloride
is reported to be less toxic to plants but is seldom used because it is
much more expensive than rock salt and more difficult to handle.
When sprayed onto plants from passing cars
and plows, salt may enter plant cells or the spaces between the cells directly.
One result of this "salt application" is that buds and small
twigs of some plant species lose cold hardiness and are more likely to
be killed by freezing.
Salt accumulation in the soil also may
cause plant injury. This frequently occurs when salt-laden snow is plowed
off streets and sidewalks onto adjacent lawns.
Anyone who has tried to get table salt
out of a wet shaker knows how readily salt absorbs water. Rock salt exhibits
the same property in the soil and absorbs much of the water that would
normally be available to roots. Thus, even though soil moisture is plentiful,
high amounts of salt can result in a drought-like environment for plants.
When salt dissolves in water, sodium and
chloride ions separate and may then harm the plants. Chloride ions are
readily absorbed by the roots, transported to the leaves, and accumulate
there to toxic levels. It is these toxic levels that cause the characteristic
marginal leaf scorch.
Measures to prevent or lessen injury from
salt include using calcium chloride, where feasible, or using sand or cinders.
Late season applications (after March 1) are most detrimental and should
be avoided if possible since this is the time plants are coming out of
dormancy and are most susceptible to injury.
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University of Vermont
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