Ask The Hort Agent: Road Salt

  • Posted: Monday, February 3, 2014 10:28 a.m.
    UPDATED: Monday, February 3, 2014 10:31 a.m.

Question: Is road salt bad for plants?

Answer: Let's just say the Romans were accused of putting salt on fields after the two year siege battle of Carthage.  They wanted to make sure the period was visible at the end of that sentence.

The modern day battle is against ice on roads and sidewalks.  Each state depends on its Department of Transportation to keep roads safe.  These departments primarily fight snow and ice with sodium chloride (NaCl), also known as rock salt or halite.  NaCl is spread on highways to lower the freezing point of water.  Think of ocean water and freshwater.  Ice will form in both, but ocean water (which is salty) freezes at a much lower temperature than freshwater.  Rock salt has been used extensively since the 1960s because it is cheaply mined.

Besides dropping the freezing point of water, high salt concentrations pull water from plant roots.  This scenario is called “burning,” but it is actually a dehydrating process.  High salt concentrations can kill plants.  Southerners rarely experience this effect around sidewalks or ditch banks because we don't have much ice and don't apply much salt.  Northerners can have tons of ice, salt and dead plants. 

Concentrations of chloride in streams and creeks fed by road ditches can become lethal for both plants and animals.  Ironically, since ice forms on bridges and overpasses before roads, these structures get even more salt.  This deteriorates the structures and goes directly into streams and creeks.       

For Southerners, I urge restraint.  Just because rock salt works doesn't mean you need to use it.  Sidewalks and driveways do not retain ice more than 3 or 4 days in NC (excluding parts of the mountains).  Lack of traction is the primary problem caused by ice.  Keep a few bags or a mound of sand on hand.  A light coating of sand will greatly enhance traction.

If you have zero tolerance for ice, consider using potassium chloride (KCl).  Also known as muriate of potash, this salt is used as fertilizer.  Bermuda and centipede love it.  With no more ice than we have, your plants will benefit from a light application on sidewalks and driveways.

NC Department of Transportation is now using a salt water solution called brine.  This product uses less NaCl and can be applied prior to an ice event with more pinpoint accuracy.  Eventually, even more environmentally friendly solutions will be obtained.   

To learn more about salt and ice, check out http://www.madsci.org/posts/archives/nov98/910675052.Ch.r.html   If you don't have internet access or you have further questions, email gpierce@harnett.org or call me (910) 893-7530.  

The best use for rock salt is for making homemade ice cream.  Again, the salt lowers the freezing point of the water mixture in the ice cream freezer. This allows a colder water to surround your ice cream canister and produce ice cream quickly.  When you are finished making ice cream, pour the leftover salt water mixture onto weeds growing on your patio or sidewalk.  It will fry them good.  

Gary Pierce, Horticulture Agent
Harnett County Cooperative Extension
910-893-7530
http://www.harnett.org/coop/horticulture-programs.asp 

 

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