Salted Earth Campaign

Legend has it that when Rome sacked Carthage in 146 B.C. the roman general ordered the defeated city to be sowed with salt.  The intent?  That no man would ever again be able to farm or eke out a living from the conquered land.  The tactic seems to have roots going back far into the ancient cultures of the Middle East.  How much is legend and how much historical fact is somewhat unclear, but one things is unquestionable: Salt kills plants.

At least most plants, save those with special adaptations for living in saline environments, are efficiently killed by salt.  This is certainly the case for plants used for farming, gardening, and landscaping.  To be more accurate, sodium chloride (table salt) donates chlorine and sodium ions in soil that deprive plants of the vital macronutrient potassium, interfere with photosynthesis, and result in other damaging effects on roots and plant health.  In short, salt is bad news for plants.

According to an article by Don Talend from the surface water journal Stormwater, Americans use an estimated 15 million tons of salt for roadway de-icing annually and Canadians an estimated 4 to 6 million tons.  According to Smithsonian.com, that's 137 pounds of salt for every American; man, woman, and child.  By spring the salt that whitens our roads and covers our cars has disappeared.  But this disappearing act is not magic by any means.  Twenty million tons of minerals don't simply vanish.  These highly soluble minerals, rather, have been carried by the spring rains down storm drains, into streams and rivers, and even been deposited into the soil itself.  No, this salt it not gone.  It's simply been relocated, and not in the form of the coarse grains laid down by plow trucks but now as a microscopic layer of precipitate blanketing individual grains of sand and silt in the soil.  We are waging a salted earth campaign against ourselves.

What is the consequence of years of burying our land in salt?  An article from Cleveland.com reports that Atlantic Coast salt marsh plants are now taking up residence along highways in Ohio as a result of the change in soil chemistry.  Native plants are poisoned or stressed to the point of failure or susceptibility to other ailments.  Landscaping is browned or killed and synthetic fertilizers with their own host of ecological problems are applied to alleviate the problems.

On the other hand, what other options do we have?  Should we leave our roads untreated and allow cars accidents and damage to person and property?  Should be shut down our schools and businesses for the winter?  Obviously these are not realistic or sustainable options.  There are, however, some principles that can be applied to reduce that 137 pounds per capita to a more manageable and reasonable amount.

The salt selection at a typical grocery store.

The first strategy is choosing the best deicer.  There are a variety of salts available for use as de-icing agents.  All have pros and cons.  Standard "rock salt" actually contains small amounts of several salts, but for the most part it is sodium chloride (NaCl).  This is the same substance most of us put on our french fries and meats.  Sodium chloride has the benefit of being plentiful, cheap, and effective to about 15°F.  (Salt works by lowering the effective freezing temperature of water, so this means sodium chloride makes the freezing point of water something like 15°F instead of 32°F.)  This means that below 15°F the salt has little effect.  The main downside is that this salt donates sodium and chlorine ions in soil, both of which have the detrimental effects already discussed.

Calcium chloride (CaCl₂) is another highly effective melting agent.  In fact, calcium chloride will melt ice as low as -25°F.  It has the enormous advantage of not containing sodium, the worse of the two ions produced by sodium chloride.  Unfortunately it is much more expensive and contains twice the chlorine of sodium chloride (though exactly how much is mobilized in soil needs to be the subject of further research).  My personal preference is potassium chloride (KCl).  Potassium chloride melts ice a few degrees better than standard sodium chloride (12°F).  Like calcium chloride, it lacks sodium but better than calcium chloride it has no more chlorine than sodium chloride.  Potassium is actually a required plant nutrient present is most fertilizers.  Unfortunately this salt is also more expensive.  Last up is magnesium chloride (MgCl₂).  This is the blue-colored "ice melter" seen in some stores.  Like all of the alternative deicers it is more expensive than common rock salt.  I am not an advocate of this salt because despite being rated as effective to 5°F, I have found it to have a very narrow effective melt area.  In other words, it may melt ice at a lower temperature but it doesn't "spread" and instead just melts grain-sized tunnels into the ice.  Like calcium carbonate it contains double the chlorine of sodium chloride.  This information, as well as more concerning lesser-known deicers, can be found in the Purdue University Extension Publication ID-412-W.

While the average homeowner can't on their own change the choice of salt used by local road crews, he or she can make better choices for their own plot of land and that is what Domicile Ecophile is all about.  Calcium chloride and potassium chloride seem like the best options from a chemical standpoint as long as budget allows.  Next it is important to understand that salt only works to a certain temperature.  This week at Domicile Ecophile we have seen high temperatures between 5° and 10°F; lower than the effective temperature of most salts.  A great way to stay in budget and reduce the amount of salt leached into the environment is to be smart about applying salt only when it can be effective.  Sand or gravel can be another option when traction is needed but temperatures are too low for salting.  Lastly, we have all seen piles of salt left on the road or sidewalk.  Let's be judicious about how we apply salts.  Reducing waste is better for the environment and our budgets.  Some walkways around the yard may not need to be cleared in winter.  Likewise some portions of very large driveways may not need to be cleared or deiced either, reducing the total volume of salt applied.  Lastly, consider the relief of your property.  Take a look at where the runoff from your salted surfaces eventually ends up.  Is there a garden directly downstream of a walkway or driveway?  If so you may want to alter your salting practices or add a berm to divert water away from sensitive plants.

So this winter take a look at that bag of ice-melter before you purchase it.  Consider which salt is actually in that bag.  Be smart about when and how much salt you apply.  Let's work to minimize the amount of unnecessary salt we add to our soil: I think the romans would approve.