Biology Question #1014

Boo, a 13 year old female from Mississauga asks on November 2, 2002,

Why do some people use salt to kill weeds? What does salt do that makes the weeds die?

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The answer

Kevin Vessey, Plant Pathologist, University of Manitoba answered on September 28, 2003

I have not heard of people using salt to kill weeds.  It certainly would, but sounds expensive and not very specific (i.e. would kill LOTS of things). 

Table salt is sodium chloride (NaCl).  Both sodium and chloride are plant nutrients at low levels, but at high enough levels can be toxic to plants.  These substances can have deleterious effects on plants by several means, but the major contributor to death is that they cause desiccation (i.e. drying-out) of the plants by interfering with the plants' abilities to absorb water.  As salt is added to soil, water is held more "tightly" by the soil, and is not as available for absorption by the plants.  In other words, the plants die of thirst.  This is often apparent by the leaves curling up, turning brown and dying.

The reason I said that salt is not a very good choice as a herbicide is because aside from killing the weeds, it would kill (or damage) all the plants where the salt was applied.  It would also probably kill a goodly number of the insects, fungi and bacteria in the soil as well. The soil could not support any more plant life for a long time until the salt was washed out/diluted by rain. In comparison, a wide spectrum herbicide (i.e. they kill most plants) like glyphosate (i.e. "Round-Up") will only kill the plants, does not have long-term effect, and would be cheaper.

Kevin Vessey answered on April 26, 2004

Reader Gina Fry says, "I'm glad I didn't use salt when it was recommended to me as safe, but how can Round-Up not have a long term effect on the environment?"

Dr. Vessey answers:

Chemically, glyphosate is a relatively simple molecule [N-(phosphonomethyl)glycine], composed of a very common amino acid (glycine) connected to a phosphate group (PO3) via a methyl group (CH2).  The reason it is considered relatively safe in the environment (relative to other pesticides) is that once it is in soil, it binds very tightly to soil particles and does not easily move or can be absorbed by plants.  In the soil, it becomes food for bacteria.  Its half-life in the soil (the time it takes for 1/2 of single application to be consumed by bacteria) has been measured at 44 days.  This is a relatively short half-life for a herbicide.

For more information, read this good report on the fate of glyphosate in the environment or see this assessment of its risk to human health.

Michael Litwak, concerned citizen, Petaluma, CA answered on February 20, 2005

When you use Round-Up, the active ingredient glyphosate is not the only chemical you're applying. Round-Up is 99% "inert" ingredients -- including polyethoxylated tallowamine and isopropylaminesome. These can be acutely toxic to some inidividuals. Be careful with Round-Up. I suggest reading the article in The Ecologist to explore Round-Up's toxicity.

[Editor: a further literature search found this recent article in Environmental Toxicology and Chemistry.]

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