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Photo Credit: David L. Morgan

Tip burn on a tropical foliage plant is a sure sign of salt damage.

This is actually a trick question, because all living things need water to survive and grow, but too much or too little can be harmful. And here’s the trickiest part: Even optimal amounts of water can be damaging – that is, if the water contains high concentrations of salt, because too much salt can kill or injure many plant species.

Salt appears in many forms. We’re all familiar with sodium chloride, or common table salt. And most of us know that salty water produces harmful effects in all but the most salt-tolerant plants. But there are many kinds of salts, and it may surprise you to learn that quite a few of the types we use are beneficial in certain concentrations but harmful in higher amounts.

What am I talking about? One word: fertilizers.
Fertilizers are compounds that contain plant nutrients. They dissolve in water into what we call ionic form, supplying nitrogen to the plant (ammonium ions) and carrying sulfur to it (sulfate ions). Together, these nutrients help the plant grow – when they’re applied in the correct amounts.

Of course, this information begs the question: At what concentration do these salts become toxic? Well, it depends on the ion. If fluorides are present in just a few parts per million (ppm), they can “burn” sensitive houseplants, like zebra plant (Aphelandra). When carbonates and bicarbonates are found in concentrations as high as 100-200 ppm – as they are in some city water supplies – they can severely affect many species.

So how do you determine if you’ve got salty tap water? The best way is to get a chemical analysis from your city water department. Many times the report will give you a very useful summary in “total dissolved solids,” or total salts in ppm (sometimes indicated as milligrams per liter, which is the same). As a general rule, if your water has 1,400 ppm, then the salts are too high to be used on many houseplants.