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Salt buildup on the soil surface is very common and harmless to most plants. However, to some plants that are extra sensitive to lime or high mineral levels, this may be an indicator that you should intervene to keep them healthy.
SIGNS YOUR PLANT HAS SALT BUILDUP
A pale, white, chalky substance forms on the soil surface, on the outside of a clay pot or around the rim of the pot or saucer
The salts that build up in plant soil aren’t quite same as our typical table salt. Rather, these could be either mineral salt deposits which are composed of calcium carbonate, sodium, and iron (limescale) or fertilizer salt desposits that come from the various components of mineral fertilizers. Most commonly, you may see a white crust form on the surface of the soil if you live in an area with hard water. It's also normal to see it on the outside of terra cotta pots. For the majority of houseplants, this isn't a problem, but there are some that won't tolerate it. These are acid-loving, or ericaceous plants.
WHAT TO DO NOW
You usually don't have to do a lot unless the buildup is extensive, or if you think it looks bad. Manually remove the buildup on the soil surface, and clean the saucer or outer pot now and then.
For plants that are sensitive to lime and a higher pH level, mineral buildup can be an indication that the water you're using is too rich in minerals. It's best to use rain water or distilled water for these plants. Improper pH levels can interfere with the plants ability to take up sufficient nutrients from the soil which results in stunted growth or discoloration of the foliage.
Note: An activated carbon filter pitcher is not sufficient to remove limescale from water for sensitive plants.
IS SALT BUILDUP BAD FOR MY PLANT? For the most part no, but it depends on the plant.
HOW DO I KNOW IF MY PLANT IS EXTRA SENSITIVE TO LIME? In Planta, you can see if the plant is sensitive to lime. You can find this on the Plant Info tab for the plant in question.
CAN SALT BUILDUP OCCUR FROM TOO MUCH FERTILIZER? Yes, but overfertilizing will usually show earlier as other more severe symptoms like droopy foliage or dry leaves, much like overwatering or drought.
If after fertilizing, you see some white spots on the foliage of your plant, gently wipe if off with a damp cloth.