A: This is a not-uncommon problem with houseplants. The issue here involves salt – several kinds of salt.
Our water supply is potable and safe, but tap water has many dissolved salts and solids. Most of the time, these are perfectly harmless, or even beneficial. But when houseplants are watered while sitting in saucers, which is common to prevent damage to furniture and floors, that water doesn’t go anywhere until the plant either drinks it all or the water evaporates away, leaving all the dissolved solids behind.
Again, no problem, until taken to excess!
Once you build up enough salt in the soil, it becomes difficult for the roots of your houseplant to pull water in from the soil. Plants pull water in through osmotic action, and too much salt build-up prevents the plant from doing so. In effect, the plant can be in moist soil and still dry out. Seaweed and oceanside plants that grow in high salt areas have specialized adaptations to help them survive this, but most plants do not.
The answer, thankfully, is simple. Take your houseplant out of its saucer, either outside or in a sink or tub, and slowly water it. Take your time! A leisurely, thorough watering to dissolve out some of the excess mineral salts and drain them out the bottom holes of the pot will help the situation. This can’t be done entirely in one session. Each week you water for the next month, take the plant to the sink to water, and saturate the root ball and let the excess flow out of the pot.
After you get your salt levels under control, you’ll only have to repeat this process every month or two. For larger plants which are inconvenient to move, simply tip out the saucers or siphon the water out with a handheld wet-dry vacuum.