As I watered my houseplants this winter, I noticed a powdery white build-up on the sides of both plastic and terra cotta pots. Is this salt? I rarely use fertilizer, and when I do it is something organic, like fish emulsion. I thought salts come from synthetic fertilizers.

Synthetic fertilizers are seldom the only culprits. The best type of water for houseplants is rainwater or melted snow. Distilled water can be used, but the cost may be prohibitive over time as well as the work to carry the jugs into the house. Tap water (well or city) is generally suitable for nearly all houseplants. However, the water should stand overnight in a container to allow the chlorine to evaporate and to reach room temperature.

Whether you live in an area with hard or soft water, both can be harmful to your plants as they contribute to a build-up of excessive salts in the soil. Hard water contains calcium, magnesium and iron salts. Softened water is worse because the softening process exchanges calcium for salts that have no use. Plants use these salts in very small quantities. In large amounts they become toxic. What the plant does not absorb from the watered soil will appear as the white crusty deposits. Repotting in new potting medium will help the already affected plant, and leaching the soil (watering to the point of runoff each time you water, or at least monthly) will help prevent future salt build-ups.