White marks can appear on an unglazed pot, around the rim, or on the surface of the mix. Mineral salts from water and fertilizer are one possible explanation, but appearance alone is not a diagnosis.
Look at texture and location
A dry crystalline crust differs from fluffy fungal growth. Check your watering and feeding history, drainage, and whether the pot repeatedly dries hard between waterings. Identify the plant's sensitivities before choosing a response.
Where salt buildup is likely and the plant tolerates it, species-appropriate leaching with suitable water may help, provided water can drain freely. This is not appropriate for a pot with blocked drainage or a root system already sitting in stagnant water. Follow reliable guidance for your plant and medium.
Prevent a repeat
Measure fertilizer carefully and avoid routine extra doses. If irrigation water is a continuing concern, obtain information about its quality rather than guessing from a kettle scale deposit alone.
Clean empty pots before reusing them and consider replacing deteriorated medium when justified. Do not simply cover the crust with fresh mix and assume the root zone has improved. Surface tidiness can conceal a continuing problem, so judge subsequent growth and moisture behavior as well as the appearance of the rim.
Further guidance & source notes
- University of Maryland Extension: Potting and repotting indoor plants
- University of Maryland Extension: Growing media for containers
These are background resources, not an endorsement of this website. Regional instructions and the needs of your plant take priority.