Christopher Satch, Plant Doctor
Calcium is an essential element for all life on earth. However, calcium is not very soluble in water, and so it tends to precipitate out of solution and become a solid. As water evaporates, the calcium is left behind, and it forms a white to off-white, chalky residue. Typically, ground or well water that comes from an underground limestone source is rich in calcium and other minerals (Areas devoid of limestone have other quirks associated with their water). These minerals in well water are used by the plants when they are in solution. They are used to fortify plant cell walls, much like they fortify our bones and teeth. Depending on your aesthetic (and the plant), you may want to leave it or take it away.
Calcium builds up where water dries up. You may have encountered calcium deposits in your bathrooms in the shower or in the kitchen sink. For your plants, you’ll encounter it in two places- on the leaves that got wet, or on porous ceramic pots like terracotta. On ceramics, there’s no harm in leaving it for that rustic look, unless you are growing salt-sensitive plants like orchids. On leaves though, while harmless, can look ugly and possibly clog pores when it builds up.
Now for a little chemistry (and the solution!).
Calcium is an alkali-earth metal with low density, resistance to heating and dissolving, and never occurs pure in nature. The term “alkali earth” is a semi-outdated term. It is called “alkali” because it forms alkaline solutions with water and other elements. The term “earth” is an old alchemist’s term that refers to non-metallic substances (think rocks and chalk).
Because of its alkali nature, it can react with acids to become soluble. You can use a simple acid like lemon juice or vinegar to dissolve the calcium salts. I recommend mixing 1:3 parts lemon juice:water (a 25% solution) OR 1:9 parts vinegar:water (a 10% solution). Wipe the leaves with a rag soaked in this solution and the calcium should come right off. If some persists, repeat process. With terracotta pots, it’s most economical to just replace the pot. Otherwise, you can soak any pot that has calcium buildup (sans-plant) overnight, and scrub the deposits off as best you can.