That’s the gardening question of the day from Tereza Becica of Van Nuys, California. Here’s the answer:
The crusty build-up can come from minerals naturally found in the water and from water-soluble fertilizers. It can also accumulate in the soil, causing root “burning” that leaves roots unable to take up water.
To prevent this build-up, avoid applying excessive fertilizer. Don’t use water from a water softener or other water with a high salt content. Don’t water from the bottom, as this encourages uptake and deposit of mineral salts on the soil surface and rim of the container.
If you have no other choice besides softened water, water thoroughly each time. Hold the pot over a sink and add water until it pours through the drain hole. That way, even though you are adding more salts, you will flush some away. If you bottom water, water heavily from the top at least once a month to leach out accumulated mineral salts.
When cleaning the crust from empty containers, soak them for at least 30 minutes in a solution of nine parts lukewarm water to one part liquid chlorine bleach. Scrub them with a stiff wire brush, then rinse well to remove all visible traces of the minerals. If they don’t come clean after one or two attempts, discard them.
Deb Brown is professor emeritus, University of Minnesota Department of Horticultural Science.