If you grow houseplants or garden plants in terra cotta pots, you may notice a white crust or film developing on the pots and even the potting soil. Here’s what it is and how to deal with it.
White crust or film showing up on clay pots signals the presence of soluble salts, which come either from fertilizer you’re using on the plants or from minerals in your water (especially if it is hard water). Because terra cotta is porous, water seeps through it. The water evaporates from the sides of the pot, but the salts are left behind—hence their visible presence. You may also find this white residue around the drainage holes of pots made of other materials or on top of the soil itself.
To remove the white markings from your clay pots, mix a solution of bleach and water, using one part bleach per nine parts water. Take the plant out of the pot and scrub the pot inside and out with the bleach solution. Rinse it thoroughly with plain water, let the pot dry completely and then plant the plant in it again. Alternatively, make a paste of baking soda and water and use that to scrub off the salts. Salts on the outside of the pot will not harm the plant, so if you can’t remove all the markings, don’t worry.
However, a buildup of soluble salts within the soil can eventually damage or even kill your houseplants. To guard against this, every few months you should leach the salts from the soil by flushing it with an amount of water that’s twice the volume of the soil in the pot. If there’s white crust on top of the soil, gently remove that thin layer first. Then stand the pot in the sink, tub or shower and pour the water through it, letting it freely drain away. Whenever you water your plants, be sure to let the water drain instead of letting it gather in the saucer where the soil may draw it, and the salts it contains, up again. Of course, there are some plants that are watered from below (e.g., the pot is stood in shallow water until the soil surface looks damp, indicating water has soaked through it completely). African violets, for one, are often watered this way to avoid splashing and thereby scarring their leaves and to prevent crown rot. It’s especially important to leach these bottom-watered plants regularly as otherwise salts will certainly build up in the soil.
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