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I've had several situations where my houseplants get a thin white dry-looking mold growing on the surface of the potting soil. I live in the desert Southwest of the U.S. Sonoran desert.

I'm diligent in my watering but show restraint and patience. The dryness can make you water more than you should but the watering frequency is certainly more frequent here.

I've had people tell me it's calcium build up but our water isn't hard.

If I can find a picture later I will update the question with an image.

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  • Hi! I haven't seen a picture of your soil and wonder how your plants are doing. Are you still having a problem? If so, do you want to add any other details to the question, like what type of soil you're using? Unfortunately I don't have any expertise in the field, so I don't know if it matters, but it might. If the problem's solved, that's great, and I'm glad your plants are feeling better! – Sue Saddest Farewell TGO GL Jan 21 '16 at 21:16
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So there are two possibilities here, one is that you are indeed seeing mold, the other is you are seeing salts and mineral deposits on the surface of your soil.

Mold is minimized by watering only when needed, but you should water deeply to prevent salts from developing on your surfaces.

Salts develop when you have hard water (including softened water). If you water without flushing the water clear through your pots and planters, the excess minerals never flush out and they start to build up. They begin to grow crystals on the surfaces (soil top and sides of terra cotta pots) because that's where the water is drying first, so the water is continuously moving to the edges carrying along those minerals.

So the answer is, if you're able water only when necessary to prevent mold but water deeply to assure a full watering and to flush the excess junk out of the soil.

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If you want to prevent mold (if it is mold you're dealing with), I highly recommend increasing your plant's (and soil's) light exposure. Certain kinds of light (especially unfiltered sunlight that still has UV rays in it) will probably help more than others. Mold generally prospers when light is dim and conditions are moist.

I've also found that sea minerals reduce the amount of mold that grows on the surface of the soil. However, although plants (peppers, anyhow) seem to love sea minerals, they probably will decrease your plants' drought-tolerance for a while, especially in your area. I do believe plants eventually get used to it, though. That seems to be the case, anyway. However, if you aren't dealing with mold, but rather mineral deposits, sea minerals will probably increase the visual aspects of the problem.

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