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Why does there appear little white stones in the soil of my plants? I had this before in other plants, and now I see it appearing together with my basil. They have a size of max 1 cm and are very porous and crunchy. We got an attack of little flies at the same time, so I was wondering if it might be the little eggs of those flies? (maybe a stupid idea, but it's the only thing I can think about.. water does not contain so much lime and it would be a strange form of precipitation)

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This is perlite, a porous mineral that is added to potting soils to help prevent them from getting compacted. They tend to rise to the surface because the soil contracts as it dries out between waterings and expands when watered, effectively squeezing bigger particles in the soil towards the top (a process known as the Brazil nut effect).

It's completely benign, and unrelated to any pests that you may have had.

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    Just to add a note to this great answer: My personal preference is to mix in some vermiculite to the soil mixture for seedlings. It doesn't float like perlite will do but it is a bit more expensive. Typically I mix perlite and vermiculite both into the mix. They help avoid compaction and the vermiculite holds onto a bit of moisture. – itsmatt May 23 '13 at 14:21

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