In general, is there any benefit (growth, soil quality, etc) if I place eggshells on top of my potted plants?
My mother often does it to provide it organic material. She's not much of a scientist. I always ask why she does it. She explains that just does it because she's been advised to by others with more experience and swears by it.
Thus my other question: If there is a benefit of egg shells, what is it? Why is it helpful to do this?
In addition to acidity (probably not a problem), and very slow calcium release, I'm going to offer a hypothesis as to why she does it...
Crushed eggshells are sometimes used as organic methods of deterring certain pests, especially gastropods such as snails and slugs (see this question). Gastropods are unlikely to be a problem in indoor pots, but perhaps she saw someone do this outside around plants in a bed, and copied it? Or even an ancestor did this, and it has been passed down through the generations from village gardening in beds to modern city gardening in pots?
Similarly, it might have been done to help with calcium deficiency in beds, although digging the shells in and crushing them would be much more effective.
My mother in law liked to put eggshells (not crushed much) at the bottom of pots when repotting. Served the dual purpose of drainage (as some people might use gravel) and possibly providing calcium. With the added bonus that eggshells are easier to come by than gravel for many people.
At the top of the pot I doubt it would do anything much. I'm not really convinced it did much at the bottom. I put my eggshells in my compost so the vegetable garden gets more calcium.
They're mostly lime (calcium carbonate), with other trace minerals. If the soil has a low pH, it will help. Otherwise, it can't hurt, but probably won't help much.
