Sea shells should not be used in plant pots because they are made of calcium carbonate (chemical formula CaCO₃), which is exactly the same substance as limestone and garden lime. They will make the potting medium extremely alkaline. Some plants don’t grow well in alkaline soils!
Any material that is added to improve drainage should be inert, it should not react chemically with the potting medium to alter the soil pH.
For this reason, it’s a bad idea to put limestone instead of gravel in the bottom of flower pots for drainage, don’t do it!
Adding a layer of rocks or any other material to the bottom of plant pots does not improve drainage at all anyway, this myth has been thoroughly discredited. For more information see the article – Should You Put Gravel or Rocks at the Bottom of Plant Pots for Drainage?
If you want to improve drainage in houseplants, the correct way to do it is to amend the potting medium to create a faster draining mix. I explain all the materials that can be used for this purpose in the article – How to Improve Drainage in Plant Pots, The Proper Way to Do It!
Crushed sea shells can be used to increase calcium levels in garden soil, and to make it more alkaline for lime-loving plants, but it’s best to keep sea shells out of your houseplants.
Got any gardening questions? Please feel free to add them to the comments below or email them to deep_green@optusnet.com.au if you would like them answered. Make sure to include pictures in your emails if they help explain a garden problem!