you do compost , you can add old sea shells , my wife puts old shells In bottom of plant box to feed the plants...we collect shells for free at beaches .

You can also use egg shells for calcium in garden areas that need it - like tomatoes.

save all the old used egg shells, I have a electric coffee grander , I put shell in and turn the shell to a power, mix with feed (foods) and give to the chicks. they need it to keep making hard eggs... :wink: