One of the best ways we’re aware of to make the calcium plant available is to treat them with vinegar. We’ll dry ours on a sheet pan in the oven on a low temp, crush them, and then steep them in apple cider vinegar for up to a few days. The remaining bits of shell can be strained out and added to your bin, and will break down much more quickly than they would otherwise. The acv solution can be diluted 10-20x with water and applied at the base of your plants or as a foliar spray.