Ask Mr. Smarty Plants

If left soaking overnight, some gardeners believe that calcium is leached from eggshells into the water in quantities that are beneficial to plants.  Since many plants need some calcium in the soil for good growth, this is viewed as a way to provide additional calcium for plant uptake. In searching the internet, there are many references to using eggshells this way, or crushing them and adding them to the soil, but I could find no scientific research to prove this procedure is providing additional calcium. Anyway, it is unlikely that doing this will harm your ferns, if you feel strongly about doing this.

Used tea leaves are fine to add to your compost with other ingredients like vegetable peelings, grass clippings, and shredded leaves, and in a moderate amount that satisfies the ratio of carbon to nitrogen needed for good decomposition.  Look at the Mother Earth News website for a good article by Barbara Pleasant on making compost.

It’s not a good idea to just use a lot of tea leaves when planting seedlings as there may be porosity and nutrient issues if too many tea leaves are close to the seedling roots. Regular seeding potting soil has a more porous texture that will allow for healthier root growth and often contains a better ratio of the nutrients needed by young plants. Small amounts of tea leaves can be incorporated into regular potting soil when transplanting larger plants though.  Also tea leaves will work well for other tasks such as using them as a mulch on the surface of the soil for container or perennial plants and shrubs to conserve moisture.