I have had a habit of drying out the used eggshells, grinding them in a coffee mill and adding then to them potting soil for growing spring starts.  Now in these mid winter days,  I add them to an oatmeal dish I feed the chickens with.  I first add 1/2 cup of organic sugar to 2 cups of boiling water and stir it till dissolved.  Then I add 1 cup of quick oats and drop the heat to a simmer.  Once I get it nice and thick,  I turn off the heat and add the fine crush eggshell for several eggs; probably about 3 tablespoons if you were going to measure it.  I let it go till quite warm but not hot and bring the saucepan out to the chicken coup.  The chickens come to the door clucking and, using  a wooden spoon,  I empty all the contents into their food bowl and leave. They immediately take to the oatmeal and finish the food bowl in quick time.  Their eggs seems large and more solid and are laid daily, even this time of year.  When summer occurs and the chickens are getting more calcium from the variety of greens around their coop,   I will start adding this mineral rich powder to new transplants and for general soil maintenance.