DougL said:People talk about eggshells as a means to add calcium to soil. Tomato blossom end rot is caused by poor calcium transport through the plant, but actually has almost nothing to do with the amount of calcium in the soil. To add to the mythology, eggshells don't really add calcium to soil. They add, well, eggshells. Eggshells are rocks that come out of chickens. Until the eggshells dissolve (which they won't for many years) they don't provide any plant-usable calcium. Eggshells and tomato gardening is one of the more blatant pieces of gardening mythology.
Potting mix and compost are good for soil structure, but include virtually no fertilizer. You absolutely need fertilizer. Your plants will die in just potting soil and compost. MiracleGro fertilizer, properly mixed and applied, will do the trick. The stuff is soluble, so will leach out as you water. You'll have to fertilize regularly. Compost doesn't really replace fertilizer. It has some nutrients, but in VERY slow-release form.