Composting combines nitrogen-rich and carbon-rich ingredients, and microbial activity breaks them down into compost rich in nutrients and organic material that will boost the health of your garden. Avid composters talk of balancing "greens" and "browns"--nitrogen- and carbon-rich ingredients, respectively--and eggshells are identified as a "green" ingredient that will promote the reproduction of helpful soil bacteria. Eggshells also add calcium to your garden soil, which prevents certain nutritional deficiencies, and which plants like tomatoes and peppers especially need to thrive.
Select a place for a compost heap or bin. Compost expert Barbara Pleasant recommends keeping it close to the kitchen or the garden, where it will eventually be used. While well-balanced compost piles don't stink, for your neighbors' sake, keep in mind how others use adjacent properties when selecting a location. If scavengers like rats, raccoons or dogs might be a problem, be sure you use a bin with a tight cover.
Begin adding ingredients. Eggshells are "green" ingredients along with fresh, wet ingredients like vegetable peels, grass clippings and coffee grounds. Brown ingredients include dry items like paper scraps, dead leaves, twigs and straw.
Consider your ingredient balance. Unless you're in a hurry for usable compost, compost happens with or without human interference, so you don't need to monitor the proportion of green and brown ingredients too closely. However, adding too many eggshells or other greens may cause unpleasant odors, while too few greens may dry out the pile and stop microbial activity altogether. Generally, a balance of about two parts brown to one part green gets the job done. Since most gardeners don't have enough eggshells to achieve an adequate proportion of green using them alone, add other green items to provide nitrogen for your soil microbes.
Add higher proportions of eggshells and other green ingredients if you want to generate hot compost. Hot compost is ready faster and also suitable for composting riskier items, like manures, weeds and diseased plants, because the heat kills any seeds or pathogens. Hot compost requires more nitrogen-rich ingredients to increase microbial reproduction, and equal proportions of nitrogen- and carbon-rich ingredients generally start compost cooking. Be sure to turn your hot compost regularly to check for ingredient balance and ensure equal cooking time.