Eggshells are roughly 95% calcium carbonate, the same compound found in limestone and antacid tablets. That simple chemistry makes them surprisingly useful in the garden, the kitchen, and even as a dietary supplement. Most people toss them straight into the trash, but with minimal preparation, eggshells can improve soil, boost your calcium intake, and serve a handful of other practical purposes.
Calcium Supplement on the Cheap
Eggshell calcium is biologically activated, meaning your body absorbs it more readily than the industrial calcium carbonate found in standard supplements. One study comparing the two found that calcium absorption from eggshell powder paired with egg-derived proteins reached 55%, compared to just 34% for pure calcium carbonate. People taking the eggshell-based supplement also showed higher bone mineral density in their thigh bones.
To make your own, rinse eggshells, bake them at around 200°F (95°C) for 10 minutes to kill bacteria, then grind them to a fine powder in a coffee grinder or blender. Half a teaspoon of this powder contains roughly 400 mg of elemental calcium, close to half the daily recommended intake for most adults. You can stir it into smoothies, oatmeal, or soup without noticing much flavor. The finer the grind, the better your body can absorb it.
Raising Soil pH and Adding Calcium
Ground eggshells work like a mild, slow-release lime for acidic soil. In trials on both sandy loam and sandy clay soils, applying ground eggshells raised soil pH by more than 1.4 units, a relative increase of nearly 30%. The effect kicked in fast: soil pH increased significantly within just seven days, plateauing within one to two weeks depending on soil type. Eggshells also improved the availability of phosphorus, a nutrient many plants struggle to access in acidic conditions.
The key is grinding them small. Whole or coarsely crushed shells can sit in soil for years without breaking down meaningfully. A food processor or mortar and pestle works well. Sprinkle the powder into garden beds or mix it into potting soil before planting. For acid-loving plants like blueberries and azaleas, skip the eggshells entirely.
Preventing Blossom End Rot in Tomatoes
Blossom end rot, the dark, sunken patches on the bottom of tomatoes and peppers, is caused by calcium deficiency in the developing fruit. Eggshells can help, but how you apply them matters. Simply burying crushed shells near roots is too slow to make a difference during a single growing season.
A more effective approach: dissolve crushed eggshell powder in boiling water, let it steep for 24 hours, then strain and spray it directly on the leaves and stems. In a field trial, tomato plants sprayed with this eggshell solution accumulated significantly more calcium in their above-ground tissues and produced zero blossom end rot. Plants sprayed with plain water developed rot on 15% of their fruits. The eggshell-treated plants also produced heavier fruit and more robust branch growth.
Slug Deterrent: A Popular Myth
The idea that crushed eggshells form a sharp barrier that slugs and snails won’t cross is one of the most repeated tips in gardening. It sounds logical, but it doesn’t hold up. Researchers at the “All About Slugs” group tested this directly by surrounding lettuce leaves with crushed eggshells. The slugs crossed the barrier without hesitation and ate the lettuce. If slugs are your problem, eggshells won’t solve it.
Reducing Coffee Acidity
If acidic coffee bothers your stomach, adding a pinch of crushed eggshell to your coffee grounds before brewing can help. The calcium carbonate in the shells is alkaline, so it neutralizes some of the acids during extraction. The result is a noticeably smoother, less bitter cup without altering the coffee’s core flavor. Just add a teaspoon or so of cleaned, dried, crushed shell to the grounds in your filter or French press. This trick is especially helpful with dark roasts, which tend to be more acidic.
Joint Health From Eggshell Membranes
The thin membrane lining the inside of an eggshell is a different product from the shell itself, and it has its own uses. Eggshell membrane contains type I, V, and X collagen, along with compounds your joints rely on: chondroitin sulfate, glucosamine, and hyaluronic acid. These are the same ingredients sold individually as joint supplements, bundled together naturally in one membrane.
A systematic review and meta-analysis of studies on people with knee osteoarthritis found that eggshell membrane supplements improved joint comfort. Commercial eggshell membrane supplements are available in capsule form. Some people also peel the membrane from raw shells and apply it to minor cuts as a makeshift bandage, though this is folk remedy territory rather than studied medicine.
Composting and Garden Beds
Eggshells are a useful addition to compost bins, but don’t expect them to vanish quickly. Their calcium carbonate structure resists microbial breakdown, so large pieces can persist through multiple compost cycles. Crushing or grinding them before adding them to the pile speeds things up considerably and ensures the calcium actually becomes available to plants once you spread the finished compost.
In compost, eggshells serve as a liming agent that can buffer acidity, keeping conditions favorable for the microorganisms doing the decomposition work. They also contribute trace minerals like magnesium, potassium, and phosphorus in small amounts.
DIY Skin Exfoliant
Finely ground eggshell powder has mild abrasive properties that make it functional as a physical exfoliant in homemade body scrubs. In formulation testing, body scrubs containing eggshell powder caused no erythema (redness) or edema (swelling) during 72 hours of skin contact testing. The shells need to be ground very fine and sieved to remove any sharp larger pieces. Mixed with coconut oil or honey, the powder creates a gritty paste that sloughs off dead skin. It’s a zero-cost alternative to commercial scrubs that use microplastics or pumice.
Feeding Eggshells Back to Chickens
If you keep laying hens, recycled eggshells are a free calcium supplement that helps maintain strong shell production. Bake the shells first to sterilize them and eliminate any egg smell (which could encourage egg-eating behavior), then crush them into pieces small enough that the hens won’t associate them with their own eggs. Offering them in a separate dish rather than mixing them into regular feed lets hens self-regulate their intake, eating more when their bodies need calcium and ignoring it when they don’t.