The Truth About Adding Eggshells to the Garden

The Folklore
Many of us believe that adding crushed eggshells to soil or compost is an effective way to boost calcium levels, helping to prevent issues like blossom end rot in tomatoes. This idea stems from the fact that eggshells are primarily made of calcium carbonate, an essential nutrient for plant growth. People often scatter crushed eggshells around plants or mix them into compost, assuming they will quickly break down and release calcium, similar to common gardening amendments like lime. However, while eggshells contain calcium, their impact on soil nutrition is far less immediate than many gardeners expect.

The Facts
Eggshell calcium is not readily bioavailable to plants because it is bound in a hard, crystalline structure that takes years to break down naturally. Unlike agricultural lime, which is finely ground and dissolves more easily in soil moisture, eggshells decompose slowly, especially in soils with low microbial activity. Studies have shown that eggshell fragments remain primarily intact after several years, meaning the calcium remains locked inside. This makes eggshells an unreliable short-term solution for correcting calcium deficiencies in soil, and they do little to prevent issues like blossom end rot, which is often caused by
inconsistent watering rather than an actual lack of calcium.

Making use of the folklore
There are ways to improve calcium availability from eggshells. Heating them to extremely high temperatures—around 900°C—converts calcium carbonate into calcium oxide (quicklime), which dissolves more readily in soil and becomes more useful to plants. While this process significantly increases the bioavailability of calcium, it requires industrial kilns or specialized equipment, as most household ovens max out at around 300°C, far below the temperature needed for this transformation. Simply throwing eggshells into a fire may help remove organic materials but won’t necessarily make calcium more accessible to plants. The best practical approach is to finely grind eggshells into a powder before adding them to compost or soil, which can slightly speed up decomposition and nutrient release.