Here's Why You Shouldn't Put Broken Up Eggshells Directly In Your Soil

Wondering if all the talk about using eggshells in your garden is all it's cracked up to be? Egg shells have plenty of calcium carbonate, an essential mineral for plant's cell structure and soil health. Tomatoes can suffer from Blossom end rot, an indication of insufficient calcium that appears on the fruits as brown or black spots. Simply adding broken up egg shells to the soil and letting them decompose doesn't equate to plants' having access to calcium. 

To complicate matters, a plant showing signs of calcium deficiency doesn't necessarily mean that there isn't adequate calcium in the soil. "Calcium absorption can be hampered by overly acidic soil, soil with very high levels of potassium, overuse of competing ammonium nitrate fertilizers, or very dry soil," says Mary Jane Duford, certified master gardener, permaculture garden designer, and founder of Home for the Harvest.

And it's possible that adding eggshells could affect the soil in a less-than-stellar way. "Plants like strawberries, blueberries, kale, and cabbage prefer slightly acidic soil and adding in eggshells can do more harm than good," says Jen McDonald, certified organic gardener and co-founder of Garden Girls, a garden design company based in Houston, Texas.

Here's what you need to know about eggshells, and why it's often best to add them to the compost pile to use as a rich amendment for your plants. 

Why Broken Up Eggshells Don't Provide Nutrients

Eggshells need to be fully decomposed or ground up for the nutrients to be accessible to plants and the soil. "The calcium contained in the eggshells, even though it's a beneficial nutrient for plant growth, isn't readily available to plants in the short term," says Chad Massura, Founder of Rosy Soil. Plants need nutrients available in a format they can use. "Plant roots absorb mineral nutrients as dissolved ions," says Durford. "Dry bits of eggshells do contain quite a bit of calcium, but it is not yet in the form of dissolved ions." So how do you get the benefits? "The eggshells need to decompose before the bits of calcium are small enough to be absorbed by plant roots," says Durford.   

Eggshells Can Attract Pests

Not only can adding broken up eggshells to your soil not provide the benefits you're anticipating, it could bring in unwanted guests. "Eggshells can attract pests like rodents or insects, which might come looking for food," says Duford. Eggshells should be cleaned before using them in the garden or tossing them into the compost pile to avoid spreading pathogens. "Rinsing [eggshells] is important to avoid salmonella contamination," says McDonald.

Why Composting Eggshells Is A Better Idea

A better option is to place your eggshells into the compost pile, although they can take a long time to fully decompose. Compost is a rich amendment to add to soil before planting or to provide a nutrient boost to your plants during the growing season. "Applying finished compost to vegetable gardens is always a healthier idea in my opinion," says McDonald. "Heavy feeders need nitrogen (tomato, squash, eggplant, and peppers) and will benefit from a top off of compost each growing season." 

Composting eggshells is a preferred method than putting eggshells directly in the soil. "Composting eggshells is a better utilization method because the composting process breaks down the eggshells into the ions that can be absorbed by plant roots," says Durford. "This breakdown makes the nutrients in the shells accessible to plants." Another reason to compost eggshells is to minimize pests from hanging out in your garden. "Composting also helps to reduce the risk of attracting pests and eliminates any potential disease pathogens, making it a safer and more nutrient-rich option for garden soil amendment," says Massura.

Grind Up Eggshells

Eggshells can take time to fully break down in a compost bin, so one way to speed up the process and to ensure you're getting the calcium carbonate is to grind them up into a fine powder first. "To speed up the process by which the nutrients can be plant-available, you can bake, dry, and grind the eggshells into a fine powder before adding them to the soil, or better yet to compost," says Massura. "This process increases the surface area of the shells, making it easier for microbes to break them down and to release their nutrients more readily into the soil." He adds that baking them at a low temperature reduces the chance of salmonella risk. Eggshells provide important nutrients to the compost and it reduces food waste ending up in landfills—a win-win. Grinding up eggshells, according to Massura, "not only enhances the nutrient content of your garden soil but also contributes to a more sustainable gardening practice by recycling kitchen waste."