Even though we don’t eat them, eggshells are full of nutrients and minerals that are important for plant health and growth. They can help balance soil pH, prevent blossom end rot — there are tons of benefits. However, there’s an art and a science to the preparation you need to do to use eggshells in the garden — whether that’s adding them to your backyard compost pile or incorporating them directly into the soil as natural fertilizer. If you’re interested in the right approach, here’s everything you need to know about composting eggshells.

Can you compost eggshells?

In short, yes, eggshells are something you can compost as a green material. And, to be totally honest, you probably should. Eggshells are a great source of calcium, an essential nutrient that supports plant health. For some plants, like tomatoes, eggplants, and peppers, eggshells can help stop bottom end, or blossom-end rot — that’s a situation where the flowering end (the side opposite the stem) of a fruiting plant rots before the fruit is ready to eat. Because blossom-end rot is caused by a calcium deficiency, eggshells help prevent it. 

In some cases, like with tomatoes, plants can benefit from putting crushed eggshells directly in the soil. However, it’s important to remember that eggshells will also raise the pH of the soil, making it less acidic. Other plants, like blueberries and magnolias, prefer acidic soil. So, before you go adding eggshells to your soil or to your compost, if you’re using that compost in your garden, just make sure your plants are compatible with less acidic soil. 

It’s also worth noting that large pieces of eggshells can take a long time to decompose, and although the risk is small, there’s also a chance of contamination from salmonella if they aren’t handled correctly. We’ll tell you how to minimize both the risk and the breakdown time below. If you don’t feel like dealing with any of that, virtually all municipal composting programs will accept your eggshells without any effort on your end. 

How to prevent salmonella contamination in a backyard pile

Salmonella, a bacteria that is closely associated with poultry and raw eggs, can be a concern for home composters when composting eggshells — especially if that compost is being used for edible plants. This is because many home compost piles don't heat up to the necessary 130°F  to 160°F needed to kill bacteria and pathogens, and so salmonella can live on in compost if it is present on the egg shells. 

It’s estimated that only 1 in every 20,000 eggs is contaminated with salmonella. So, while the risks of salmonella are low, you should take precautions to minimize it. Here’s how you should go about preparing eggshells for compost. 

• Rise and dry the eggshells. After cracking the eggs, rinse the shells thoroughly to remove any leftover egg whites or yolk. This helps reduce bacteria and eliminates any odors that could attract pests to your compost. After rinsing, dry the eggshells completely by laying them out on a sheet pan lined with paper towels.

• Compost in a hot pile. For all the expert composters out there, if you maintain a hot compost pile (hot composting is when a pile is between 131°F and 160°F), the high temperature will kill harmful bacteria, including salmonella. 

• Bake the eggshells. For added protection, you can also bake eggshells in the oven. Lay washed eggshells on a sheet tray and bake at 200°F for about 20 minutes. This kills any potential bacteria and makes the eggshells more brittle, which makes them both easier to crush and quicker to decompose.

• Pre-process with Mill. Mill’s food recycler reaches the time and temperature required to reduce pathogens, including salmonella. You can skip all the above steps and just toss the egg shells in your Mill, which ensures that the eggshells are pathogen-free before they even reach your compost bin or pile. This is one of the safest, most efficient, and most effortless ways to handle eggshells at home.

How to speed up decomposition when composting eggshells

The key to faster eggshell decomposition is to make the pieces as small as possible, which reduces the total mass that needs to be broken down while increasing the available surface area. Here’s how to do it. 

• Crush or grind the eggshells. After washing and either air drying or baking the eggshells, use a food processor, mortar and pestle, or coffee grinder to process the eggshells and break them down into a fine powder. Then, add these crushed eggshells to your compost pile or directly to soil. 

• Pre-process with Mill. Mill Food Grounds have a higher surface area for microbes to munch on, which leads to faster composting. Using Mill means you can skip the washing and drying portion of the process because you can toss them directly into Mill — it takes care of the rest. Mill Food Grounds can then be added to compost piles or used immediately as a soil amendment.