In a landfill, eggshells produce odor pollution and attract microbial growth, which is why the European Union has declared them a hazardous waste.
Keeping those eggshells out of the landfill by using them in your garden is thus a good thing. Indeed, using eggshells in your garden can improve soil quality and encourage plant growth.
While composting eggshells along with other food waste is wise, most other common tips exaggerate the benefits of using eggshells in the garden. In fact, directly adding eggshells to your garden has few proven benefits.
How Eggshells Impact Soil
Approximately one-third of the mass of eggshells is composed of calcium, and the rest is small amounts of magnesium, sodium, potassium, iron, zinc, and copper. While eggshells also contain a fair amount of organic matter (remainders of the yolk and albumen), the shells themselves are very slow to decompose and release their nutrients into your soil.
To speed up the shells’ decomposition, some garden sites suggest pulverizing the shells and adding them directly to the soil. Others suggest making “eggshell tea”—soaking the shells in boiling water overnight (to kill pathogens such as salmonella), then straining them the next day to create a liquid plant fertilizer.
But eggshells’ most important benefit is their mass, not their nutrients. You will get more benefits out of eggshells by adding them to your compost, not directly to your soil.
Compost Your Eggshells
In a compost pile, the shells’ organic matter attracts microorganisms necessary for the decomposition process, while their enzymes speed that process up. Broken (not pulverized) eggshells create air pockets in your compost, which also encourages the breakdown of organic matter.
Your compost will be ready to add to your soil long before the eggshells in it have completely broken down. That’s a good thing since the increased aeration from the shells create helps the soil retain water. The shells’ surface area also binds nutrients from other decaying organic matter, preventing them from leaching out of your soil and into the groundwater, beyond the reach of plant roots.
What Eggshells Can’t Do
Here are some common misconceptions about gardening with eggshells.
Prevent End Rot
End rot is often caused by a calcium deficiency in plants such as tomatoes, peppers, eggplants, and squashes. But given how slowly eggshells release their nutrients, adding them directly to your plants is unlikely to help. Most soil contains a suitable amount of calcium to prevent end rot. End rot is more likely caused by irregular watering, which reduces the plant’s uptake of calcium in the soil
Transform Your Soil’s pH
Calcium carbonate is a good additive if you’re trying to make your soil less acidic. An eggshell compost can raise the soil pH and also reduce levels of lead and other heavy metals in your soil—when the eggshells are used on an industrial scale.
You’ll need to add quite a bit of eggshells (such as from an egg-processing plant) to re-balance the pH of your soil. There are far easier (and cheaper) ways to reduce the acidity of your soil.
Control Slugs and Snails
A common assumption is that eggshells’ sharp edges act as a deterrent to garden pests such as slugs and snails. But a study by the Royal Horticultural Society found that crushed eggshells did not protect plants from damage inflicted by snails and slugs.
The thick slime that the pests produce acts as a protective shield, allowing them to slide right over sharp objects. If you have the stomach for it, you can find videos on YouTube of slugs and snails crawling over knife blades, razor blades, and other sharp edges.
Make Great Seed Starters
Sure, you can use eggshell halves to start seeds, but why bother? The seedlings will soon enough need to be transplanted into larger pots or into the garden, as their roots are not strong enough to penetrate the shell and breakthrough into the soil. If you want an inexpensive, repurposed seed-starting pot, try newspaper or recycled paper.
Better yet, the egg carton itself is a perfect seed starter, without the need to carefully break eggshells in half. When it’s time to transplant the seedlings, just cut the carton into individual cups and plant the seedlings directly into the garden. The cardboard will break down as the plants grow.
The Carbon Footprint of Eggs
If you really want to do right by the environment and still have a healthy garden, skip the eggs altogether. The carbon footprint of egg production is similar to other basic foods of animal production such as milk, primarily through the production of animal feed for egg-laying hens
Even if you’re raising your own backyard chickens, switching to a (primarily) plant-based diet is not only more sustainable for the environment as a whole, but the food waste from those plants will break down more quickly and add a wider variety of nutrients to your garden soil.