Composting Garden Myths

Garden Myth: Eggshells Are a Good Source of Calcium for Garden Soils

You’re probably heard that eggshells are a good source of calcium and are an excellent addition to compost or even to add directly to the soil in your garden. In fact, eggshells are even recommended as a way of preventing blossom end rot in tomatoes*, a disease related to the unavailability of calcium in the soil. 

Now, this sounds very logical: eggshells are 95% calcium carbonate, after all. Doesn’t it stand to reason that they’d up the amount of calcium available to plants if you added them to compost or soil?

But that’s without considering the fact that eggshells don’t break down readily. In fact, archeologists regularly dig up middens (refuse mounds) hundreds of years old and the eggshells are still there, largely intact. As a result, very little calcium from eggshells is actually returned to the soil in any way useful to plants, at least during the lifetime of the gardener. 

Actually, if you’re an astute composter, you’ll already have noticed this: when your compost is “ready” for garden use and all the other ingredients have decomposed, you’ll still see abundant bits of undecomposed eggshell. 

Unusual Conditions Can Help Decomposition

Under special circumstances, eggshells will break down much faster.

Under very acid conditions (a pH of less than 4), for example, they will decompose quite readily, especially if reduced into powder, but you’ll find very few compost piles or garden soils in that range. (Actually, you wouldn’t want your garden soil to be that acidic: very few plants can tolerate extremely acid soils.)

If you do need calcium, you can dissolve ground eggshells in vinegar. Photo: food-hacks.wonderhowto.com

A rather extreme way of actually getting some value out of the calcium carbonate eggshells are made of is to “melt” them in acid! Pour plain kitchen vinegar, highly acid (a pH of about 3), over ground-up eggshells and let sit overnight (or longer, depending on the quantity and the size of the shell fragments). The acid will cause the shell bits to dissolve, releasing carbon dioxide and leaving calcium and water you could then add to your garden’s soil.

They will also decompose faster in hot compost; that is compost where the temperature rises to 40 to 60 °C (100 to 140 °C). Even then, they’ll need to be ground up very finely to decompose within a reasonable amount of time. And of course, most home compost bins, as well as garden soils, don’t get anywhere near that level of heat.

If there is a municipal compost program where you live, don’t hesitate to include eggshells. It is, in fact, probably the best way to recycle them. Ill.: pandaenvironmental.com & www.stickpng.com, montage laidbackgardener.blog

If your municipality has a compost pickup program, definitely make sure your eggshells go there. They’ll be doing hot composting and will make sure your eggshells are properly composted.

Much Ado About Nothing?

It’s important to realize that, in fact, most garden soils do not lack calcium, so you almost never need to add more. It’s one of the most common soil elements, abundant nearly everywhere. So all this concern about the calcium in eggshells is largely theoretical. Your garden will probably get along fine if you never add calcium, whether it be from eggshells or another source.

That said, you can still add eggshells to soil or compost. They’re harmless, after all, and it’s a good way of recycling them at home, especially if the alternative is sending them off to a municipal dump where they’ll just take up space.

Grind eggshells even more than this before you add them to the soil. Photo: http://www.comment-economiser.fr

If you can, reduce eggshells to powder or at least to the smallest pieces possible. Try using a hammer, a pestle, a blender or, best of all, a coffee grinder. Small pieces will have the greatest chance of breaking down to something useful within your great-grandchildren’s lifetime. 

As to the common garden myth that eggshells can be used to control slugs, read more about that here.

Chicken Fodder

One truly useful thing you can do with eggshells is to feed them to chickens. They need a lot of calcium to produce eggs and their digestive system will break eggshells down. Yes, that does sound a bit cannibalistic, but chickens actively seek out anything containing calcium, even their own egg shells.


So, eggshells are not all that useful to the soil you garden in, but at least you can recycle them there and reduce your environmental footprint!

Larry Hodgson is one of Canada’s best-known garden communicators. After studies at the University of Toronto and Laval University where he obtained his B.A. in modern languages in 1978, he succeeded in combining his language skills with his passion for gardening in a novel career as a garden writer and lecturer. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He is a regular contributor to and horticultural consultant for Fleurs, Plantes, Jardins garden magazine and has written for many other garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening, Rebecca’s Garden and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 50 other titles in English and French. He can be seen in Quebec on French-language television and was notably a regular collaborator for 7 years on the TV shows Fleurs et Jardins and Salut Bonjour Weekend. He is the President of the Garden Writers Association Foundation and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. An avid proponent of garden tourism, he has lead garden tours throughout Canada and to the gardens of over 30 countries over the last 30 years. He presently resides in Quebec City, Quebec.

3 comments on “Garden Myth: Eggshells Are a Good Source of Calcium for Garden Soils

  1. There are no myths safe with you around. 🙂

  2. I like to use them in the bottom of flower pots. I use them for drainage material, they work great and are lighter than pebbles. I also dump ground up shells around my wisteria, if nothing else it’s great mulch! Thanks for the article.
    https://www.powerequipmentwarehouse.com/diy-tips

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