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Eggshells – Decomposition After One Year

A year ago I started a study to see if eggshells decompose in soil; Eggshells – Decomposition Study. It is now one year later and time to have a look at the buried eggshells.

eggshell decomposition study - year 1

Eggshell decomposition study – year 1


Decomposition After One Year

I dug up one of the baskets holding the eggshell and slowly removed the soil from around the egg. I then put the eggshell in water to rinse out the soil inside the egg, as shown in the picture above. In the process two pieces broke off the egg.

I scraped the inside of the egg to see if the inner membrane was still there, but it was gone. It had completely decomposed. This is not surprising since this membrane contains a lot of protein which easily decomposes.

The remaining shell was intact and showed no visual signs of decomposition.

Archaeological Eggshells

One of the commenters in the post Eggshells – Decomposition Study, suggested I look at some research on eggshells found at archeological digs.

I found a report entitled “An Analysis of the Avian Fauna and Eggshell Assemblage From a 19th Century Enslaved African American Subfloor Pit, Poplar Forest, Virginia“. This is quite an interesting read from a historical perspective. The study looked at a property in Virginia that was at one time owned by Thomas Jefferson. It was a tobacco plantation that contained a small community of slaves from 1840 to 1860. Excavation of the site found thousands of eggshell fragments from both chickens and ducks, which had been raised by the community.

The key point for us is the fact that over the last 165 years, the eggshells in the soil did not decompose very much. In fact the researchers could still distinguish chicken eggshells from duck eggshells.

Do you still think eggshells decompose in the garden in a year or two?

As reported in my last post, they do decompose if they are crushed to a very fine powder in something like a mortar and pestle.


To learn more about eggshells, have a look at these posts:

Eggshells – Do They Decompose in the Garden

Eggshells – How Not to Use Them in the Garden

Eggshells Control Slugs – Do They Really Work?


  1. Thesis for Kathryn Elizabeth Lamzik;


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Robert Pavlis
Editor of
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

24 Responses to 'Eggshells – Decomposition After One Year'

  1. Okay, so… Forgive me if this sounds dumb, but… There are tons of birds flying around any given neighborhood. All those birds nest and lay eggs every year. So if eggshells don’t decompose for hundreds of years, why aren’t we seeing accumulations of wild bird egg shells?

    • They easily break into smaller pieces. I have some chicken egg shells in the garden that have been sitting in a protected area for at least 5 years – still there. Just because you can’t see something does not mean it is decomposed.

    • Machelle Okey says:

      I am 61, and have lived by a woods and/or wooded property all of my
      life. I have, of course, seen countless birds shells that have been pushed out of nests after the peeps emerged from them. These fragile
      bird shells get stepped on and crushed, (sometimes into a fine powder-like substance), and walked into the ground. Okay, so…Forgive me if this sounds ‘logical’ to you, but try letting your
      common sense ‘kick-in’, and just think about how obtuse your question really is.

  2. Manon says:

    My understanding – which could be dead wrong – is that the inner skin is strongly antibiotic, so if you you don’t cook (low oven or microwave) or dry them first, there will be no microbial activity to aid the breakdown. Unless the breakdown is actually done by the acid itself as opposed to microbes that thrive in acidic soil?

    Re: the ducks they *do* eat the shells – it’s not Thomas Brophy’s imagination. That’s an excellent use for them.

    • Any antibiotics will be quickly decomposed by microbes. In fact I would guess the inner skin – having a high nitrogen level would aid in microbe decomposition it is were taking place.

  3. Ronney Phillips says:

    I was pleased to see this post about eggshell decomp, mostly because I didn’t think they’d decompose much, either. I wasn’t sure about the length of time it would take, though.

    It didn’t stop me from collecting eggshells and adding them to my compost, though, although I didn’t really see them decompose much.

    Recently, I’d started taking the extra step to pulverize them with a coffee grinder as I’d heard other people doing. Somewhere between grinding and collecting the shell dust, I asked myself what the heck I was doing. “Chalk is made of calcium carbonate. So are antacids. Why am I collecting all this gross stuff and grinding it up? Why not pulverize chalk or antacid tabs?”

    And yeah, I’d heard the nonsense about grinding up antacid tabs to add as a soil amendment for a quick fix to blossom end rot, which kind of connected everything for me.

    I thought, “Oh…well, I’ll just add my pulverized shells to the garden area a few weeks prior to planting. That can’t hurt.” And it probably won’t hurt.

    But it probably won’t help, either.

    In the future, I figure I’ll probably do something more productive than grinding eggshells like….well, anything else.

  4. Ron says:

    In past years I noticed that my composted egg shells never seemed to disappear. So, now I dry them then grind them to as fine a powder as possible in a coffee grinder. My thinking is that as a powder and heated in my compost, they should degrade.

    • One of the references in one of my egg shell posts did show that as the particle size got smaller the shell did decompose faster. But this was in acidic soil. In alkaline soil they may still not decompose. I doubt the heat has a big effect on the egg shells.

  5. Brian Tremback says:

    When doing soils investigations here in the Champlain Valley of Vermont, I occasionally come across small sea shells left over from the Champlain Sea. The beach deposits in which the shells occur are slightly alkaline. However, they’ve also been there for about 10,000 years. Considering that eggshells and sea shells are similar in chemical composition, unless you’ve got acidic soil, I wouldn’t count on eggshells doing much for at least a few millennia.

    I do throw them in my compost with the rest of my kitchen scraps. Local birds snap them up in the spring, probably using them as a supplement for the calcium they’ve lost making their own eggshells. Their acidic guts should have little problem dissolving them.

  6. rogerbrook says:

    Your point is very well made Robert
    … but some gardeners are very patient

  7. I do slow composting where it takes 3 years for it to be ready for the garden and I always still see eggshells floating around in it. So yeah, I agree that they don’t break down quickly in soil.

    Heard you on the Root Simple podcast and loved hearing you break some myths there. I live in Alberta now but was able to spend 7 years in Guelph going to university and then working for a while. I really enjoyed my time in Guelph.

  8. Andy Konecny says:

    Certainly this is conclusive that undisturbed egg shells don’t readily break down, but it doesn’t prove any lack of value of putting egg shells in compost.
    Crushed egg shells will still add nutritional value to the soil, and birds will pick up bits for their own calcium needs. Crushing to a fine powder feels like overkill to me, I just make sure they are broken up to smaller than 1cm size before adding them to compost. My Mom keeps a jar by the stove that she just mashes them in with a smaller jar until it is full, then we bring them over to a local hobby chicken farmer to help reduce the amount of crushed limestone (crushed oyster shells is other option) he needs to feed the chickens.
    Sure beats adding them to our overflowing landfills.

    One question set though, is there such a thing as too much egg shell for a given garden bed? and what difference does crushed vs powdered make?

    Andy in Toronto

    • If your soil is alkaline – crushed and powdered do the same thing – nothing.

      If your soil is acidic powdered decomposes faster than crushed. There is a good study on this in this post; Eggshells – Do They Decompose in the Garden

      Guelph has alkaline soil, and I suspect most of Toronto is also alkaline.

      Feeding chicken or other birds with it is a better use of eggshells.

  9. Bonny says:

    I go grind up my egg shells in my nutri bullet and mix them in with the worm food.

  10. Duncan Crow says:

    The science is that calcium phosphate needs to be in an acidic solution in order to decompose and disassociate its minerals. That’s how digestion and also composting works….so all you need is the acidic solution/surroundings that normal composting would produce.

    • You are correct that it is the acid condition that decomposes eggshells.

      Your statement about compost being acidic is only partially correct. It generally becomes acidic for a while and then it becomes alkaline. In Eggshells – Do They Decompose in the Garden? I discuss this and show the resulting eggshells from a two year old compost pile.

      Animal digestion works with acids, but composting works because the microbes break down the organic matter. They do not need an acidic environment to do that.

  11. Ann says:

    I found this very interesting as I have been gathering my eggshells.

  12. This is a great site, and one of the best I know on gardening so thank you very much. I appreciate it.

  13. Thomas Brophy says:

    Before I had the small flock of ducks I have now had for 1 1/2 yr. , I used to crush egg shells and distribute among garden raised beds–just crushed by hand-rolling pin, not to powder. I find no traces whatever of these shells currently in the garden. Now I feed them back to the ducks, so they get to the garden more indirectly.

    • Just because you don’t see them does not mean they are not there.

      Consider this. Your soil contains sand particles, but you don’t see them. No one claims they have decomposed. just like sand, once the eggshells are small pieces you don’t see them.

  14. janet says:

    Wow! Digging up egg shells seems to be fun!

  15. Danielle says:

    I’m guessing there’s no benefit to putting all eggshells in the composer then?

  16. “disposable” diapers decompose even more slowly