Eggshells – Decomposition After One Year

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Robert Pavlis

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.

Compost Science for Gardeners by Robert Pavlis

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?

Eggshells – Decomposition After Three Years


  1. Thesis for Kathryn Elizabeth Lamzik;


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Robert Pavlis

I have been gardening my whole life and have a science background. 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!

26 thoughts on “Eggshells – Decomposition After One Year”

  1. 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.

  2. 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.

  3. 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.

  4. 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.

    • The inner skin will decompose and be of some benefit. The outer shell will not add much unless it is crushed very fine and you have acidic soil.

      • Possibly an aid to drainage in heavy clay soils?
        Would take a LOT of eggshell to make much difference though.
        Interestingly, I find chicken bones DO break down when hot composting (over 60°C for >21 days)) as do rat carcases, fish bones/heads/guts & what’s left of rabbits after butchering.
        Not tried deer bones yet, though I should – probably broken up first.


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