Eggshells – Do They Decompose in the Garden – 5 Year Study

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Robert Pavlis

The advice to add egg shells to the garden or compost pile is very common. In my last post I looked at some evidence that suggested eggshells do not break down in a compost pile or in soil – at least not very quickly. The one exception where eggshells do break down is very finely ground eggshells added to acidic soil .

How quickly do eggshells break down in soil? Is it 6 months or 5 years? Maybe it’s 100 years? No one seems to know. In this post I will describe a 6 year study that has been started to find out if eggshells decompose in that period of time.

Eggshells - Do They Decompose In The Garden 1
Eggshells – Do They Decompose In The Garden – supplies, by Robert Pavlis

Experimental Design

The goal is to see if there is any visual decomposition of eggshells in the soil over a 6 year period. Do they get soft and brittle over time? Do they slowly disappear?

I picked up 5 nice containers from the dollar store that had lots of holes in the side walls. I added more holes in the bottom. The purpose of the plastic container is to protect the eggshell while I bury it and then later unearth it. I want to a make sure any degradation is from natural causes, not my clumsiness.

Compost Science for Gardeners by Robert Pavlis

The holes will allow water, chemicals, and microbes to move in and around the eggshell as if the container was not there. Larger rodents should be kept out.

A crepe breakfast provided lots of eggs. I tried to keep half eggs intact as much as possible. They were not washed, and each one had a bit of hardened egg white in the bottom. This extra organic material should improve any microbe activity taking place on the eggshell. The inner skin was also kept intact.

Each of six containers got one half eggshell with soil at the bottom of the container, and inside the eggshell. The eggshell is fully covered by soil. The containers were then dug into the soil near a blue spruce, in an area that should be safe from my wonder shovel. My soil pH is 7.3.

The plan is to unearth one container in each of the next 5 years.

Hypothesis – Eggshells Do Not Decompose in the Garden

Lots of people, in fact most people, say that the eggshells degrade in composts bins and in soil. My hypothesis is that they don’t degrade, except very slowly. Instead, what happens is that the act of handling the compost, spreading it, digging it into soil etc, breaks the eggshell into small pieces. Once the pieces are small enough – people do not see them, and they think, that they have decomposed.

I expect that even after 5 years, the eggshells will be complete and showing very little degradation.

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Eggshells used in the experiment, by Robert Pavlis


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Eggshells – containers partially buried at equal heights and spacing, Robert Pavlis


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Eggshells – containers fully buried at base of tree, by Robert Pavlis

Eggshell Decomposition Study – Year One

After a year under ground, one shell was dug up and examined. Details can be found in Eggshells – Decomposition After One Year.

The inner skin was completely decomposed but the outer shell was intact showing no evidence of decomposition.

eggshell decomposition study - year 1
Eggshell decomposition study – year 1

Eggshell Decomposition Study – Year Three

After three years under ground, one shell was dug up and examined. Details can be found in Eggshells – Decomposition After Three Year.

The outer shell was intact showing no evidence of decomposition.

Eggshell decomposition study - year 3
Eggshell decomposition study – year 3

Eggshell Decomposition Study – Year Four

Watch all the details in my video.

YouTube video

Eggshell Decomposition Study – Year Five

The 5 years is now up and I removed the last container of eggshells. They don’t look any different than after 1 year.

Eggshells in my soil will last many years before they decompose. They are adding no nutrient benefit to the soil.

Eggshells Found in Archeological Digs

After I started this study I found several references to archeological digs that examined the food people were eating at the time, by looking at eggshell fragments. some of these were over 100 years old. Eggshells last a long time in the soil.


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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!

64 thoughts on “Eggshells – Do They Decompose in the Garden – 5 Year Study”

  1. plant roots can cause ph changes in their vicinity through exudates. did you test plant root attack with your gardening myth experiment? how many kinds? what other common garden forces may your experiment not have included? could beetles & larvae scratch and bite eggshells into submission (e.g. eggshells alongside beetle & larvae attracting elements like wood)? birds? moles? more diverse microbe or fungal activity via manures and other inoculants? acidifying inputs such as sulfur? pine needle or bark mulch? many things accessible and common to gardening situations that could change the game remain untested and unmentioned here. this experiment appears to have less value than many readers are led to believe.
    an effort is a good thing, but putting an effort on a pedestal before its time is premature.

    • The post is quite clear about the fact that this is a test in my soil under the specified conditions. It was not an experiment to prove egg shells will never decompose. It was done mostly to give people a visual about what happens in soil.

      But other scientific studies have shown egg shells are very stable in anything by acidic conditions.

      • Nice experiment but I think this commenter’s concern that the article’s conclusions are more general than the data can support may still be valid as long as the article’s title casts a question more broad than your experiment’s research question. You do, indeed, make it clear the article that the conclusion only applies to “my” garden, but the title that a search engine used to lead me to this article asks a more general question about “the” garden. Thus I expected it to be more of a metastudy or large-scale trial with generalizable conclusion than this small but wonderful backyard study. If you change that word to “my,” it will probably lower the website’s SEO a little but also diminish the chance that potential visitors will be somewhat misled. In my case, I consciously searched the web for results like yours that would confirm my own foregone bias—to me, it already seemed obvious that eggshells in the soul don’t do much more than make my flowerpots rather ugly.

        • This experiment was mostly done to demonstrate what scientific studies have already concluded. The science on this is fairly clear, but people don’t always believe science – they might believe a visual demonstration.

  2. I’m using eggshells as containers to start peas, the idea being I can break them up when planting. And yes, I do have too much time on my hands.

  3. In India, buttermilk is often used as a fertilizer for certain plants. I wonder if the calcium in buttermilk is more easily available to the plants?

  4. Interesting to read, and useful to know. I have a couple of ostrich eggshells I’m going to make into pots, and I was trying to find out if they’ll decompose on any timescale I have to worry about. From this article, I’m gonna say they will not. These shells are probably a good quarter-inch thick, so they’re not going anywhere. Thanks for the info.

    Eggshells are primarily made of calcium carbonate, which is also what seashells are mostly made of. There’s a place near me where one can collect Eocene era (54 million years ago to 33 million years ago) shells. They closely resemble modern seashells in shape, but not in color. Despite being technically considered fossils, they still contain a lot of original material, rather than having been fully replaced by foreign minerals like older fossils are.
    Which is to say that seashells, at least, will fossilize before they decay. Seashells don’t rot- they either get ground into sand by the waves, or they hang out underground ’till they fossilize. I’d wager a lot of the same applies to eggshells.

      • Just like plastics turning into microplastics, only without the environmental issues.
        After 7 years of at least 20 shells a week going into my 40’x11′ veg patch, there’s no more shell visible than at the end of year 2.
        At 4 shells to the ounce (yes, I’ve weighed them…), that’s around 115lbs of eggshells added, plus whatever my neighbours put in their compost bucket (they leave it by my back gate once a week).

  5. We have had a problem with small slugs in our garden. I started putting egg shells into the garden and the slugs disappeared. The egg shells are very small but seem to disappear. If your study is right, perhaps the lack of decomposition is uncomfortable for slugs to move on because of the small sharp shells. Haven’t studied this, so only observation.

  6. I personally live in the pacific northwest and I made my own soil myself with a lot of materials I gathered, foraged or ate what was in it and I can say that I’ve been adding egg shells to my garden for 3 years now and every year at the end of Sumner before I begin adding more for the season, you wouldn’t be able to find a single one. They all been decomposed. Idk where you guys live but my soil is alive as can be out here.

      • Part of the decaying process includes fragmentation of most elemental compounds. I compost eggshells with coffee grounds which really helps to speed up the decomposition due to high pH.

        • Most people do not mix egg shells with coffee grounds to try and compost them, although they do add them to a compost pile.

          Neither egg shells nor coffee grounds have a “high pH”! Even if they did – that would slow down decomposition of eggshells. Coffee grounds are not very acidic either:

          Do you have some evidence that egg shells decompose quickly in a compost pile?

  7. I just started using a compost bin inherited with the property (we arrived 3.5 years ago). Though given the high plastic content it could more accurately be described as a mini landfill. Having sorted and disposed of the biggest plastics the next two components were almost complete eggshells (lots) and millions of teabag skeletons. I think given the numbers of teabags the bin could have been being filled for up to a decade. I’m just starting to try bokashi so I will be interested to see how egg shells fare in that. I think I certainly agree that eggshells degrade poorly in composting but I think I will still add them as I have no chickens and it feels like they’ll be better in soil than landfill and plus they might help the soil composition/less claggy when digging.

    • Elmy, see if your tea bags are made from paper and not another material. Many bags are surprisingly made from plastic (polypropylene) and will not break down. Have had this issue in my vermicomposter, and it was a shock to find that out! Also scary to know that our bodies are unknowingly ingesting microplastics.

  8. Excellent study Robert. It backs up what any composter or gardener should know already. They simply don’t decompose.
    Eggs have developed over millions of years to serve a purpose. If they broke down quickly they’d be useless.
    I’ve done my own “experiments” here too by burying them deep into my veggie patches over the years. Every time I dig soil over they come back up. They’re doing no harm but not helping either.
    I’ve tried all the various suggestions through the years by heating/boiling/mashing them. It’s all the same really.
    Nowadays I just feed empty shells back to my ducks who gobble them down. The hens are pickier.

  9. Problem with this experiment is that is it implies that composting will happen if you bury organic matter in the soil in the same way it happens in a compost heap. That’s entirely incorrect.
    A properly composed (browns vs greens) hot compost heap is teaming with the bacteria, moulds and critters that break things down. Once you have “stale soil” 99% of that is gone, and the process will slow down immensely. This is why organic matter doesn’t rot away quickly in landfill rubbish sites.
    Also, you didn’t weigh the eggs, how do you know that 25% or even 50% of the shell didn’t go already?

    • Composting does happen exactly the same way in soil as a compost pile – the only difference is the time horizon. Soil is also teaming with microbes.

      You are correct, I did not weight the shells. I never claimed NO shell decomposed, only that any decomposition was insignificant enough that you could not see it.

      • I was thinking about weight, too—your replies to comments on eggshells’ diminishing visibility in soil over time emphasize an unclear definition of decomposition. You propose that, if a shell is visibly intact, it hasn’t decomposed (much.) You also posit that, if it has been ground into fine, barely-visible powder, then it has not decomposed either. But in the experiment’s design, weren’t you mainly checking for visibility? Supposing the shells in your project had, indeed, disappeared, that would have probably led to a different conclusion. Therefore, if decomposition’s definition involves only chemical changes exclusive of physical changes, then it might be good to know how much calcium and other minerals have leeched out of the five-year-old shells while they maintain their unchanged physical appearance.

        I do agree with you, by the way, that commenters comparing shells in their garden compost to shells in your experiment seem to be overlooking the confounding variable of mechanical forces grinding their shells into powder, whereas your experiment uses undisturbed soil. The premise that their observations derive from the same experiment as yours is false, so the conclusion that their observations disprove your findings does not logically follow.

        • I was using the visible view of the shells as a rough test for decomposition. When things decompose they change visibly. I was also checking the structure. If the shells were decomposing in small areas you would expect to see at least small holes after a time.

  10. I heard you can either microwave the shells to break them down not sure if it’s true. Doesn’t sound like it would be. What about soaking them in vinegar before putting them in the composting and then crushing them up? New to this composting but excited to start! Thanks

    • heating does not do much. Vinegar will decompose them since it is acidic – buy why do that? If you need calcium in the soil there are better sources.

        • Eggshells have benefits if they decompose. In most soil they don’t, so in those situations they have almost no benefits.

          • I’m minded to think even if they only very slowly break down they do have a minor benefit in that they help open up the structure of clays & heavy loams in the same way adding coarse sand & grit does.

          • Well, I’ve noticed that isopods eat grounded eggshells readily. The calcium in them goes to their exoskeleton which they molt many times, especially when they are juvenile, so the calcium gets usable + chitin as an extra. I mean the decomposition by soil bacteria is not everything that happens in a compost pile. The pile is full of all kind of invertebrates. I am sure the isopods are not the only one which would feast on ground eggshells.

      • I think decomposition’s probably the wrong term here, as we’re dealing with something largely inorganic & thus unaffected by bacteria & fungi.
        Once shell particles are small enough to be ingested by worms, they’ll certainly be broken into even smaller fragments in the worms’ gut but at what point they become available as a base mineral (mostly calcium carbonate).
        They’re microporous, therefore helpful in the development to mycorrhizal activity within the soil.

  11. Is there a possibility that the processes happening inside a well maintained hot compost bin (aside from movement, as you mentioned) would change the results any? I’m very new at this, but really appreciate your experiment. Thanks!

    • Finished compost is neutral. so eggs don’t decompose. Early in the process a pile can become slightly acidic, but it only lasts a short while.

  12. Here in the UK, I dug out an old compost bay this spring – untouched for 6 years.
    Whilst the compost itself was superb, all the roughly crushed eggshell pieces looked exactly the same as those which were in 12 month old compost.
    Maybe a tiny amount of the calcium will leach out but with shells weighing 5g & my consumption dozen or more a week, I don’t think the 20kg/44lbs of shell added over 6 years will have even altered the soil structure over my 12’x50′ veg patch.


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