Eggshells – Decomposition After Three Years

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Robert Pavlis

Three years ago I started a study to see if eggshells decompose in my soil. Details of the study can be found here: Eggshells – Decomposition Study. It is now year three of a six year study and time to have a look at the buried eggshell.

Eggshells - Decomposition After Three Years, by Robert Pavlis
Eggshells – Decomposition After Three Years, by Robert Pavlis

Eggshell Decomposition After Year Three

I dug up one of the baskets holding the eggshell and removed some of the soil. The whole thing was soaked overnight to soften the clay soil. I then gently removed the soil and extracted the eggshell, which is shown in the above picture.

It does have a few cracks, but I may have made those putting the eggshell into the basket three years ago, or I could have made them today. The inner protein skin was decomposed even after 1 year and without this the shell becomes quite fragile.

You can see that the shell is essentially intact. The surface is still smooth. If it were decomposing I would expect to see pot marks on it, and even small holes where the soil has eroded the shell. Nothing like that is visible.

Microbe Science for Gardeners Book, by Robert Pavlis

After three years in my soil, which has a pH of 7.3 and 40% clay, there is no visible sign of decomposition.

One can only conclude that adding them to my garden is adding nothing significant in the way of nutrients.

Eggshells After Four Years

I decided to report year 4 as a video.

YouTube video

If the above does not play, try this link: https://youtu.be/V_AYLqaHNxQ

What About Year Two?

Eggshells - Decomposition After Two Years, by Robert Pavlis
Eggshells – Decomposition After Two Years, by Robert Pavlis

For those who follow this blog closely, you might realize I missed year two. I actually forgot to post the pictures. I did dig up one of the baskets last year and extracted the eggshell, but rushed the job too much. As I was trying to get the soil out, I broke the shell. The picture to the right shows the results.

This basket had quite a few roots in it, and I suspect they might have added extra cracks making my job harder.

In year three I was extra careful not to repeat the mistake.

Even though it broke into pieces, you can clearly see the the shell has not decomposed.

 

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 One Year

 

 

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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!

34 thoughts on “Eggshells – Decomposition After Three Years”

  1. Robert, you said “The eggshell in this experiment was readily available to microbes – they did not do much, if anything to decompose the shell”. Can you validate this claim in some way, perhaps measurements of shell thickness, or some comparative photographs taken under a powerful microscope, or is your claim simply anecdotal evidence?

    As proven through the papers mentioned in my previous post, microbes DO decompose eggshells. All five of the different bacteria strains mentioned in those papers began to decompose the shells.

    In your reply to my comment above you state “If the surface is stable, there will still be no decomposition”. This was proven incorrect by the peer reviewed papers I mentioned. In those experiments there was microbial decomposition over the entire surface of the egg shells. Again, if you have some way to validate your claims I would love to read it.

    In your reply to my comments above you comment “If they don’t show it degrades quickly, they don’t help the discussion”. This is utter nonsense. It helps the discussion greatly by casting doubt on your unverifiable assumptions. The studies prove that microbial decomposition of egg shells occurs in soil. People keep claiming that eggshells decompose in their compost and you keep claiming that is simply mechanical damage due to your assumptions. Those studies above highlight the inaccuracies in the assumptions you made.

    You claim “The problem is that the size of particle needs to be extremely small – finer than what most gardeners will have”. What size do they need to be and where is your proof? What size will most gardeners have and where is your proof of this? You also claim “This study shows that even though you don’t see them – they are not decomposing”. Where is your proof of this, or is it anecdotal evidence based on your false assumptions? You should spend some time pondering this and decide what you think instead of contradicting yourself in replies like this.

    I sure hope that this experiment is not included in your book! If I were you I would remove this page, spend an afternoon reading papers that have been published, then re-write this page and base it on proof rather than the anecdotal evidence it seems to be currently based on.

    Reply
    • It will be in my new book – Garden Myths, Book 2.

      Brix measures mostly sugar content. It certainly does not indicate nutrient dense food. Not sure what it shows about plant health, but I doubt it shows you anything of value.

      Reply
  2. Take egg shells grind them up 1oz egg 1oz white vinegar let sit,for 30minutes mix,in 1gal water.then the plant can take in the calcium.

    Reply
  3. I put egg shells into my aquaphonics ebb and flow grow tray and next planting season I found some slimmy things which I knew were the inside of the shells. The shells themselves are gone.

    Reply
  4. Hi, I tried to follow up post on a slug article, but it’s now closed. I recently learned that ants protect the aphids, which were one of the many critters feasting on my garden veggies. I sprinkled cheap cinnamon over the garden to rid the ants and noticed within days an explosion of undamaged growth, and no sign of ants or aphids. I sprinkled again after a heavy rain and covered a snail on the stem. It stopped moving and within about 12 seconds it sort of keeled over and fell off the stem. This may not be news to an experienced gardener like you but I was astonished and feel like I have an effective tool in my arsenal. I’m wondering if cinnamon can be made into a spray that would be just as effective but cheaper?

    Reply
  5. Can you realistically conclude that no new nutrients are being added unless you have a control and then test the nutrient composition around each of the baskets with eggs in them? Or would you only be hypothesizing?

    Either way I concur that egg shells take a long time to decompose if left intact. I have put egg shells in a coffee grinder to put them into dust.

    Thanks for your study.

    Karl

    Reply
    • I can’t say they don’t add any nutrients to the soil. But since there is not much in the egg to start with – they weigh next to nothing, we can be fairly sure that at most very little is being added. If a significant amount is added they would decompose.

      Reply
  6. Judging by my compost pile, egg shells take a very long time to decompose if left semi-intact. I now save my egg shells, allow them to air dry, and then toss them into a bag to be crushed finely once the bag is full. I then add the crushed shell powder directly to the soil around my plants. Works wonderfully.

    Reply
  7. I compost them without crushing and find that after 3 years (when I harvest the compost) they have broken down enough where I can only spot the odd shell here and there – lets say 10% of added shells. I add 1000 eggs a year for the last 4 years I only air-ate my compost once per year hence it takes 3 years to decompose everything.

    Reply
    • This study shows that even though you don’t see them – they are not decomposing. But if you think they are decomposing, please duplicate my experiment in your compost pile and let me know the results. You have to be sure they don’t get damaged by physical means.

      Reply
      • No Robert, that is incorrect. This study does not show that “even though you don’t see them – they are not decomposing”. I am a little disappointed that you would extrapolate the results this way.

        This test shows that large pieces of eggshells do not decompose quickly. You can not (or at least should not) take this to mean that when ground into tiny pieces that you will get the same results.

        Earthworms use sand and other tiny rocks as gastroliths. Do they also use ground eggshell as a gastrolith? If used as a gastrolith by worms do eggshells decompose fast? I am not aware of any study that investigates this but I do know your study does not determine the answer.

        Plant root exudates are often acidic – citric acid, oxalic acid, and malic acid come to mind but there are others. There are many classroom experiments showing eggshell calcium carbonate reacting with acids. When ground into tiny pieces, do eggshells decompose faster due to plant root exudates? I am not aware of any study investigating this reaction but I know that your study does not determine the answer.

        When ground into tiny pieces do eggshells decompose fast? When ground into tiny pieces do eggshells release significant amounts of calcium or phosphorus into the soil? I don’t know the answer, but I do know that your study does not answer these questions.

        Reply
        • I never said this study shows “even though you don’t see them – they are not decomposing”. The only conclusion I made was that in my soil there was no visual evidence of decomposition.

          If large pieces don’t decompose, what makes you think smaller pieces will? The study I referenced in the original post showed that even in quite acidic soil, the eggshell had to be ground extremely fine before it added calcium to the soil. Crushed eggshells are not fine enough and in more alkaline soil even fine powder does not decompose any time soon.

          It is possible that earthworms speed up the process. How fast do they speed it up? The gut of the earthworm has a pH of 6.9, which would not speed up chemical decomposition of eggshells. The digestion process might cause some physical damage to the pieces. How many times does a piece need to pass through an earthworm before it is decomposed? What is the turn over rate of soil through earthworms? Without this kind of information it is only a guess that eggshells decompose at any reasonable rate going through an earthworm. But id you have some data I’d love to see it.

          One of the reasons for selecting the plastic containers was so that plant roots could reach the eggshell, and they were buried next to a spruce tree. There were certainly roots inside the container. Their exudates did not speed up decomposition.

          Reply
          • Well technically you also concluded there were no nutrients being added to the soil as well.

            “One can only conclude that adding them to my garden is adding nothing in the way of nutrients.”

            Karl

          • You got me on that. Changed the post to read “One can only conclude that adding them to my garden is adding nothing significant in the way of nutrients.”

          • You did say exactly “This study shows that even though you don’t see them – they are not decomposing”, I cut and paste it from your reply to ‘Pauliewaulie’ above. If you don’t believe me, go and have a look.

            Re: would being used as a gastrolith in an earthworm speed up decomposition. I mentioned in my first response “I am not aware of any study that investigates this but I do know your study does not determine the answer”. I stand by that statement. If you have any data that shows if this does or does not speed up decomposition I would love to see it.

            You mention that root exudates did not speed up decomposition, but then in your previous post you mentioned the eggshells that were covered in roots crumbled and were difficult to get out in large pieces. Was this due to rough handling or due to decomposition near the roots? Do you have proof that root exudates did not do this, or are you making another assumption?

            I have not said that eggshells decompose fast (because I don’t know if they do or not). I have simply started that your response to ‘Pauliewaulie’ claiming that “This study shows that even though you don’t see them – they are not decomposing” is a false assumption and I am disappointed that you would have extrapolated the results like this.

          • The eggshells from this year and last year were in the same spot and right beside each other. Both had roots around them.

          • You are asking why I believe that vastly increasing the surface area of a substance may cause it to decompose much faster? Seriously? You and I both know that is the wrong question.

            Back to the topic at hand, I had a quick browse through google scholar while at work and came across the following:

            DICKSON, K.M., and HAYWARD, J.L., 1998, Experimental eggshell taphonomy in a controlled fluvial environment: Journal of Vertebrate Paleontology, v. 18, Supplement to No. 3, p. 38A

            SMITH, D.L., 1998, Bacterial degradation of avian eggshell: taphonomic and ecological implications: Unpublished M.S. Thesis, Andrews University, Berrien Springs, 68 pp.

            Bacterial decomposition of avian eggshell: a taphonomic experiment
            DL Smith, JL Hayward 2010

            These independent peer reviewed papers show that microbial degradation of eggshells occurs in soil. They do not show how quickly it occurs in chicken eggshells, or if noticeable amounts of minerals are released to the soil, as that was not the purpose of these studies.

            Only further testing (and not the extrapolatory wishful thinking/anecdotal evidence from your experiment) can determine if greatly increasing the surface area of eggshells through finely crushing increases the decomposition rate to a point where appreciable amounts of any nutrients are made available.

          • Increasing the surface area of something does not automatically mean the surface will react more. If the surface is stable, there will still be no decomposition.

            “They do not show how quickly it occurs in chicken eggshells” – well that really is the question. If they don’t show it degrades quickly, they don’t help the discussion.

            We already know that increasing the surface area of egg shells will speed up the decomposition. I posted a link to this research in an earlier post. The problem is that the size of particle needs to be extremely small – finer than what most gardeners will have, and the soil has to be very acidic – more acidic than most gardens.

      • This study doesn’t show Pauliewaulie’s eggshells aren’t decomposing, it shows that your eggshells in your very specific conditions aren’t decomposing.

        I also add 3-6 eggshells/day (also about 1000/year) to one of my compost piles with only stray shells visible in the finished product.

        How would you suggest duplicating your experiment in an actively managed compost pile?

        Reply
        • I never said that the study shows “Pauliewaulie’s eggshells aren’t decomposing”. I agree with “it shows that your eggshells in your very specific conditions aren’t decomposing”. However, the study I referenced in the original post indicates that unless the soil is very acidic, and the eggshells are powdered to a very fine consistency – they don’t add calcium to the soil. It showed crushed eggshells don’t decompose even in acidic soil.

          I would put the eggshells in a container with holes. Surround the shells with something like leaves, or other soft vegetation that will prevent physical damage to the shells. Make the compost pile. Dig down about a foot, and place the container in the center of the pile. Then remove it before any turning of the compost pile takes place. After turning replace the container.

          Reply
    • I think that you’re right, Pauliwauli. I hope Robert remembers that calcium carbonate can’t naturally decompose beyond it’s nuclear structure, and, that whether or not it’s visible to the human eye, it still remains gigantic and readily accessible to microorganisms’ appetite, not to mention mycorrhizal fungi.

      Reply
      • An eggshells in not a pile of calcium carbonate. When the eggshell decomposes it will release calcium and carbonate ions.

        The eggshell in this experiment was readily available to microbes – they did not do much, if anything to decompose the shell.

        Reply
      • They don’t necessarily “decompose” via micro organisms. they dissolve at the edges. If they are crushed or powdered, there are a LOT of edges. I get shells from a local restaurant. I cook them in a big pot to kill anything that might be growing. When the water cools I put it on my tomatoes and peppers. When the shells are dry, I grind them in the coffee mil I have just for stuff like that. I mix it with compost. Nothing has died as yet that I can tell. I add some to worm food as someone already pointed out.

        Reply

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