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.
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.
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.
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 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 Four
Watch all the details in my 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.
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.
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.
So is your point to disprove the benefits of eggshells or to simply suggest people throw them in the garbage?
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.
But as you said, with vinegar they will decompose.
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.
Do you have a link to some science to support that claim?
You forgot something VERY IMPORTANT. Worms use eggshells in their gizzards to help with their digestion, much like hens do.
How effective are they at decomposing eggshells in the soil?
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.
I agree with the word decomposition – what is a better word? I am not sure.
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.
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.
A good experiment even if not everyone “gets it”. Came across this as I was looking into the oft repeated claim that baking eggshells increases calcium bioavailability for chickens.
About to grind up a 6 month stash of eggshells for the worm bins. Keep up the good work.
Try baking the egg shells after collecting a bunch. They will decompose
Prove it? show me any evidence that baking changes the eggshell chemistry.
Today is May 31st 2020. We moved into our home in Oakland County Michigan in July 2015. It was vacant for at least a year prior to us moving in. I found a fully intact buried egg today in a garden area On our property. We cracked it into a container and it still had yoke and amniotic fluid. So it has been in the ground for at least 6 years. Probably longer than that based on the prior owners. I suspect it was the owner prior to them who had the reputation for being a great gardener . I know it’s anecdotal, but figured I’d pass it on as you have taken scientific interest.
Do you happen to know the pH of your soil?
Hi, I live in western Washington, and we have many old logging camp sites in the local woods from the steam era.
While hiking and exploring I have found logging camps and every camp had a kitchen. When you find a kitchen waste site the thing you find most of is clam shells, egg shells, bones, tin cans that are almost gone and A1 sauce bottles with glass stoppers.
I’d have to say that a 70 to 80 year old egg shell looks pretty dang intact even the bones are spongy and easily broken up just using your hands.
I haven’t tried to date A1 bottles with glass stoppers but if you do it would give you a closer age.
What about it worm bin? Do worms consume eggshells?
Once shell pieces are small enough, worms will eat them as part of the soil they consume. The action in their gut will break pieces into smaller pieces. But their gut system is alkaline, so they won’t dissolve the egg shells.