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.
I think the possible conclusion that eggshells don’t decompose quickly is right. It’s like micro plastic, it may be small but it’s still there and not decomposed to it’s building block. The more interesting question then is what biological process breakdown the shell quicker. I am putting some into a aquarium to see if they break down.
Faulty methodology, assumes the visual decomposition of the physical shell is the primary determinant. 1) The egg shell has a complex structure. 2) The calcium can leach out of the shell and the visual decomposition may not be obvious. 3) Better to sample the soil and test for calcium at defined intervals. Since you need to show the relative change you should start over. 4) Also, need to finely ground the eggshells to dramatically increase the cross sectional area interface with the soil.
The design of the experiment needs to be matched to the results being measured and the conclusions one wants to reach.
I am not trying to prove no calcium is lost. Most people think the egg shells compost because they can’t see them. This experiment was designed to show that they do not decompose to a point where people don’t see them.
I doubt soil samples would work. for one the amount of calcium lost to the soil is very small, and second, my soil already contains high levels of calcium. Adding small amounts to a high level won’t be measure able.
Grinding would speed up the process – see the reference about this in one of my egg shell posts. But grinding would not produce the results I am looking for – a visual display of the intact shell.
So then is there any use to the eggshell or just throw them away?
They don’t do any harm, but they don’t add much either. I still add them to my compost because the inside skin and a bit of egg white provides some nitrogen.
Chickens and other birds. Soak and rinse them, grind it into a powder, cook it to sterilise it, and mix it in with their feed. They it is a good substitute for grit so they can make healthy eggs. If you don’t have chickens find someone who does they’d probably appreciate it.
One more comment about eggs/eggshells – compost heaps with any kitchen waste of any sort do tend to attract vermin, but eggshells are the waste most adored by rats.
All my kitchen waste now goes in a pair of rat-proof bins, and only when well rotted do the contents get turned into my normal garden compost bins. I do notice that despite it being a reasonably acid environment, the eggshells are by far the most recognisable remains!
Totally UNTRUE. We have used Egg shells for 60 years. We crush them but do not make a powder of them, We use a lot of eggs so our garden would have egg shells showing everywhere if they did not decompose. They add Calcium which Tomatoes particularly need. It takes about 2 years for all of them to totally dissappear in any area but the fact that they will last the entire growing season is good. When used they are buried in the ground as you till it. anything wrapped in plastic even with holes will not decompose very fast so it was a flawed expirment. Another good thing for gardens that takes a long time to decompose but is a steady source of acid for acid loving plants is Orange peel. Just break it is smaller pieces and let it dry then put it into the ground, NOT very deep so it does not disturb the roots of rhodies and other acid loving plants and shrubs. Things that decompose fast only put nitrogen into the ground and gardens grow best with less nitrogen and more of the other vitamins and minerals needed.
You present no evidence to support your opinion – which is not supported by science.
The experiment is not flawed as you claim since the eggshells are NOT wrapped in plastic – please read the post again.
Archeological digs from the mid 1800s find intact eggshells – so intact that they can identify the bird species that laid them.
I think he meant the plastic in the ‘Holey Pots’ is equivalent to “wrapped in plastic”
I never knew that pseudo science was such a big thing
Last week I broke down a compost pile that I started about 18 months ago. I had never aerated or turned it. The top 2/3 of the pile went into another bin for further decomposing. At about the 18″ level it was nicely decomposed, with only two things recognizable to the eye: a small round fruit label, and some eggshells.
I look forward to your results, Robert! We grind our eggshells into a powder and add them to worm bins. I suspect this speeds up the decomposition process significantly, especially after the particles pass through a worm several times.
I wonder how the digestion system of a worm works? It may not use acids – but I have no idea.
It’s an interesting thought experiment. I toss my eggshells in the compost bin usually, but the advantage strikes me as folklore (or apropos to your blog – a myth!).
An XL egg weighs about 8 grams. So if you saved a dozen eggs a week, it would take almost 3 years to get the same calcium as a 30 lb bag of lime. Assuming it breaks down quickly.
That’s a lot of omelets.
Good point. My math is even worse. An egg is about 5.5 gms calcium carbonate http://www.neospark.com/images/eggshelldefects.pdf So 12 eggs per week for 52 weeks, over 3 years will produce 5.5x12x52x3 = 10,000 gm calcium carbonate, or 23 lbs.
What fun
I love these sorts of experiments!
Hi Robert.
I’ve been following your blog for months. A lot of things you write here has clear some thougt in my mind especially about the compost.
When I read about this, you do spark some thinking of me about this.
In Malaysia, we use SRI techniques. We crushed and fry the egg shells. Then we dissolved it in organic vinegar for about a month.
The liquid then will be use as plant booster to add up calcium in plant.
Egg shells are basically calcium carbonate, and acids will break up this compound and form soluble calcium ions. This is also why egg shells decompose faster in acidic soils and vinegar is much more acidic than acidic soils.
Crushing helps them dissolve, and heat treatment also seems to help – although I have not confirmed the latter.
I’d be interested to know the strength of vinegar you use, and how dissolved the egg shells are after a month?
What is the difference between regular vinegar and organic vinegar? Nothing chemically! It might be important to meet some regulatory requirements to be certified “organic”, but the reality is that there is no difference. If you search this blog for “what is organic” you will find several posts that explain this point.
I don’t see any problem with your procedure until you get to the last sentence “The liquid then will be use as plant booster to add up calcium in plant“. Your solution is NOT a plant booster! Why? Because there is no such thing. Plants take the nutrients they need from the soil. Any nutrient that is deficient in the soil will slow down plant growth. Adding this deficient nutrient speeds up plant growth. This is true for all nutrients.
In your case the solution is only a plant booster if your soil is deficient of calcium. If it is not deficient, the solution will do little to help plants. Many garden soils are not calcium deficient and adding more may actually increase the the amount of calcium to toxic levels. This is not likely to happen using a few eggs produced in homes, but can be a real problem by people adding fertilizer booster in larger quantities.
You are also adding some acetate salt from the vinegar, which will feed bacteria, and this adds some benefit to the soil.