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.
Northern Utah. ph 7.2…. I have gardened and composted for 60 years with just about every kind of mix you can imagen. The egg shells just break apart, they don’t decompose. I have piles from 1 to many years old. As you dig into the piles, the egg shells are still there. I place the compost on the garden and even after working it in, you can find a small broken up piece from time to time, many of them from many years past. I don’t like throwing away anything, so I still add them to the compost piles.
Now instead of relying on the calcium in egg shells, I use Calcinite, by YARA. It’s a greenhouse calcium source that is readily absorbed by the plants, it even has some nitrogen in it. Works terrific on tomatoes especially when they are formed and somewhat grown and need to start ripening. I use it in a foliar spray and also directly into the soil.
One last thought. We have a Water recovery system we call sewer ponds for the city. Every 10-15 years or so they drain the pons, dry them and clean them out. Where the tubes of raw sewage enter the ponds, there are large mounds in a doughnut shape circling the output. These are piles of mostly egg shells after they have gone through the Insinkerator or garbage disposal of homes were they are basically crushed and blended, then down the drain into the sewer and on the path to the water recovery ponds. as they leave the supply pipes, the egg shells quick separate from the water and drop to the bottom along with other heavy materials. But the other materials decompose, leaving these piles blended eggshells.
These piles can be six feet high and 30 feet in diameter. one doughnut pile at each pipe termination. They are at least 10 years old with no sines of deterioration after 10 years or more.
I’m very much inclined to agree with this experiment.