Lots of people add eggshells to the garden or compost pile. It is claimed that they add important calcium to the soil for plants. Is this true? How well do they decompose? What happens to them in a compost pile? Do they add any value to the garden?
Eggshells in The Garden
Why would eggshells be good in the garden? To limit the discussion, this post will only look at chicken eggshells. It turns out eggshells contain a variety of nutrients that plants can use (calcium 50 ppm, sulfur 39 ppm, magnesium 12 ppm and potassium 12 ppm) (ref 1). They also contain 21 ppm sodium and 5% organic matter.
The organic matter might be a surprise since it is not mentioned by any gardening sites. Eggshells consist of a hard outer shell, and a soft inner white skin. The inner skin contains the organic matter. The organic content can be even higher than 5% if they are not washed out. This organic matter contains nitrogen in the form of proteins, which is very useful to plants after it decomposes.
From a nutrient point of view, eggshells would definitely be a benefit to the garden soil if they decompose so that the nutrients are made available to the plants.
Eggshells – Do They Decompose?
I put some kitchen scraps in the garden more than year ago and I looked at the eggshells the other day. They seemed to be completely intact – see the picture at the top of the post. Even the inner protein layer was still there.
I don’t use a compost pile very much any more, preferring the cut and drop method instead (Composting – The Cut And Drop Method). So when I emptied the compost bin the material had been there for several years. The picture below clearly shows the eggshell pieces. The pieces are quite hard and there seems to have been very little decomposition.
Eggshells Don’t Dissolve in Water
Dr. Jeff Gillman, in his book, “The Truth About Garden Remedies” (ref 2), discusses an interesting experiment. He took 1 eggshell, boiled it, and then let it sit for 24 hours. The minerals in the water were then analyzed. About 4 mg of both calcium and potassium were released into the water.
An eggshell contains about 2,000 mg of calcium. The boiling and soaking process released 0.2% of the calcium. Boiling water did not have much effect on the eggshell – why would rain and soil water have a bigger effect?
Eggshells – What Happens in Soil?
Charles C. Mitchell, Extension Agronomist-Soils at Auburn University , tested crushed eggshells in soil (ref 3). He wanted to see if eggshells add calcium to the soil, and if they change the pH of soil. If they decomposed while in the soil, you should see both changes to the soil.
The testing found that hand crushed eggshells did NOT change the soil pH, and they did NOT increase the level of calcium in the soil. This is after being in the soil for three weeks.
When the eggshells were ground very fine, they changed the soil pH and they added calcium to the soil.
The soil used for this test had a pH of 4.9, which is quite acidic. This is a very important point. Eggshells are essentially calcium carbonate which dissolves in acids, but not in alkaline solutions. Even finely ground eggshells will have a small effect on less acidic soil.
The study in (ref 4) found that eggshells stop affecting pH once the pH of the soil is around 6.8. They stop changing the pH because they stop breaking down at a pH of 6.8.
Eggshells Found in Archeological Digs
I found a report entitled “An Analysis of the Avian Fauna and Eggshell Assemblage From a 19th Century Enslaved African American Subfloor Pit, Poplar Forest, Virginia“ (ref 5). This is quite an interesting read from a historical perspective. The study looked at a property in Virginia that was at one time owned by Thomas Jefferson. It was a tobacco plantation that contained a small community of slaves from 1840 to 1860. Excavation of the site found thousands of eggshell fragments from both chickens and ducks, which had been raised by the community.
The key point for us is the fact that over the last 165 years, the eggshells in the soil did not decompose very much. In fact the researchers could still distinguish chicken eggshells from duck eggshells.
Do you still think eggshells decompose in the garden in a year or two?
5 Year Experiment Looks at Decomposing Eggshells
I wanted to see for myself what eggshells really do in the garden. I buried some and have dug some up, every year for 5 years. The inner protein layer decomposed after one year. this video shows you what I found after 5 years.
Do Eggshells Decompose in Compost or Soil!
The above information is interesting – at least to some of us.
Eggshells are extremely stable and don’t break down very fast without some help. Water alone does not seem to break down the eggshells. Acidic soil will break them down, but only if the soil is acidic enough and if the eggshells are very finely powdered. Most gardeners don’t powder the eggshells before putting them into the compost bin or spreading them in the garden.
As explained in Compost Creates Acidic Soil , compost does go through an acidic cycle during which some of the eggshell might decompose. But soon after starting the compost pile, it becomes alkaline, and during that phase very little of the eggshells will dissolve. Since the eggshells are mostly intact at the end of the composting process it seems clear that composting does little, if anything, to decompose them.
I don’t believe eggshells decompose in any reasonable period of time, either in compost or soil. If that is true – they add very little value to the garden. The exception is very finely ground eggshells (down to 60 mesh), added to acidic soil.
Where Do Eggshells Go?
If eggshells do not dissolve and they don’t decompose, where do they go? Gardeners have been adding them to soil for years and you don’t find huge piles of eggshells in their garden.
Is it possible that the microbes in soil decompose eggshells? Possible, but I could not find any reference that discussed this issue.
It is my belief that eggshells simply break up into smaller and smaller pieces while people work the soil, until you don’t see them. You think they are decomposed, but they are still there in small pieces. Pieces that are much larger than finely ground.
This conclusion is my belief, and is not proven in any way. What to do?? I know – it’s time for an experiment, which I will discuss in my next post.
References:
1) Characterization of Avian eggshell waste: http://www.scielo.br/scielo.php?pid=S0366-69132006000400004&script=sci_arttext
2) “The Truth About Garden Remedies” by Dr. Jeff Gillman
3) Crushed Eggshells in Soil: pdf of Crushed Eggshells in Soil
4) Can Crushed Eggshells Be Used as A Liming Source: http://www.agronext.iastate.edu/soilfertility/info/eggshell-lime.pdf
5) Thesis for Kathryn Elizabeth Lamzik; http://trace.tennessee.edu/utk_gradthes/1635/
My mom was the gardener in the family since her death our gardens & lawns have been on their own for over a decade. She use to hand ground egg shells all the time. I was told to recycle veggies (except onions, meat & dairy[are’t eggs considered dairy?] ) to help my soil. I live in SoCal and 15” of rain for the is an excellent season. Thus I would have to water the gardens once/Bi-weekly in hopes to attract the worms and help with decomposition. I do NOT want to live without greenery around the home – like those in SunCity, CA. with green/white rock lawns or total cement. I noticed there it just increases homesteads temperatures- Summers are 105-110 degrees already. I am not sure if I am wanting to bother but this post has given me a wealth of information. Thanks.
I just have to say are these eggs 100% organic or they just genaric brand because i could sware non organic eggs have plastic in them
There is no difference between organic and non-organic eggs – its just marketing. There have been many studies looking at both taste and nutrition of organic and non-organic products and I have not seen one yet that shows a benefit.
flavor if nothing else, but I doubt that commercial eggs and home raised/free range/pastured eggs contain the same quality and quantity of nutrients.
What is the science on this? Organic food is not more nutritious – don’t know about free range eggs.
The best thing eggs shells do in a garden is keep the soil loose. When broke up and tilled into the soil. We buy ans speak several 100 tons of crushed egg shell shells on or fields for corn, soybeans and hay fields. We don’t put them on for calcium or nitrogen. Only to keep to soil from compacting
“We buy and spread several 100 tons”
I was happy to find this post, because I’m sick of having compost that seems to be 75% egg shell. (We use at least 2 dozen a week.) But now I don’t know what to do with the shells. Putting them in the garbage is smelly and seems wasteful. I don’t have extra energy for drying and blending them. I don’t have any place to bury them. Perhaps I’ll buy an inexpensive mortar and pestle and try that.
Whether egg shells actually are a beneficial soil amendment , meaning whether they provide needed sustenance to plants, is open to question, perhaps actually doubtful. But I still add them on the outside chance they are of value ( can’t hurt). But I dry them in a low oven ( about 250 deg f.), for about an hour, then very easily grind them to tiny bits in a (cheap) coffee/spice grinder. Very quick and easy, sprinkle on beds.
If you use heat energy to heat them, and electricity to grind them, and they add nothing of value to the soil – is adding them an environmentally friendly thing to do?
As I have stated previously I baked my eggshells and run them through a coffee grinder to enable me to feed the pulverized eggshells to my composting earthworms. I have recently learned that birds can use these same eggshells as a feed supplement sprinkled on a suet cake or in amongst the birdseed. I am going to learn to build birdhouses to house specific birds that devour insects in an organic line of defense at a local community garden. As far as energy saved or spent, walk, ride a bike or skip a shower to find out who your true friends are, to make up the difference. What is the cost to mine calcium and ship it to you? You have a link that states ground to a certain size crushed eggshells are as effective as lime for increasing the pH.
Only in acidic soil and only when ground very fine.
Maybe a good experiment might be to leave some uncrushed eggshells in rain water which has a pH of 5.6 to see if there is any change in the eggshells or in the pH if an acidic environment facilitates their decomposition. The eggshells do add many of the minor and micro nutrients need by the plants and one report does state even large pieces of eggshells do no harm in the soil. There seems to be evidence that eggshells finely crushed are underestimated as a liming agent but are also less water soluble which can be a good thing as the calcium will remain longer in the soil before being washed out. I do think eggshells have value and it is up to gardeners to find the best way and where to plug them into our scheme of growing things while eventually getting to the point of 100% recycling, especially kitchen and yard waste.
Collect half eggshells and put then in the egg box. Come Spring – fill them with soil and plant seeds. When you have seedlings plant them intact with the shells. Read about this recently and will try this.
Not a good idea. The shells are far too small to grow seedlings. Secondly, since the shells do not decompose, they will prevent roots from growing in the soil outside of the shell.
Well! I could poke holes in the bottoms of the shells before planting!
I don’t think heating the eggshells in an oven the is already on to cook food is excessive waste. And the electric coffee grinder reduces them to tiny bits in 5 seconds doesn’t seem to me to waste much. Agree Robert?
Maybe not excessive waste – but still waste. Everything we do impacts the environment.
I dry them in the oven then crush a little then put them in a $12 coffee grinder which turns them to powder, then I mix with water and leave for 24hrs then water my plants with it, it takes only 3 weeks before the calcium is available to the plants, I do this once a month in total it only takes about 10mins so I think that’s better than working 30mins to earn the money to buy products containing calcium plus I means I don’t waste eggshells
But the science does not support the statement “3 weeks before the calcium is available to the plants”.
There is a way to use pulverized eggshells in a rather quick way although in this factsheet the eggshells are used in a slightly larger size. I used apple cider vinegar rather than rice wine vinegar and the mistake I made I didn’t note that it was a 1000 to 1 dilution and it was rather harsh on the seedlings, it killed them at a 10 to 1 dilution. The important thing is to realize eggshells are indeed a garden resource, as a bird diet supplement, an essential ingredient in any worm bin and as a diluted drench or foliar spray for plants. http://www.sciencedirect.com/science/article/pii/S0956053X1300576X
It is only a garden resource if the garden needs it. Do birds really find and eat the shells? Do you have proof? Mine sat in the garden on top of the soil for several years and are still there – I have lots of birds. If the calcium does not dissolve, then it is not a good foliar spray. Also, calcium absorbed into leaves will not be transported to other parts of the plant so it is not a great nutrient to be sprayed on.
Too much of anything can rapidly turn into a bad thing. Can eggshells be used in the garden and the answer is yes if pulverized as through a coffee grinder, they are as effective as Ag lime for raising soil pH.
As far as a wild bird feed supplement, the shells can be larger 1/8″ to 1/4″ in size sprinkled into the faces of suet cakes or mix in the seed of a bird feeder. It is especially important during mating season and will help to ensure the increase of beneficial birds.
A foliar spray containing calcium can help to lessen the occurrence of BER in tomatoes but there are precautions with any foliar spray. Here is a link. http://www.soiltest.uconn.edu/factsheets/FoliarFertilization.pdf
I just look at pulverized eggshells as a valuable, renewable and sustainable resource and as with any resource use wisely. I use a good amount in my vermicomposting. I not only save mine, I have family members saving me theirs.
Calcium foliar spray will not lessen BER. Low levels of calcium in tomatoes is a consequence of BER, not the cause of it. https://www.gardenmyths.com/blossom-end-rot/
The calcium foliar spray is for the leaves and hopefully is freeing up calcium for the fruit but it has to be sprayed when first buds appear. By the time a person observes BER it is too late to do anything. I myself only experience BER with paste tomatoes that are determinates and only the first fruits are affected. Very rarely on any other type of varieties even if they are determinates. A low level of calcium can be a source of BER but so can an abundance of nitrogen or potassium or magnesium, so without a soil test one can only guess. pH can be a factor along with soil temperature, planting in cold soil can lead to an occurrence. A 30 minute moisture stress can be a cause for an instance of BER. There are many causes of BER which is an abiotic disorder that usually looks worse than it is unless it is most of the tomatoes and not just a select few. Also BER is usually or can be followed by a secondary infection. The reason I pick the affected fruit immediately. I grow Roma tomatoes and no matter what I do I will get a few with BER at the beginning of the season but by the middle of August there is no sign of the disorder. I believe and this maybe a myth but the plants stem system along with the roots haven’t grown big enough to transport the needed calcium to the fruits in time. The important thing is to start with a soil test because it gives you a place to start. Unless you want to be like my dad, a Canadian farmer’s son and taste the soil in the fall and early in the spring. In the fall if it is tart add lime and if sweet in the spring add manure. No, I don’t do it because I always felt my dad was trying to get me to eat dirt but this might be the year I taste the soil before I send out the soil sample for testing.
BER is not caused by low levels of calcium in the soil or in the plant. It is a low level in the fruit, and the latest research suggests that the low levels is a consequence of BER, not a symptom.
It is also my understanding that calcium sprayed on as a foliar spray will only add calcium to the point at which it is absorbed. Calcium is not translocated from leaf to fruit and since fruits absorb almost no calcium from a spray, it would not help BER.
i am glad i am not the only one questioning why ppl tell u to compost eggshells, like u have said, you still had many shells in the bin, yrs later, my mom had been doing the whole egg and compost thing in the 90’s and its scary to see the slimy acidic sludge that comes out of them even yrs later, as no paper or chuff was ever added, Peesonally as i have grown up and done many 100’s of kilos of it now, i wont add them either as my own experiance they dont break down inany type of environment nutral or acidic, wet or dry heat. only thing i would use em for is slugs now.
Dissolving egg shells in vinegar gives calcium acetate which is water soluable and may be available to plants.
I have only just started adding egg shells to my garden. My mother use to do it. From reading all the above maybe the best egg shells do is add some body to the soil. Loosen the soil up bit. Better in the garden than the bin.
I use baked eggshells run through a coffee grinder in my worm bins and doubled the population of Canadian nightcrawlers one winter on nothing more than shredded newspaper, coffee grounds and eggshells. I now have to see if the digestive tract of an earthworm makes the nutrients in an eggshell plant available. Thanks Robert, great site.
I was wondering that too as I researched vermicompost for a future post – have not found the answer yet. If the gut is acidic then it might decompose eggshells more quickly. They also use them to grind their food – so there is added abrasion in their gut which will help.
I don’t add eggshells for calcium but to the surface to deter slugs and sbnails- they hate the sharp edges and leave seedlings along- and also the cabbage white butterfly thinks that area is taken if it sees white things down there.LOL Not scientific but garden lore yes?
Egg shells do not deter slugs. https://www.gardenmyths.com/eggshells-control-slugs/
I have never heard that they keep cabbage butterflies away – but I doubt that is true.
I collect a large number of eggshells, then bake them on a sheet at a low heat to make them brittle, grind them in a coffee grinder. Put them in a bowl and add some vinegar. They bubble up. After a few hours I add the bowlful to the compost.
Vinegar will dissolve them. but do you need more calcium in the garden? Most soil has lots, in which case this is a lot of work for little gain.
HI Carole, your procedure sounds promising to me. Having been adding crushed egg shells to my garden beds for years , I’ve not seen traces of them after a while. So, I assume they are either dissolved, or ingested ( maybe by earthworms) hence to become part of the mineral component of soil, hopefully available to plants.
However, since having a few ducks, they consume all the crushed eggshells I make available (after oven roasting at 300 degrees for an hour). Guess they’re then part of new eggs, or contained in manure, which also goes into garden beds or compost, depending on time of year.
Yes! I read about this way to break them down a few years ago in another forum. I bake them at 170F for a couple of hours. I hate to throw organic material away and noticed that the eggshells in the compost bin did not break down. I also put coffee grinds and veg water in the garden, along with some of the bath water which has epsom salts in it!! My husband, not a gardener, thinks I’m mad!!!
Epsom salt has been used for years to increase fruiting I believe– probably because of the magnesium, which can otherwise be in low supply in some soils. I have used it successfully, sprinkled around the base of tomato plants to increase yield. You’re not mad, just conscientious.
Actually most soils are not deficient in magnesium. If there is no deficiency, there is no benefit to adding more magnesium.