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/
Egg shells being mostly calcium carbonate, so subjecting them to heat in a hot fire has the potential to convert it into calcium oxide, which is much more soluble in water and potentially improve the absorption of the calcium by plants.
That being said, calcium is a micronutrient that isn’t need in high quantities for most plants – usually accounting for less than 1% of a total mass for healthy specimens, and a little more for things like fruit or nut trees (but still under 3%). So quantitatively, adding eggshells might not provide a lot of excess calcium, but proportionally it could still be a help to our plants when augmented into compost or soil (especially when pulverized adequately and utilized with optimal soil pH).
I think Mr. Pavlis is being incredibly pedantic in the comments. You made your point: eggshells are resistant to composting, especially if not ground fine.
I can’t see that this equates in any way to putting your eggshells in the landfill being a better option, which is the major alternative to composting them.
Blathering on about how running a coffee grinder for a few seconds or drying your eggshells in the oven while you are cooking something else is somehow a bigger waste of energy that a garbage truck is utterly ridiculous.
There’s a bigger picture here. I fully intend to continue putting my small quantities of finely ground eggshells into my compost mix, whether or not they do anything except add to the ultimate inert mineral content of my garden soil.
Absolutely nothing said anywhere here argues scientifically against the wisdom of this.
Hi mate, I love chickens, eggs, soil and worms.
I have found if you put half egg shells into your soil….
Worms will Seek them out and breed inside that space ….crazy but true
I heard that in Germany, they would always burn their eggshells in the fireplace, then add the wood ash and burnt, pulverized eggshells to the garden. Even if the burning doesn’t do much chemically, smaller particle size will greatly increase the rate of decomposition in the soil, no matter how slow that may be. Extend applications over multiple generations and eggshells make a fine soil amendments. For example, today’s vegetables may contain eggshell from 5 generations ago, and all one has to do is keep adding to the soil’s bank of inert eggshell particles. The same applies to crushed rock amendments of all kinds. There exist very cheap sources for all the necessary minerals, it just takes them a while to mature when deposited directly into the soil. Rock phosphate would be one such example.
This has to be the best article and thread I’ve found on the discussion of egg shells. I break them up and use them in the bottom of pots for my ivy, palms, and violets, it makes great for great drainage material. I also put them around the wisteria, does it help? No idea but it makes me feel better than I’m not trashing them!
https://www.powerequipmentwarehouse.com/diy-tips
Egg shells, bones, banana’s etc are but old wives tales. Their immediate effects sounds about as useful as a tit on a bull. Their contribution to a healthy soil takes about as long as it did Methuselah to die. I’m grateful that science has applied its factual knowledge to debunking, time-wasting and useless gardening tips from the amateur “experts”, with deepest respects to their progeny. And, before it’s asked; no, I’ve never plated a seed in my life. Hmm, might have something to do with the fact that my mum got us kids, 12 of us, to pee on every thing that ever grew, whether green yellow or red. They all went the same way though – to the garden gremlin in the sky!
Ok. Ok. Foliar spray of eggshells, ground-up, dissolved, soaked, or whatever, no good (useless). Putting on ground also probably useless, unless soil, however unlikely, needs calcium, and shells are finely ground, in which case limestone probably more utilitarian. Maybe, if finely ground, whether by small seconds in coffee grinder type appliance, or mortar and pestle, intestinal tracts of earthworms and consequently garden soil could benefit.
But it would seem reasonable that birds should benefit from consumption, based on how domestic fowl like chickens and ducks adore them. I’m assuming they know instinctively what they nutritionally need. I’ve never seen any harm come to them upon eggshell consumption, and their shells always seemed harder, stronger than those of commercially produced eggs, whose farmers were probably adding precise amounts of calcium to their feed. So……guess from now on I’ll just try to feed the wild birds, since I no longer have domestic fowl. I have seen wild birds messing about in the tray I hang just for eggshells, but can’t claim with certainty to have witnessed consumption.
As always, Robert, thanks to you for sponsoring this informative forum, and to other contributors as well from whom I’ve learned much, and by whose knowledge I so often have been humbled.
Adding cider vinegar to finely ground egg shells works well. The egg shell is primarily calcium carbonate (which is not water soluable). If vinegar (acidic acid) is added it will start a chemical reaction that produces carbon dioxide and water soluble calcium. This nutrient is immediately available to any plant that needs it; namely, tomatoes with blossom end rot. I dilute this tea before applying it as a folial spray as acid is not needed but water soluble calcium is.
Acetic acid will dissolve eggshells, but BER is rarely a soil calcium deficiency. It is a watering issue.
The fruit does not absorb calcium as a foliar spray very well, and if leaves absorb it, it is not transported from leaves to fruit.
http://www.gardenmyths.com/blossom-end-rot/
I know a foliar spray is pointless, but would it make sense to use a strong acid such as vinegar to dissolve the eggshels, then add them to the compost or the soil?
Does your soil really need calcium? In not – it is a waste of time and resources. If it does need calcium, add some lime – much easier. Most soil does not have a calcium deficiency.
I am wondering if it maybe a good idea to put out some eggshells this time of year as a source of calcium for the birds so they can make their eggs. Earthworms also need calcium. So as you increase their populations you may get to use the some pulverized eggshells. Pulverized eggshells, run through a coffee grinder have the same effect of raising the pH as lime. Why buy what you have on hand. Store the eggshells and use when needed.
Might be good for birds, but in alkaline or neutral soil they stay as eggshells. If you have very acidic soil and need to lime, they will help.
If you learn to keep the right birds in and around your garden they can be a great line of defense against garden pests, keeping them in check and the garden environment harmonious.
I try and use eggshells in any way I can here rather than throw them in the garbage because this to me is waste that can never be recovered.
I guess I just look at eggshells as a resource that has many uses and possible benefits. I use a tremendous amount in vermicomposting. I have many family member saving eggshells for me and usually do several pounds at a time (5-7). They do add up rather quickly.
I do like the fact that you do take a look at garden myths. I just watched your video on container soil/mix and am in complete agreement. The only thing I could add is if someone begins taking about container gardening, the first question should be, how big is the container? and the second is what are you trying to grow?.
If we focus on the similar compounds in teeth enamel, then there is a clear indication of the effect of bacterial breakdown of apatite. Acids produced by bacteria in the plaque of your teeth dissolve the calcium phosphate and calcium carbonate and give you cavities and toothache. There is nothing particularly unique about the bacteria living in the plaque so other bacteria, such as those living in the soil, will also be producing acids that can break down minerals like apatite whether they are in eggs, shells, teeth or bones or in inorganic minerals in stones and rocks. Once they are dissolved in the ground water they are then available to organisms in the soil, including plant roots. The solubility of calcium phosphate and calcium carbonate is very important in biological processes that include the formation and resorption of hard tissues such as bone, teeth, shells and eggs. These compounds must be carried around the body dissolved in the blood or carried in transport proteins. The regulation of both calcium and phosphorus in the body is very important and can lead to the dissolving and reconstitution of bone particularly after injury and the production of kidney stones. The reabsorption of unfertilised eggs within birds and reptiles, while not being common is sometimes seen. The quantity of calcium phosphate and calcium carbonate that can dissolve in the soil depends on the pH and concentration of acids and bases. It is obvious that these compounds will dissolve in the soil and are carried away in the ground water because this is how stalactites and stalagmites are formed. The fact that egg shells and other apatite like containing structures will weather and breakdown in the soil relatively quickly, given that the acid producing microbes have the luxury of moisture and warmth, cannot be disputed if cavities in teeth are considered and that calcium compounds dissolve in the ground water and will produce stalactites and stalagmites if the correct conditions prevail. Egg shells, bones, teeth and shells of things like molluscs will break down and dissolve in the ground water and the nutrients within them made available to plants.