Eggshells – Do They Decompose In The Garden?

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/