Egg shells, a natural and plentiful byproduct of our society and natural ecosystems, seem an unlikely candidate for controversey.
Yet, controversey there is. On one side, natural gardeners and farmers recommend egg shells for improving soil calcium, effecting soil acidity, enriching compost, and deterring slugs. On the other, there is a small group of very defensive IPM advocates who are deeply skeptical of anything that doesn't come from a corporation. They claim that egg shells are pseudo-science and that they will not break down in soils, even over millions of years.
There are some folks out there who just don't seem to understand how ecosystems could have possibly provided such services for the couple of years before Monsanto evolved to take care of such things for us. And quite remarkable that we're not all up to our necks in non bio-degradable egg shells.
Discussion
The controversey of egg shells is really two different questions: the first is its effectiveness as a soil additive. The second is as a slug deterrent.
First, as an additive: Egg shells have been recommended as a soil and compost additive since the early days of Organic gardening, including in Rhodale's Book of Compost (1960.) It is also commonly recommended by university extension offices, including my own state's MSU Extension. But does it work?
There are indeed a few studies demonstrating that egg shells can have an effect. One such, from Charles Mitchell of Auburn University, found that finely ground egg shells were as effective as pure Ca(OH)2 at effecting soil acidity and calcium. Additional studies have backed this proven effectiveness of egg shells, such as this one. from Berkley has found that egg shells are digestable and accessible by the human body, proving that they can indeed be broken down by organisms in ecosystems. This is an important factor not covered in Charles Mitchell's research, which just examined the breakdown of eggshells in soil, that plants, bacteria and fauna utilize acids to digest materials. To me, it seems quite an extraordianry claim that egg shells will not break down in healthy ecosystems and be cycled through plants, even when they are initially added as hand-ground instead of finely ground.
The second issue, slugs, has not systematically been researched. However, there are numerous annecdotal reports of its effectiveness. But there are also tests demonstrating that roughly broken egg shells will not directly kill slugs. I did not see any attempts to test the effect of finely ground egg shells on slugs. I have personally used egg shells to deter slugs and observed them to be at least somewhat effective, but I have yet to test this and my observation my be incorrect. Which is the very point of Research Based Permaculture.
Conclusions:
Soil Additive or Compost Additive, finely ground:
Finely ground egg shells are a proven way to provide calcium to plants and effect soil acidity.
Soil or Compost Additive, crushed:
Egg shells will biodegrade in soils, especially when acidic. However, the process may be slow, or even very slow, so appropriate expectations are necessary. There is no research to indicate how quickly the calcium will become available. So if calcium is needed immediately it is recommended to finely grind egg shells and mix them with soil.
Slug deterrent:
The use of egg shells to deter slugs is not backed by research. However, there are no risks involved with experimenting with egg shells, other than the risk of it not working. Because it is would be a cheap DIY source of slug prevention, it would be a good candidate for a well-desiged experiment.
Discussion Updates: (3/21)
1. Biodegradability in soils and ecosystems. The idea that egg shells do not biodegrade and cannot act as a slow release fertilizer is apparently widespread in the "scientific gardening," community despite the research proving otherwise. Plants and animals are also active in breaking down egg shells in ecosystems and biologically active soil. What happens when one puts these incredibly unbiodegradable egg shells in vinegar? The calcium breaks down: CaCO3+ 2H+ -> Ca+2 + H2O +CO2. How was this miracle achieved? Acetobacter found in soil, just one organism, including plants, which which secrete acids capable of having an additional effect on egg shells in an ecosystem, but were excluded from the studies.
2. After publishing this post, I became aware of another criticique of using egg shlls in the garden, from the very same people claiming that egg shells are not biodegradable. They were now convinced that egg shells were such a potent source of bio-accessible, effective calcium that including small ammounts of them in compost or adding them to soil would quite definitely create calcium pollution.
I disagree with this sentiment and would add there's no research-basis for this supposedly "pro-science" statement.
Certainly, anything can become pollution in large enough amounts, and this applies to egg shells. As per the studies linked above, and following the standard recommendations for application of agricultural lime, it would not be recommended to exceed 2.5 tonnes of powdered egg shells/acre per year. For most home gardeners looking to add their eggs back to their soil, I doubt this would be much of a problem. For those who already have well-limed soil, it may not be recommended to add powdered egg shells directly to soil, however adding them to compost should be benign as compost should ahve an acidifying and buffering effect on soil acidity. But there will also not be a need for additional calcium in those soils. Still, application of crushed egg shells would probably have to be extreme to negatively impact soil acidity or become a pollution problem
Recommended Usage:
For most gardeners seeking to add calcium to their soil, or to lime their soil, working on an Organic theory of gardening using slow-release fertilizers, egg shells can be added direcly to the soil or compost. I will follow the recommendation of MSU Extension in advising to grind first. Powdered egg shells will have a measurable immediate impact on soil acidity and calcium, whereas a more varied grind will act to feed the soil over a longer period of time. We use a coffee grinder to grind ours.
Recommended dosages of commercial organic fertilizers with similar calcium contents are 1-2 T per plant, or 3-4 lbs for 100 feet of garden bed. With such application rates, use of egg shells is unlikely to become a pollution issue. A family which uses a great deal of eggs might expect to produce a few lbs of egg shells per year.
Citations:
Egg Shell Powder a Good Way to Get Calcium, Berkley. http://www.berkeleywellness.com/supplements/minerals/article/eggshell-powder-good-way-get-calcium
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