I am relatively new to composting and still have not figured out how to generate heat so for now it is mainly cold composting. I was wondering if it is a good idea to put egg shells in the compost bin both to make sure it decomposes fast but also to ensure it does not attach mice or voles.

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    You can add them thoughtlessly as long as they're dry. Those won't attract vermin. However, you should pulverize them to the smallest grain size. I've found egg shells in the humus after years past; an egg shell is just too sturdy when only split in half. When ground to powder, they're a great addition to your compost mix.
    – dakab
    Mar 26 '17 at 0:28

Yes,they can be added to your compost bin or pile - quicker to break down if you smash them up, but they don't have to be. Some people rinse out the inside first so as not to encourage rodents, but if you have an actual closed bin, that shouldn't really be necessary. More info here: Eggshells In The Garden: Using Eggshells In Soil, Compost And As Pest Control.

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    Just an anecdote, but I put them in unwashed and I only make a minimal effort to break them in smaller pieces and everything works fine. Pieces show up in my finished compost but they can just finish decomposing in whatever soil I add it to.
    – Philip
    Mar 26 '17 at 18:14

There's considerable doubt that egg shells decompose in any significant fashion especially if the soil is not acidic.

If you want to benefit the soil at all, best would be to grind into a fine powder before adding to your soil or compost, and maybe the worms will use it.

  • So, is gardenmyths a site where myths are perpetuated, or debunked? The author based the premise on "It didn't look to me like they did," and seems to think that composting = dissolving in the soil, and completely ignores microbial activity, which is almost the entire composting mechanism. Having said that, yes, a finer grind will probably increase how quickly it gets used or broken down, though having the shards there probably affords some protection from slugs and types of grub/worm pests. Mar 29 '17 at 16:05
  • I'm not aware of any benefit from egg shells against any pests :( Which bacteria breaks down shells? Mostly the decomposing bacteria in compost heaps and the ground break down organic compounds, and shells are almost entirely calcium carbonate crystals i.e. inorganic. Mar 30 '17 at 4:30
  • Where do you get the idea that microbes don't effect inorganic materials as well as organic? - "Microbes interact with metals and minerals in natural and synthetic environments, altering their physical and chemical state, with metals and minerals also able to affect microbial growth, activity and survival. In addition, many minerals are biogenic in origin"..."Most biominerals are calcium carbonates, silicates and iron oxides or sulfides..." - Metals, minerals and microbes: geomicrobiology and bioremediation - microbiologyresearch.org/docserver/fulltext/micro/156/3/609.pdf Mar 30 '17 at 14:10
  • And these are the same bacteriae that perform nitrification?? Mar 31 '17 at 3:09

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