Egg shells..... A step further
If you take your ground up shells and put them in a 5 gallon bucket, and add a 2 to 1 ratio of white vinegar (by volume) to shells and get, at first, a lot of foaming! But if you let it sit for a day the CO2 will be gone and you will have reduced the shells to calcium with a bit of phosphorus and water that tastes a bit vinegary. pour off the water and use on your plants. (Do this in a green house and let the plants harvest the CO2 that you released!)
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