You can use eggs shells to make a cheap calcium supplement for your garden. One thing that we have a lot of in our home is eggs! I have been trying to come with alternate ways of using the shells that we discard to the compost for several reasons and have had very good luck with this fertilizer as an option.
For the past year as our chickens matured and we started really getting full scale egg production I have been just throwing the egg shells in the compost. There is nothing wrong with doing this, however, with the amount of eggs we are now using now the compost is starting to show signs of too many shells. This technically isn't going to hurt anything but I have noticed that it takes quite some time for the shells to fully break down and they also are bringing the pH down quite a bit in the compost. So, with that being the case I needed to come up with a better way.
In our aquaponics system I am currently growing tomato plants which do require some calcium for proper growth especially during the flowering stages. I figured I would start by adding some calcium to the system using the egg shells that we already have. As you know, egg shells are made up of mainly calcium carbonate. The problems is getting rid of that carbonate which I do not need in my system and which is responsible for bring down the pH in the compost and garden soil. The easiest way to separate the carbonate from the calcium is to add an acid to the shells to break them down. The cheapest and safest type of acid that we use around our home is everyday white vinegar. Vinegar is normally only about 5% in a water solution but this does the trick just fine.
So the first step is to break down the shells by grinding them up into small pieces. But if you have ever tried to grind fresh egg shells you quickly realize it is no easy chore. So, to the oven for about 15 minutes first to bake them until brittle. This also cleans off any left over egg residue. Once baked throw them in a glass bowl and grind them using whatever method you choose.
Once you have the egg shells ground up dump the grit into a jar and then add your vinegar. Now sit back and watch the fizzy show! This is fun for the kids too because the acetic acid (vinegar) reacts with the Calcium Carbonate (egg shells) and makes lots of carbon dioxide (CO2) bubbles as it breaks them down. This can be a pretty wild bubbly reaction for about an hour and it is quite entertaining to watch:) The acetic acid and calcium carbonate are reacting to form calcium acetate, water, and carbon dioxide.
Once this reaction has had time to settle out for a couple of days you are left with a calcium rich solution that is not alkaline for your soil. You can then dilute just a bit f this with water and use it on your garden beds, in an aquaponics system, or hydroponics system. This is a much more available nutrient form then calcium carbonate and allows for the plants to instantly have access to the calcium instead of waiting years for the egg shells to break down in nature.
You can test the results of the calcium fertilizer with the API calcium test kit. Just add some of this solution to a bucket of water and test the before and after levels of CA2+ (calcium). You will be surprised at how much it raises the levels of calcium with just a small amount added. I also use this kit to monitor the levels of calcium in my aquaponics system. While this is a micro nutrient and not as important as the big three (NPK) it is still important for certain plants, especially tomatoes.