Water soluble calcium made from egg shells

Not too long ago a friend asked us why we separate our egg shells form the rest of our compost and the answer is because they are so precious!

There are many applications for eggshells that are more efficient to recycle the nutrients that they contain since they don’t break down readily on their own. If you have ever made compost, even when your compost is finished and you have turned it several times in the process you always find small broken pieces of eggshells. This happens because they don’t really break down into simpler components.

In Costa Rica and Latin America presence of snails in gardens is common, this is not only bad because they eat leafy greens but because snails are hosts/carriers of a parasite related to rat lung worm, this nematode called Angiostrongylus costaricensis can cause severe intestinal damage in humans if it or its larvae are ingested accidentally. Keeping snails away from vegetable gardens is therefore very important. It is a common practice amongst gardeners to dry and sometimes toast eggshells and routinely crush them and sprinkle them around vegetables or garden beds. It is said that snails have a hard time crossing over crushed eggshells as it may cut or harm them.

This is one of the simpler applications for eggshells that may be more helpful than composting them alongside the rest of organic refuse. However, the more interesting way of recycling eggshells that we use is rendering the calcium of the shells available in a water soluble compound through a chemical reaction.

Since eggshells don’t really break down or at least it takes a very long time for them to do so, you could say they are almost petrified in a way. Their components are not readily available for plants to uptake and it will take fungi a longer time to process them and break up into their core elements that then plants can absorb. Through a reaction process of eggshells and acetic acid you can obtain a substance called WCA (Water Soluble Calcium) in a short time. In combination with Epsom salts you can obtain CAlMag. WCA also can be mixed with many other inputs according to KNF (Korean Natural Farming) recipes. WCAP (Water Soluble Calcium Phosphate) is another substance that is made through the reaction of acetic acid and charred bones. Some recipes I have read say that if you char your eggshells to the point where they are black you can obtain some phosphate and thus WCAP in the process. But I have not confirmed this with other sources.

Calcium is important to build cell walls and can help with fungal problems; phosphate is important during the flowering and fruiting stages of plants. We have used WCA successfully in a preventative mango treatment (during the pre-flowering, flowering as well as fruit setting stages) that we used to reduce powdery mildew and anthracnose in combination with alternating applications of LAB (Lactic Acid Bacteria), Neem oil, mild fish hydrolisate (as we want the minerals vs. high nitrogen) and FPJ (Fermented Plant Juice)/foliar fruit fertilizer.

To the method!


  • Start by separating your eggshells from the rest of the compost. A good tip is briefly rinsing them and placing them in a basket or tray where they can dry off. Also, try not to place one half of a shell inside the other half as this will make the process more complicated later on. I try to place them side by side one facing belly up the other belly down. No biggie if some end up stuck together.
  • Once you have accumulated enough, get a large metal tray/pan and crush your dry egg shells on it. You want them in small pieces around 1/4 inch or 1cm no need to be exact at all. I crush them with my hands wearing thick gloves.
  • Then proceed to toast/char them, you can do this in the oven, on a grill, open fire/embers or with a torch. I use a torch. You want to proceed little by little, they should evenly turn brown. Mix your shells often so you get them on all sides. During this process some of the inner membrane of the eggs will be released. As you are toasting the shells you can blow this membrane off as you want as little of it as possible for the next step. All shell pieces should be browned and brittle.
  • Once your shells are evenly charred and have brown all over and feel brittle it is time to mix with vinegar. Some recipes say to use distilled vinegar I prefer vinegar with the mother and if possible one that has been made onsite from leftover fruit. Making vinegar is a simple process that involves fermenting fruit, with cane juice or sugar and water. You will need approximately one part charred eggshells to ten parts vinegar. You can mark your container with 10 equally separated lines to know the ratio. The mixture of these two ingredients will create a reaction. You will need a glass container and it should have enough headspace as the mixture will bubble a lot. Put your shells in the container first. Then slowly add the vinegar and watch it fizz. The eggshells should move up and down and there should be bubbles coming from in between the eggshells that are sitting in the bottom of the jar. If you do not see this movement you did not cook the shells enough.
  • Proceed to cover your jar with something that lets air out, e.g. paper towel and rubber band or cheesecloth and rubber band. Place it in a dark place away from direct sunlight. You should let the mixture sit for at leas 7 days. You can let it react for longer too, longer sitting will not degrade the end product. 10-15 days is optimal.
  • WCA is shelf stable. Sift mixture. You can use the leftover shells as a soil additive. If you use a live vinegar it is recommended to store in a dark bottle, e.g. wine bottle and with a lid that allows pressure to be released.
  • WCA is used at a rate of 1:1000 or 1:500. Test and see what works best for you.

Here is a document from the University of Hawaii on the topic: https://www.ctahr.hawaii.edu/oc/freepubs/pdf/SA-10.pdf.