How to extract plant available calcium from shells
This season in our home garden, the oranges, tangerines, and pomelos are developing nicely. At this stage, the trees require a relatively high amount of calcium (Ca), an essential mineral for fruit development. An adequate amount of calcium makes the fruit skins firmer and slows down the ripening process (resulting in higher sugar content). The result is that fruits stay on the tree longer and can be stored for longer after harvest. In addition, fruits are less likely to be affected by pests compared to those from calcium-deficient trees.
Why not just throw eggshells or clam shells directly into the soil?
Eggshells and clam shells (or other shell-bearing aquatic animals) are composed of 99% calcium carbonate. However, plants cannot absorb calcium in this form. Calcium carbonate needs to be converted into a more soluble form before plants can absorb it. If you scatter the shells in your garden, they will take at least 3 years to decompose—possibly even longer if the soil has low microbial activity and low acidity. Doing it this way won’t guarantee that the plants receive enough calcium during fruit development.
How to extract calcium from eggshells or clam shells
The easiest way to extract calcium from shells is to mix them with vinegar or another type of acid. When calcium carbonate comes into contact with vinegar (acetic acid), the two react. During this reaction, CO₂ is released, and the resulting solution is calcium acetate (liquid calcium). This solution contains 7% calcium in a form that plants can absorb and can be used in the garden during the fruiting stage to support fruit development.
Recipe / Instructions
Wash the shells thoroughly to ensure that the liquid calcium can be stored for a long time without going bad. Then, dry-roast the shells in a pan or oven until they are completely dry and crush them into small pieces. After crushing, roast them again briefly to ensure they’re fully dry. Once cooled, place the shells in a clean, dry glass jar. Pour in vinegar following a ratio of 1 shell : 5 vinegar. You’ll see an immediate reaction between the shells and vinegar. Close the lid and leave it in a cool place for 3 days. After that, strain out the solids, keep the liquid, and store it in a cool place. This solution can be used for up to 6 months.
How to use
This liquid calcium can be used from the time the fruits begin to form until they start to ripen. To apply, dilute the liquid calcium in clean water at a ratio of 1:500, and spray the fruits 1–2 times per month, or as needed. Calcium does not travel easily within the plant through stems and leaves, so spraying liquid calcium directly on the fruit helps the plant absorb it more effectively. Additionally, to support nutrient absorption, you can mix in fish amino acid or worm tea (which act as chelating agents), also at a 1:500 ratio. This liquid calcium can be applied to all fruit-bearing trees.
References:
Effect of calcium on fruit quality: A review
https://www.researchgate.net/.../662c30d006ea.../download...
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