Last Updated: April 7, 2023
You’ve probably heard that adding eggshells to your garden soil is a good way to improve it. This old gardener’s trick is rooted in some fact, but what about the proof?
Eggshells are touted as being particularly good for tomato plants. The reason is that they can (apparently) help prevent blossom end rot. This would be great, but is it true?
What are eggshells made of?
First, let’s look at what an eggshell is actually made of. The shell is almost entirely composed of calcium carbonate, which is also what seashells are made of.
Elements found in eggshells:
- Other trace elements
When you see these ingredients, you may think, “that looks like a nice fertilizer-booster for the garden!” It is true, these elements are all essential for healthy plant growth.
The problem is that these elements are bound up in chemical compounds in the eggshells. In order for plants to use them, the shells need to be broken down first.
Plus, bone meal and all purpose fertilizers can easily add calcium and other nutrients to your soil without the hassle involved with using eggshells. However, I understand the desire to use ingredients that would otherwise go to waste.
With that said, if your soil needs calcium, eggshells can provide it. Get a soil test and understand what your garden soil already has in it. If there is plenty of calcium, then there is not really a need to add eggshells.
Do eggshells break down in soil?
So, can soil break down eggshells and make those important nutrients available for tomatoes and other vegetable plants? The answer isn’t as simple as yes or no. Here are some important points:
- Eggshells are not water-soluble. Since eggshells don’t dissolve in water (think about what happens when you hard boil an egg), they won’t just break down when it rains. This means that some other mechanism will have to release the nutrients.
- Hand-crushed eggshells don’t break down. In this study, hand-crushed eggshells were added to soil and compared with untreated soil. The eggshells did not change the nutrient makeup or the pH of the soil. In other words, if you are simply crushing your eggshells up into small pieces, they are not going to break down and release their nutrients.
- Finely ground eggshells can break down. In the same study above, finely ground eggshells showed a change in soil pH. However, the original soil was very acidic (pH 4.9). This is important, because calcium carbonate dissolves in acidic conditions. Without an acidic environment, the eggshells would not break down, finely ground or not.
So, in order to see a benefit from eggshells in the garden, they first need to be ground into a fine powder. This isn’t a big deal, a simple spice grinder can get that done in about a minute.
However, if your soil is not acidic (below ~6.5 pH), then the powdered eggshells still won’t break down. So if you want to guarantee the eggshells will actually provide some benefit, you have to ensure an acidic environment.
How to use eggshells in the garden (properly)
With all of that information to consider, what is the best way to use eggshells for growing tomatoes and other veggies?
First, you need to perform a soil test. Broken down eggshells primarily add calcium to the soil. Therefore, there isn’t a need to add them unless you know that your soil lacks calcium.
If you need to add calcium to your soil, there is a method that quickly frees up the calcium in eggshells.
The vinegar method
Like I said earlier, calcium carbonate breaks down in acidic conditions. Therefore, we can use common white vinegar to break down the eggshells into water-soluble calcium.
How to break down eggshells for the garden:
- Sterilize and dry the eggshells. Rinse, dry, and lightly crush the eggshells and place them on a baking sheet in a single layer.
- Bake at 200°F for 30 minutes. Baking the eggshells will dry them out and will also kill bacteria that may be on the eggshells. If the eggshells are still moist after 30 minutes, continue baking until completely dry.
- Pulverize the eggshells in a coffee grinder. This step is important. By finely grinding the eggshells, you are making the most of them. Powdered eggshells will break down much more quickly and will provide the most nutrient content in the end.
- Add 1 part vinegar to 1 part eggshells. In a bowl, add 1 tbsp of vinegar for every 1 tbsp of ground eggshells. The mixture will quickly begin to bubble and fizzle as the chemical reaction begins. Essentially, the vinegar and calcium carbonate react to produce carbon dioxide and water soluble calcium.
- Allow the mixture to sit for 1-2 hours. After a few minutes, the bubbles will subside. Allow the reaction to complete by letting it sit for a few hours. Some even recommend waiting up to a week before use.
- Add 1-2 tsp per gallon of water. Add 1-2 teaspoons of the vinegar/eggshell mixture per gallon of water. This will infuse the water with calcium, and can be used to water your tomato plants.
While using vinegar is relatively simple and works to add calcium to the soil, it can also be washed away by heavy rain or irrigation.
Garden lime is a cheap alternative that can add calcium and raise soil pH more effectively than eggshells.
The composting method
If using vinegar sounds like too much of a hassle, you can try composting your eggshells. Once again, it is still important to grind the eggshells into a powder first.
While compost is “cooking,” it goes through swings in pH. To overly simplify it, compost typically starts out more acidic, and ends up more alkaline when finished.
As I discussed, eggshells require acidic conditions in order to break down, so compost can help achieve this. However, it is important to add eggshells to a brand new compost pile, not to an older pile or bin.
This ensures that the eggshells are exposed to as much possible acidity during the process, releasing the maximum nutrient content.
Top tips for composting eggshells:
- Don’t add hand-crushed eggshells (finely grind them instead)
- Add powdered eggshells to a new compost pile, not an old one
Is it worth using eggshells in the garden?
With all of the steps that are required to use eggshells for gardening, is it worth it? I think that will vary from one gardener to the next.
- Uses up more kitchen waste
- Free resource
- Most soil does not need calcium
- Must use a spice grinder
- Larger particles may not break down for many years
- Vinegar solution can wash away in rain
Essentially, eggshells are often thrown away, destined for a landfill, so why not save them? With a bit of chemistry, or help from composting conditions, they can release their helpful nutrients.
So what is my opinion? I would recommend using the vinegar method (above) for potted tomato and pepper plants. This can potentially help avoid deficiencies in calcium, especially over longer growing seasons in warm climates.
However, if my ground soil was lacking in calcium or was too acidic (it isn’t), I would prefer to use garden lime as a reliable solution. Lime has been tested, is widely available, and we know how much to add to change pH.
Don’t let my opinion change what has worked for you. If you add eggshells to your compost, feel free to keep doing it. It shouldn’t harm the soil, and under ideal conditions, it may help amend it with beneficial nutrients.