Many strange gardening hacks circulate the internet, from coffee grounds to banana peels to tea bags. Purportedly, adding these household materials to your garden soil improves plant growth. Many sources recommend adding eggshells to tomato plants for a fertilizer boost. Does science back this claim, or is it an urban soil myth?
Eggshells are a rich source of calcium carbonate and trace amounts of magnesium, phosphorus, and other minerals.
They can be a helpful mineral fertilizer for tomato plants, but there are some key caveats to understand. Let’s dig into everything you need to know about using eggshells in your tomato beds.
The Short Answer
Eggshells can be a good source of calcium and organic matter for tomatoes, but they are unlikely to yield a major improvement in soil calcium levels. You can add eggshells to your garden by drying, crushing, and incorporating them into the soil, or by composting them.
University research indicates that composting is the best way to make the minerals in eggshells available to your plants. Some anecdotal reports also suggest that dried eggshells can also deter slugs from feeding on young tomato transplants because the sharp edges of the shell dehydrate the slug’s body, but there isn’t much evidence or science to prove this claim.
The Long Answer
Eggshells can supplement calcium for tomato plants.
Depending on how you add them to your garden, eggshells may provide some nutrient and pest-deterrent benefits. The most notable reason you’d want to add eggshells to tomato plants is as a calcium enhancement to reduce blossom end rot.
Some evidence suggests that calcium deficiency is linked to blossom end rot in tomatoes, while others find that a lack of calcium does not cause blossom end rot, and the issue is far more complex. Both studies have the same basic causes listed for why a plant cannot uptake calcium from the soil, so in both cases, they’re right!
Typically, the issues that impact tomatoes have less to do with the quantity of calcium in the soil and more to do with environmental concerns, as both studies state. Inconsistent watering or extensive drought can produce blossom end rot, for instance.
Similarly, an excess of nutrients in the soil (particularly magnesium, ammonium-based nitrogen, or potassium overloads) can reduce the ability of a plant to absorb calcium from the soil. Plant stresses caused by diseases can also be the culprit, particularly any diseases that girdle the plant’s stem and cause a reduction in moisture transfer through the plant’s tissues.
So, does calcium from eggshells help to prevent blossom end rot? Yes and no. Eggshells are almost pure calcium carbonate, but they have to be fully broken down to be absorbed by the plants, so there’s a time issue involved.
In addition, most garden soil isn’t lacking in calcium, despite the symptoms of blossom end rot in a plant. In most cases, something else is a key factor. The extra calcium generally won’t hurt (unless you add an excess to the soil), but generally, it’s not going to fix the issue!
Benefits of Eggshells
Eggshells benefit gardens with calcium and nutrients, aiding soil health and plant growth.
These kitchen scraps can benefit your garden in a few ways, but there isn’t significant scientific evidence supporting any of them. Eggshells are mostly made of calcium carbonate crystals, which can contribute to soil health. This rich source of calcium helps plants build stronger cell walls. The shells also add nitrogen, potassium, and chloride.
Additionally, an egg’s alkaline shell and membrane may aid in balancing soil pH in acidic environments, but the effects will be very small and localized.
The best above-ground benefit of eggshells is reputed to be pest control. Many gardeners use them to protect young tomato plants from slug damage.
The jagged texture may deter and dehydrate slugs by damaging their tender skin. However, if it rains, the shells become soggy and less effective; any preventative effects are very short-lived and often do not work.
Role of Soil Calcium
Soil calcium is crucial for plant growth. Imbalances impact crops as vitamin excesses affect humans.
Soil calcium is a crucial nutrient for plant growth. Too much or too little calcium can cause major problems in crops, just like excesses and deficiencies of specific vitamins can cause problems in humans.
Soil calcium plays a role in:
- Forming plant cell walls
- Uptaking soil nutrients
- Activating plant enzymes
- Improving disease resistance
- Creating soil aggregates (soil texture that clumps together)
- Regulating plant hormones like auxins (which influence root development)
- Developing fruits and seeds (important for tomatoes!)
In the soil, calcium interacts closely with magnesium. The ratio of these two minerals can affect how plants uptake other nutrients (like potassium) or how the soil clumps (“flocculates”) together. Without getting into complex soil science, it suffices to say that soil calcium is vital for a thriving garden and healthy tomato plants.
Calcium deficiencies are most efficiently addressed with agricultural lime (calcium carbonate) or dolomitic lime amendments. Eggshells can help improve soil calcium levels but may take longer than traditional fertilizers.
How to Use Eggshells as Fertilizer
Eggshells in tomato beds need microbial decomposition to become beneficial organic fertilizers for plants.
Throwing an eggshell into your tomato bed may seem like a quick fix, but it’s unlikely to contribute anything to plant health. The key to any organic fertilizer is decomposition.
The shells must be broken down by microbial activity to become available to your plants. Once decomposed, they can also buffer the soil against changes in acidity.
Follow these steps to use eggshells as a tomato fertilizer:
1. Clean Residues (Optional)
Gather eggshells from your kitchen and clean them before storing them.
Begin by collecting eggshells from your kitchen and washing them off before storage. After each use, I like to rinse the shells and collect them in an empty egg carton until I have at least two dozen.
You can also collect them in a kitchen compost bin without washing them off. As long as you provide the proper nitrogen-to-carbon ratio in your compost, the membranes and remnants of egg white should not be an issue.
2. Bake Them in an Oven (Optional)
Crush and bake eggshells for better garden use.
Some sources suggest baking the shells to make them easier to crush. On an industrial scale, eggshell lime is put in an oven to make them more brittle. Then, the residues are crushed and pass through 2 mm sieves.
This is a lot of work for a home gardener, but remember that the smaller you crush your eggshells, the better. If you have time to dehydrate in the oven, go for it!
3. Crush the Shells Into a Fine Powder
Crushed shells aid microorganism decomposition, influencing the breakdown of large objects.
Crushing the shells makes it easier for microorganisms to break them down. This explains why giant watermelons or big tree limbs take forever to decompose, while grass clippings seem to compost in mere days!
Some studies suggest that hand-crushing eggshells is insufficient to ensure adequate soil incorporation. Instead, blend them in a food processor or blender to make a fine powder. If they haven’t fully dried, they may clump together rather than form a powder. This material can still feed the soil microbes but may be more difficult to spread.
A helpful hint here: when you blend your eggshells, do not immediately open the blender or food processor afterward. Blend them to a fine powder, then let them sit for 10-15 minutes before opening the blender. This allows the airborne particulate in the blender to fall back down rather than come out in a cloud of fine dust in your kitchen!
4. Add to Compost or Add Directly to Soil
Crushed eggshells can enhance the soil with calcium when added to compost.
Adding your eggshells to a compost bin is the best way to make the calcium readily available. Some gardeners put the crushed powder into the planting hole with their tomatoes, but the efficacy of this method depends on how many microbes are present in your soil.
If you have a rich, loamy garden bed, your tomato plants will quickly absorb and utilize the powder. However, in poor soils without much life, pre-composting the shells will make them a far more beneficial amendment.
These shells are also a great addition to a worm bin. The slow-release calcium improves the vermicompost’s quality and prevents acidic conditions in your worm bin. The worms can also use powdered eggshells as grit to aid their digestive process. Mix crushed eggshells in with worm bedding (like shredded newspaper) or sprinkle the powder in with your food scraps.
Do Eggshells Deter Slugs?
Using slug bait like Sluggo effectively keeps slugs away from eating plants.
The ragged edges of dried eggshells supposedly deter slugs by cutting through their soft bodies. This could be useful for spreading around young tomato plants attacked by these slimy pests.
However, in this experiment with lettuce, the myth-busting team at AllAboutSlugs put shells to the test and found that they don’t do anything to stop slugs from eating your plants. You are better off using an organic slug and snail bait.
This garden hack may improve soil calcium around your tomatoes and prevent problems with blossom end rot. The key is to crush and compost the shells first to make the minerals available to your plants. All in all, there is no risk of adding eggshells to your tomato beds. They cannot harm your tomatoes, but they may not provide a noticeable benefit.