Are Eggshells Good for Plants? (Numbers and Facts)

More people are composting their kitchen scraps than ever before. Eggshells are often added to compost piles or in the planting holes to increase calcium levels in the soil. Does this really work?

Eggshells have many uses in the garden. Here is what you need to know to make the best use of them.

Are Eggshells Good for Plants?

Eggshells can be used to deter pests in the garden so they don’t eat your plants. They also make biodegradable seed starting pots, aid drainage in potted plants, and give birds needed calcium when mixed with birdseed. Finally, they can add calcium to the soil and can raise the pH of the soil, but only when ground into a fine powder.

Why are Eggshells Good for Plants?

Eggshells are traditionally used when gardening by many people. According to Butcher and Miles, the average commercially produced eggshell contains:

  • 95% Calcium Carbonate
  • 0.3% Phosphorus
  • 0.3 % Magnesium
  • Traces of sodium, potassium, zinc, manganese, iron and copper

Here are the most common ways eggshells are used.

Biodegradable Seed Starting Containers

Biodegradable Seed Starting Containers

Use can your deeper eggshells to start seeds in. Heat them in a 200-degree Fahrenheit oven for 30 minutes to sterilize them. After they cool, make one hole in the bottom of the eggshell. Fill with seed starting mix and plant your seeds in them. 

When the plants are ready to put outside, simply dig a hole and place the eggshell in it. The roots will grow through the drainage hole in the bottom and split the eggshell, which them slowly decomposes to add calcium to your soil. 

Plants with slow-growing roots may not be able to break through the eggshell but just grow in a circle inside the eggshell. This will eventually girdle the plant and kill it. 

Prevent Snails 

Snails and slugs can be very destructive to a garden. Many people put crushed eggshells around plants as a barrier to snails and slugs. This works, but only if the soil is dry. When it rains, the eggshells become too soft to work. Fresh eggshells must be put out after a rain. 

Help Drainage in Potted Plants 

Coarsely crushed eggshells can be placed in the bottom of pots to improve drainage and air circulation. Place a layer about an inch thick in the bottom of the pot, then fill the pot with potting soil. The shells let water drain and provide space for airflow, which helps prevent diseases. 

Feed Birds Calcium 

Just like chickens, wild birds need calcium in their diet to make their eggshells. Crush sterilized eggshells and mix them with birdseed. When the birds feed, they can also get extra calcium by eating eggshells. 

Deter Cats and Deer 

Both cats and deer are said to be repelled by the smell of unwashed eggshells. Cats are also said to dislike walking on crushed eggshells, potentially keeping them from using your garden as a bathroom. However, rats love eggshells, so you may cause more problems with this repellant than you solve.

Add Calcium

Eggshells Add Calcium

Blossom end rot is caused by a problem with the plant’s calcium level. Since eggshells contain calcium, many people put the crushed shells in a hole before planting tomatoes and other vegetables. 

However, while it is true that blossom end rot is caused by a calcium deficiency, the problem is not usually a lack of calcium in the soil. The problem is inadequate water or damage to the roots. Calcium enters the roots and is carried throughout the plant. 

If there is no water to transport the calcium, the plant has a problem. Damage to the roots when removing weeds is a common cause of inadequate water uptake, as is drought. 

Properly prepared eggshells will add calcium to the soil or a compost pile. However, adding crushed shells when planting or to the compost pile will not help until they decompose. Crushed eggshells decompose slowly over a year so do not help the immediate problem of blossom end rot or other low calcium problems.

Increase Soil pH

According to Mitchell, an agronomist with Alabama Extension, eggshells can be used to increase the pH of the soil. However, crushed eggshells are no better than a control area that was not treated at all. Shells must be finely ground to raise the pH of the soil.

Feed Soil Micro-Organisms

Ground eggshells placed in a worm bin give worms badly needed grist so they can digest their food. In a compost pile, they are available to other microbes so they can break down the organic matter in the compost.

How Do You Prepare Eggshells to use in the Garden?

While crushed eggshells may have a use as a pest deterrent and for some other uses in the garden, they do not help add calcium to the soil or raise the soil pH. For those uses, you will need to prepare the eggshells in one of two ways.

Grind the Eggshells

Grind The Eggshells

Grind the eggshells into a fine powder. A cheap coffee grinder or a mortar and pedestal will work for this project.

  1. Dry the eggshells. Putting them in the oven for 30 minutes at 200 degrees Fahrenheit will dry the eggshells and sterilize them. Alternatively, you can spread them out on some newspaper and let them dry for a few days.
  2. Crush the eggshells by hand.
  3. Put the eggshells in the coffee grinder or mortar and pedestal.
  4. Grind the eggshells into a fine powder.
  5. Store the ground eggshells in a moisture-proof container such as a glass jar.
  6. Sprinkle the ground eggshells around your plants, in your worm bin, or on the compost pile.

Boil the Eggshells

As an alternative to grinding the eggshells, you can boil them.

  1. Put 10-20 eggshells in a pot.
  2. Cover the shells with water.
  3. Bring the pot to a rolling boil.
  4. Let the water sit overnight.
  5. Strain the eggshells out of the water.
  6. Use the water to water your plants.

This process makes the calcium water-soluble so the plants can take it up easily. Each eggshell adds four milligrams of calcium. Using this water as a foliar spray makes it easy for plants to take in the calcium and use it.

Where to use Eggshells?

From a food safety presentation, make sure you sterilize the eggshells before using them. The best use is to add calcium to your compost pile rather than add it directly to the plants in your garden. You can then add compost to your garden without worrying about spreading salmonella or other egg-borne diseases to your plants, especially to vegetables. 

If you are trying to raise the calcium level of your soil, it is better to have a soil sample analyzed to see if you need it first. The same is true if you are trying to raise the pH level. When you send in your soil sample, write down what you hope to grow in this soil and the soil lab will specify the amount of calcium to add to the soil for either purpose. 

You can use eggshell powder as your calcium source. Eggshells contain 59% calcium as pure calcium carbonate. You will need to increase the amount of eggshell powder you use to compensate for the difference between the calcium in eggshells and pure calcium carbonate. 

What Plants Do Eggshells Help? 

Eggshells are said to help many plants.  Here is a partial list: 

  • Tomatoes 
  • Roses 
  • Cucumbers 
  • Indoor plants 
  • Flowers 
  • Squash 

However, eggshells do not help specific plants, they help specific soil conditions. As mentioned above, they add calcium to the soil when finely ground. They can also be used to raise the soil pH. When added to worm bins and compost piles, they feed beneficial soil microorganisms. 

Cautions about using Eggshells 

Like anything, eggshells can be overused. Too much calcium is toxic to plants. In addition, eggshells will increase the soil pH if added regularly. Again, the best use for eggshells is putting them in the compost heap. 

Frequently Asked Questions

We receive many questions about using eggshells. Here are some of the most frequently asked questions. 

Should You Wash Eggshells Before Composting? 

Salmonella is a type of food poisoning that you can get from raw or undercooked eggs. According to food safety experts, you should always wash your hands after touching eggs or eggshells with egg residue. If you sterilize the eggshells, Salmonella isn’t a concern. 

However, if you process unsterilized eggshells into dry powder, you may worry if you are contaminating your compost pile or garden area. You may have a problem if you apply the eggshell powder directly to your plants. 

Composting usually solves the problem. A hot compost pile, which reaches a temperature of 140-160 degrees Fahrenheit, sterilizes all the pathogens in the compost pile as well as weed seeds, so salmonella is not a problem. Even if you use a cool compost method, the composting process leaves the level of salmonella in the compost at the same as in the surrounding soil.   

How Long do Eggshells take to Decompose? 

Crushed eggshells take approximately a year to compost. Ground eggshells compost in a few months. 

Are Eggshells Good for Indoor Plants? 

No, crushed eggshells are not good for indoor plants except at the bottom of a pot to increase drainage. Ground eggshells added to the soil in a pot add too much calcium for the small amount of soil and can make the potting soil pH too high. Enough eggshell powder can cause a toxic level of calcium, killing the plant.

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Stephanie Suesan Smith

Stephanie Suesan Smith has a Ph.D. in psychology that she mainly uses to train her dog. She has been a freelance writer since 1991. She has been writing for the web since 2010. Dr. Smith has been a master gardener since 2001 and writes extensively on gardening. She has advanced training in vegetables and entomology but learned to garden from her father.

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