I eat a fair amount of eggs at home, and while we all know ways to recycle the egg cartons, I was wondering what to do with the leftover eggshells. Specifically, I wanted to know if it was possible to compost eggshells in my compost bin. After some research and testing, here’s what I found.
Eggshells are great for compost as they contain many nutrients, especially calcium. Calcium benefits plants by increasing disease resistance and longevity of plant cells. Before adding to compost, eggshells can be crushed or powdered. Since crushed eggshells can take years to decompose, powdered is the preferred way.
So, while eggshells are good for compost, what exactly do they do to the soil and how are you supposed to use them in compost? Let’s take a further look.
Do Eggshells Make Good Compost?
Eggshells are a good addition to compost as they can balance out the pH, provide high amounts of calcium, and can be quickly decomposed while in powder form. Even though many items added to a compost pile contain various amounts of calcium, eggshells are by far some of the highest at 2.2 grams of calcium per egg.
Adding eggshells to your compost pile is especially beneficial as eggshells not only balance the pH of compost, but they can balance the nutrients, and in particular—the ratio of calcium compared to other nutrients found in compost. Most of the common items that go into compost have a high nutritional value, but few rival the potency of calcium in eggshells.
For more context, here are some of the most common items most add to their compost piles:
While any of the items on this list are great for compost (and contain high amounts of other nutrients such as nitrogen and potassium), if you find your soil is lacking in calcium, increasing the number of eggshells in your compost can be very beneficial.
How do you know if your soil is lacking in calcium?
Calcium deficiency can be noticed if the plant’s growth is slowed or stopped. Additionally, young leaves can be stunted with brown spots.
Is there a test to find out if your soil has too little or too much calcium?
As it turns out, most soil tests only analyze the levels of the main plant nutrients—nitrogen, phosphorus, and potassium (NPK). If you’re concerned about the overall nutrient balance in your soil, testing your soil is a good idea.
Testing the NPK of your soil sounds complicated, but it isn’t too difficult. If you’d like to see a test in action, check out the video below by California Gardening. You can find the mentioned soil test kit here on Amazon.
While testing for these three nutrients, and the soil’s pH, is a good place to start, it doesn’t really answer our question to determine if the calcium levels are sufficient in compost and soil.
As mentioned, currently, the best way to determine an over or under-abundance of calcium is to observe your plant and see if any of the new growth is stunted or developing brown spots. Fortunately, it’s very difficult to go overboard on the eggshells in compost as they’re a naturally occurring fertilizer and not a potent synthetic fertilizer.
So, at this point, I found that eggshells are great to add to my compost bin. But I still had questions. Specifically—what other nutrients are in eggshells, and how exactly should I process or use the eggshells?
Which Nutrients Are in Eggshells?
The primary nutrient in eggshells is calcium which makes up 95% of the nutrient profile. Trace nutrients include magnesium, phosphorus, potassium, sodium, zinc, manganese, iron, copper, fluoride, strontium, and selenium. While eggshells don’t offer a complete balance of nutrients for plants, calcium is a vital piece.
As mentioned, eggshells primarily contain calcium which makes up 95%, or 2.2 grams of calcium per egg (from commercial layers). The calcium in the eggshell is in the form of calcium carbonate, which is also what limestone and chalk are made of.
Other nutrients in eggshells aren’t nearly as concentrated, with phosphorus and magnesium being the next two largest nutrients, coming in at 0.3% each.
While these two nutrients are only found in small amounts in eggshells, they still bring their benefits to the compost.
Phosphorus promotes root and blossom health, while magnesium promotes overall plant growth and photosynthesis.
Even though eggshells have some phosphorus and magnesium, there are far better ways to get these nutrients in your compost (for example, adding bone meal and Epsom salt). So, aside from calcium, I wouldn’t suggest relying on eggshells for many nutrients.
What’s the pH of Eggshells?
The pH of eggshells is 6.8, which is just slightly acidic. With 7.0 being neutral on the pH scale, eggshells are a good addition to help balance the pH of compost. The goal is to get compost’s pH as close to neutral as possible. When applying compost, the pH can then be adjusted to match the needs of the plant.
For example, citrus trees like more acidic soil (a pH of 6.0-7.0), so just before applying the compost, you can mix it with more coffee grounds, peat moss, sand, or pine needles to increase the acidity.
Unfortunately, pH is an often overlooked part of the health of compost and soil.
Without a proper pH balance, the plant’s roots will become blocked from absorbing the proper levels of nutrients from the soil. Over time, this can lead to issues such as leaf loss and eventually, killing the plant.
So, when you’re using eggshells and other compostable items in your pile or bin, it’s a good practice to occasionally check the compost’s pH. If you’d like an easy way to do this, you can get a simple pH soil meter. To see which pH meter I recommend, check out my recommended tools page here.
How To Use Eggshells in Compost
- Crush the eggshells into small pieces (you can do this with your hands or a potato masher)
- Scatter them evenly over your compost pile or mix them in
- Spread the eggshells over a baking sheet
- Bake the eggshells on the sheet at 200ºF for 30 minutes or until completely dry
- Put them into a blender with a pulse setting until the eggshells are finely powdered
- Sprinkle evenly over your compost pile
As you can see, the two main ways to use eggshells in compost is to either crush or powder them.
But how do you know which one to do?
When deciding on crushing or powdering, the deciding factor comes down to how fast you’d like the eggshells to decompose.
How Long Does It Take for Eggshells To Decompose?
Powdered eggshells can take between a few weeks to a couple months to completely decompose, depending on the type of composting involved. Crushed eggshells have a much larger surface area, and similar to seashells—they can take many years to completely decompose. For the fastest decomposition, powdered works best.
If you ever see snail shells or seashells in your garden, you might notice that it takes a long time for them to break down. This is because they’re made of calcium carbonate. Since eggshells are almost entirely made of this same material, they have a similar, if not identical decomposition process.
So, should you crush or powder eggshells?
As a general rule, if I’m adding eggshells directly to the soil, I prefer crushed eggshells. If I’m adding them to my compost bin, I powder them.
If you’re frequently applying compost to your plants, using crushed eggshells will be just fine as they’ll break down in the soil over time and release a slow and steady supply of calcium.
However, if you have a vermicompost bin (like me), or need to infuse a fast and strong dose of calcium into your soil, then powdering them is a better idea.
In vermicompost bins, crushed eggshells can quickly add up and pose as an obstacle for the worms.
Additionally, if you find your plants have a calcium deficiency from stunted leaves or lack of growth and would like to apply the amendment sooner than later, then powdering them is the better method.
Why Do Powdered Eggshells Decompose Faster Than Crushed?
Powdered eggshells have a much smaller surface area than crushed eggshells, which makes their decomposition rate much faster.
For example, when cooking a chicken breast, it can sometimes take 30-60 minutes to fully cook. But if you cut the chicken breast into strips, it takes a fraction of the time to cook. The same is true for the decomposition of eggshells in compost. The smaller the surface area, the faster the soil microbes can get to work.
While crushed eggshells will do just fine in your compost, if you eat a lot of eggs (like me), then consider powdering them to prevent them from piling up.
How Many Eggshells Should You Use?
For the standard compost pile size of 3 feet long, wide, and deep, you should be using no more than 2/3 cup of powdered eggshells. This is based on the average calcium level found in compost (3%). However, if you happen to go a bit over this amount, it likely won’t cause any harm to the compost or the plant.
Here’s the math in case you’re interested:
Since the average compost pile contains 3% calcium, and eggshells are 95% calcium, eggshells should also make up about 3% of the compost. Because the average size of a compost pile is 3 feet long, wide, and deep, it has a volume of 27 cubic feet. 3% of this or, about 10 cubic inches, should be eggshells. This is equal to 5.5 fluid ounces or 2/3 cup.
According to the state of California’s recycling website, the average nutrient levels in compost typically contain 3.28% calcium (if you want to be exact). Again, this is for the average compost pile of 3 feet long, wide, and deep (see image below).
Still, since eggshells are almost entirely calcium, and excess calcium has less impact on plants than other nutrients (like nitrogen), it’s hard to go overboard.
If you happen to go over the recommended amount of calcium from eggshells, it will most likely not harm your plant. Just make sure to add other items to your compost with varying nutrients to balance it out (especially carbon-rich brown material such as dry leaves, branches, and pine needles).
Can You Add Eggshells Directly to Soil (Without Composting)?
If you don’t need the eggshells to decompose fast, you can add eggshells directly to your plant’s soil. The best way to do this is to crush and place them under the first 2 inches of soil. Since some plants have shallow roots, be careful not to damage or undercover them in the process.
As you can see in the photo above, I added a few eggs worth of eggshells to my potted Meyer lemon tree and proceeded to bury them a couple inches below the top of the soil. They’ll break down slowly, but now I don’t have to worry too much about improper calcium in the soil.
As an extra measure, I recently applied two inches of compost to the top of the soil.
Remember, other food and plant scraps in compost will provide also calcium, so there’s no need to rely solely on eggshells for this nutrient.
Either way, eggshells are a great addition for compost and promote the hardiness of plants as well as make sure their new growth doesn’t get stunted. Choose whether you’d like to crush or powder your eggshells and add to your compost as you see fit. Happy gardening!