If a soil test reveals that your soil is lacking calcium, then you are probably looking for a way to supplement this important nutrient in your garden. Luckily, there are many options for high-calcium fertilizers.
So, what fertilizer is high in calcium? Fertilizers that are high in calcium include shells (egg, clam, or oyster), lime, gypsum, wood ash, bone meal, and calcium nitrate. Some of these will affect soil pH, so keep this in mind when you apply them, and get a soil test before doing so.
Of course, you can use a mixture of any of these sources of calcium, depending on what you have available.
Let’s get into more detail about each of these sources of calcium.
What Fertilizer Is High In Calcium?
Here is a table with some of the best fertilizers with high calcium content. You can find more detail about each type of calcium fertilizer after the table.
|Shells (egg, |
|Lime||37 to 40|
|Gypsum||20 to 23|
|Wood Ash||7 to 33|
|Bone Meal||15 to 22|
For more information, check out this page on fertilizers from the Oregon State University Extension, this page on fertilizers from the Penn State University Extension, and this page on soil amendments from the University of Maryland Extension.
Shells (Egg, Clam, and Oyster)
Shells from eggs, clams, and oysters contain lots of calcium, in the form of calcium carbonate. According to Scientific American, the shells are mostly calcium carbonate (about 98%), with the remaining 2% made up of protein and other trace minerals.
(As a side note, calcium carbonate is the same compound that makes up agricultural lime).
As with many fertilizers, shells are more potent and act more quickly when they are crushed into smaller pieces. Ideally, you should grind shells into fine powder to get the maximum benefit as a calcium supplement in your garden.
At this point, you are probably wondering how to crush shells into powder – a fair question!
For eggshells, break them up into smaller pieces and put them in a blender or coffee grinder. After they are crushed into powder, you can mix into water and pour the mixture into your compost pile.
For more information, check out my article on composting eggshells.
For clamshells and oyster shells, you are better off breaking them into small chunks with a hammer first (wear goggles if you do this!). Then, use a mortar and pestle to break the chunks of shell into smaller pieces.
Lime (calcium carbonate)
Lime is another excellent source of calcium for a garden, containing 37 to 40% calcium by weight.
Agricultural lime is often used for reducing the acidity of soils by raising the pH. However, you can also use it to supplement calcium in your garden.
Just make sure that the resulting pH change doesn’t throw your soil off balance. Keep in mind that if soil pH is too high or too low, certain nutrients become less available to plants.
For more information, check out this article from Research Gate on the effect of soil pH on nutrient availability.
Also keep in mind that there are various types of lime that you can use, including:
- Aragonite – this lime comes from oyster shells, and is mined off the coast of Bermuda. It is 96% calcium carbonate. The calcium is less available than agricultural lime, but it lasts for 4 to 5 years.
- Agricultural limestone (ag lime) – a fine granulated calcitic limestone. A finer powder will result in a faster increase in pH, and a faster increase in soil calcium levels.
- Burnt Lime (Quick Lime) – calcium oxide, which is very caustic (it can burn you, so avoid handling it without gloves!) It is made by heating up limestone to high temperatures. If you apply it directly to plant roots, it will burn them.
- Hydrated Lime – calcium hydroxide is a fast-acting source of calcium, produced by burnt lime and water.
- Dolomitic Lime – contains 22% calcium, which is less than pure lime. This is due to the fact that dolomitic lime contains both calcium carbonate and magnesium carbonate. Dolomitic lime is a good choice if you want to supplement both calcium and magnesium to your soil, to avoid a deficiency in either one of these nutrients.
Gypsum (calcium sulfate)
Gypsum, or calcium sulfate, is another good fast-acting source of calcium for your soil. Unlike lime, gypsum will not raise the pH of your soil, so you can use it without worrying about pH imbalances.
Gypsum is 20 to 23% calcium by weight, and also contains also 15 to 18% sulfur by weight. This makes it a great choice to supplement calcium if you are also worried about a lack of sulfur in your soil.
(On the other hand, Epsom salt is a good choice if your soil lacks magnesium and/or sulfur).
As an added bonus, gypsum will leach sodium from soil. This is great if your soil has high salt concentrations due to de-icing (in Northern climates) or ocean spray (in ocean side communities).
Wood ash is another solid source of calcium to supplement your soil. However, there are a few drawbacks to using it in your garden.
For one thing, the calcium percentage by weight varies considerably, from 7% up to 33%, depending on the type of wood that was burned.
Also, wood ash will raise your soil pH, just like lime. However, wood ash is only about half as effective as raising pH as lime (you would need 2 pounds of wood ash to get the same pH change as 1 pound of lime).
You can also consider putting wood ashes into your compost pile and transferring the mixture to your garden later. For more information, check out my article on adding wood ash to your garden.
Bone meal is just what it sounds like – it is made by grinding up the bones of animals that come from slaughterhouses. Bone meal contains 15% to 22% calcium by weight, and lasts for 6 to 12 months in the soil.
Bone meal comes in liquid, powder, or pelleted forms. As with shells, the finer the bone meal, the more quickly it will act to supplement calcium in the soil.
Calcium nitrate is another good source of calcium for your soil, containing 19% calcium by weight. It also contains nitrogen in a form that plants can use (nitrate).
Calcium nitrate is produced by combining nitric acid with limestone. For more information, check out this article on calcium nitrate from Wikipedia.
I decided to give an honorable mention to worm castings, since they do contain some trace nutrients, including calcium. They are also a great source of organic matter for any garden, and they help to give your soil more structure.
What Does Calcium Do For Plants?
Adequate calcium also prevents blossom end rot in tomatoes and peppers.
Does Epsom Salt Add Calcium To Soil?
No, Epsom salt does not add calcium to soil. Epsom salt is magnesium sulfate, so it contains magnesium, sulfur, and oxygen (along with some water).
So, there is no way that Epsom salt can add calcium to soil, or do anything to prevent blossom end rot. For more information, check out my article on Epsom salt.
What Else Can Cause Calcium Deficiency?
Too much magnesium in the soil can cause calcium deficiency in your plants. This can happen even if there is “enough” calcium in the soil.
The reason is that calcium and magnesium compete for uptake by a plant’s roots, so too much of one comes at the expense of the other.
Calcium and magnesium both have a +2 electrical charge, and both are found on the same column of the periodic table. This means that they share some of the same properties and react similarly with other elements and compounds.
Uneven watering can also cause calcium deficiency in plants. Without consistent watering, plants can have trouble absorbing calcium. Again, this is true even if there is enough calcium in the soil.
Can Plants Get Too Much Calcium?
Yes, you can have too much calcium in your soil. Too much calcium in soil can prevent a plant from absorbing other important nutrients (such as magnesium) from the soil.
In addition, too much calcium in the form of lime (calcium carbonate) raises soil pH. So, adding lime as a calcium supplement is something else to be careful about!
The moral of the story is this: always get a soil test before adding any supplements to your soil. Make sure you really do have a calcium deficiency before adding supplements!
A soil test will also tell you if your soil is too acidic (low pH) or too basic (high pH), which can help you to decide which supplement to use.
For more information, check out my article on how to do a soil test.
By now, you have a much better idea of which fertilizers (both natural and man-made) have high calcium content by weight.
I hope you found this article helpful – if so, please share it with someone else who can use the information. If you have any questions about fertilizers that are high in calcium, please leave a comment below.