Calcium: Eggshells, Oyster Shells, and Agricultural Lime

If you're keeping composting worms, you already know that managing your bin's pH is critical to keeping your population healthy. Calcium Carbonate pH buffers, like eggshells, oyster shells, and agricultural lime are useful tools in a worm farmer's belt to maintain healthy pH levels and healthy worms. This guide will cover these different amendments and how to use them to keep your worm bin healthy.

Besides buffering the pH in the organic matter in the worm bin outside of the worm's body, earthworms depend heavily on bioavailable calcium for happenings inside their body.

Red wigglers utilize calcium in two major ways. First, they have specialized calciferous glands along their esophagus that use calcium from their diet to neutralize acids in their food before it hits their intestine (otherwise they get protein poisoning), and secondly, earthworms rely on calcium to be able to produce viable cocoons.

No calcium = no worms.

Birds found out how to make eggs and have stuck with a very consistent recipe. It does not matter if you use brown or white eggshells, or shells from ducks, quails, or turkeys, eggshells, they are all nearly 100% calcium carbonate through and through. Especially in a vermicomposting context, there is no difference.

The easiest way to use eggshells in a worm bin is to dry them out to the point that they can blend up into a powder easily. You can dry them either with an oven or just naturally letting them air-dry. Then crush them up into as fine of powder as possible. The particles need to be small enough that a worm can fit it in its mouth. You can use a blender, coffee grinder, or mortar and pestle. Just note that blending eggshells in a blender may scratch up your blender bottle.

Ground oyster shell flour is about 95% to 98% calcium carbonate and is functionally indistinguishable from eggshells in a worm bin. It is one of the most popular commercial amendments for large-scale vermicomposting and is much easier to obtain in large quantities than eggshells.

You need to keep your wits about if you are deciding to use agricultural lime for you calcium, as some products with very similar names can kill your worms.

Calcitic Lime (Agricultural Lime) is pure calcium carbonate. It is safe, cheap, and works functionally equivalent to eggshells and oyster shells.

Dolomitic Lime: This contains a mix of calcium carbonate and magnesium carbonate. It is pretty much the same as Agricultural Lime, but over several years of it ending up in your soil can cause an excess buildup of magnesium that could be detrimental to plant growth. I would not shy away from using this until you can find a better option.

Pelletized Lime: Pelletized lime is just agricultural lime pressed into small balls using synthetic or organic binders (like lignosulfonates or starch) so it doesn't blow away in farm equipment. While the binders themselves aren't toxic to worms, pelletized lime dissolves very slowly in a worm bin compared to loose powder. You can use it if you grind it to a powder, but there's no reason to pick it over regular agricultural lime.

Hydrated Lime & Quicklime (STAY AWAY): Hydrated lime (calcium hydroxide) and quicklime (calcium oxide) are extremely caustic. When mixed with bin moisture, they trigger an aggressive chemical reaction that can kill your worms on contact.

Rock dusts are ground-up igneous or sedimentary rocks. Natural rock adds a broad spectrum of macro and trace minerals including silica, iron, magnesium, potassium, calcium, manganese, and rare trace elements which feed soil microbes and eventually end up in your plant beds via finished castings, but they do not add calcium.

Who woulda thought you could feed your composting worms chalk? 

Pure natural chalk is mined calcium carbonate and works functionally equivalent to eggshells or limestone. However, modern white school chalk is rarely made of real calcium carbonate anymore, it is usually made of synthetic gypsum (see the next section) which provides calcium to your worms, but does not buffer pH. 

Colored school chalk also contains synthetic dyes. While non-toxic classroom dyes won't poison worms, school chalk is an inefficient pH buffer and isn't worth using over real agricultural limestone or eggshells.

People frequently mix up gypsum (CaSO4) with agricultural lime, but they do different jobs. Gypsum supplies calcium and sulfur to the bin, but gypsum does NOT raise or buffer pH. Because it lacks a carbonate ion, it dissolves neutrally. If your bin is turning sour and acidic, adding gypsum will supply calcium for the worm's own biological uses, but it will not buffer the pH in the bin itself

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Natural gypsum is mined mineral calcium sulfate. Synthetic gypsum from industrial processes is often extremely pure and highly soluble, but recycled drywall sources can contain unwanted adhesives, paper binders, or construction additives. Stick to agricultural-grade natural gypsum if you choose to use it in your beds.

Preparing it for the Worms

Whatever your chosen source of calcium is, it will do best when ground to a fine powder so it has more surface area to react with its surroundings, and so the worms can fit it in their mouths.

For eggshells, they will first need to be dried. You can oven-dry them or just let them sit out long enough to dry. You don't need to wash them or strip out the inner membrane; the worms handle any pathogens or organic material just fine.

Once dry, you can place the eggshells or oyster shell flour into a spice grinder, blender, or mortar and pestle. Be aware that they may scratch any plastic bottles you use for your blender - but the blades should be fine.

Once it is a dry powder (like how agricultural lime is when you buy it) you're good to go.

How to Add it To a Worm Bin

Because pure calcium carbonate is self-buffering, it is virtually impossible to overdose a worm bin with eggshells, oyster shell flour, or calcitic ag lime. It naturally caps out near neutral (7.0 to 8.2 pH) and stops dissolving until more acid is present.

If you are making some fresh bedding from acidic bedding, like peat moss, you'll definitely need to buffer it with calcium. Mixing in 1 to 2 tablespoons of agricultural lime, eggshells, or oyster shell flour per gallon of bedding before introducing worms gives them a safe, pre-buffered start.

But for maintenance, dusting a light layer of powdered calcium over your food feedings every few weeks is plenty to keep calciferous glands working and bedding buffered. You can also just add some whenever you notice issues with the bin.