Different Kinds of Lime

I’ve decided to include two reader questions here since they are somewhat related.

Our first question comes from Hibou:

I’ve read that a bit of lime is very good for garden soil. Is it good for my vermicompost bin? If so, which type? Wikipedia tells me that “agricultural lime is” mainly calcium carbonate (CaCO3). However, the lime I’ve got my hands on is “slaked lime,” i.e. calcium hydroxide (Ca(OH)2), the by-product of acetylene gas. Thus it’s mixed with a bit of calcium carbide (CaC2) as well. What do you think? Will my worms appreciate it?
Thank you,

Hi Hibou – I’m really glad you wrote in with this question since this is an important topic to talk about. Firstly, yes you can certainly add lime to your worm bin in moderate amounts to help keep the pH balanced somewhat and provide a source of calcium for the worms. Is it vitally important to do so? Definitely not – composting worms actually tend to prefer a somewhat more acidic environment (similar to the one they are used to in their ‘natural’ habitats such as compost heaps etc), so you really don’t need to get hung up on the pH issue. That being said, if you ARE giving your worms a lot of acidic foods such as tomatoes or juicer waste (citrus etc), you will more than likely need to provide at least a little lime supplementation. One thing I definitely DON’T recommend you do is wait until your bin goes ‘sour’, then try to rectify the situation by dumping lots of lime in – you’ll more than likely just end up making matters worse for yourself. Rapid shifts in pH can wreak havoc on your bin ecosystem, and ultimately your worms.

Ok – now on to the different types of lime. This is REALLY important since there are a variety of different materials called “lime”. You want your lime to be CaCO3 – 95% or higher. One exception is ‘dolomitic lime’ which basically has a 50:50 ratio of CaCO3 and MgCO3 – it is ok as well. You definitely DO NOT want ‘hyrdrated lime’, ‘builders lime’, or ‘slaked lime’ (all similar) since these can be caustic and thus not worm-friendly.

Hopefully you have not added your slaked lime to your bin yet! Your worms will definitely not appreciate it.

Next, we have a question from Nick:

Are seashells (Calcium carbonate and chetin) OK to use in my
bedding, instead of eggshells or limestone? I live close to a beach full of seashells, and would be criminal not to take advantage of the millions of seashells washed ashore every day. Hopefully, there will be no adverse effects on my red Wigglers. I am a beginer in vermicomposting and would feel really bad, if I inadvertently harmed the breaders.

Hi Nick,
I am a big fan of slow-release sources of CaCO3, so I would give a big thumbs-up for seashells. I use eggshells myself, but I suspect seashells (that have been washed with fresh water to avoid adding salt to your bin, and perhaps crushed with a hammer) would provide a similar source of calcium. Moderation would be important, as always – you certainly don’t want a worm bin full of sea shell shards (try saying that 3 times fast!!), as these would likely be quite abrasive.

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    • Nick
    • October 24, 2008

    Thanks Bantley!
    I appreciate your timely reply.
    I also must tell you , I find your blog invaluable!
    Keep up the good work.

    • John H. from Orlando
    • October 26, 2008

    Based on this posting, I just pulverized a few of the sea shells that I had brought back from the beach. First, I used a hammer to break them. I was somewhat surprised how easily the shells and small pieces of (dead) coral cracked and soon became a coarse powdery mixture. I took this mix and placed it into my old coffee blender. After a couple pulses, the mix was a very fine powder.
    Normally I use my blender to pulverize my dried egg shells. I like that I have the opportunity to use sea shells as a source of CaCO3 since my family doesn’t eat as many egg as I think my bin could use.

    Now I wonder if I can manually crushed other easily obtained rocks to create my own rock powder. I have heard that volcanic rock dust (azomite) can greatly enhance the mineral content of your vermicompost and thus improve its effect on plant growth.

    • linda caylor
    • November 2, 2010

    i asked for the different types of lime and was told all about worms why cant i get the right answers to what i ask-this happens all the time

    • terry yeary
    • September 8, 2011

    I am new to worming. What is the best formula of foods to feed them? I have Norwegian night callers. Everyone seems to have a secret formula. What’s best?

    • Will
    • September 22, 2011

    I just wanted to ask if you’ve ever heard of anyone using reptile calcium powder sold in pet stores to raise the pH of their composting bins? There are varieties that claim to be 100% calcium carbonate and they are ground into an ultra fine powder. One variety is made by the company Zoo Med, which sells a product called “Repti Calcium without D3” that is “precipitated” calcium carbonate. Claims to be free of lead and all other impurities, and it is solid white and honestly it looks like flour. I use it to dust crickets prior to feedinmg them to my geckos. What is your opinion of using such products to raise pH? It’s extremely cheap and easy to find at local pet stores or online. Thanks!

    • Bentley
    • September 30, 2011

    Hi Will,
    I have not heard of that, but it’s likely because these specialty materials are likely more expensive than other versions of basically the same thing.
    If it’s 100% CaCo3 (doesn’t even need to be quite that high) and it is fairly cheap then by all means use it – I’m sure it will work very well. Just don’t go to crazy with it!

    • Mark
    • February 6, 2013

    I am new to the vermicomposting process. Could you give me a few tips on how to quickly build my worm population?


    • Adam
    • March 4, 2014

    Hello Bentley…Adam from Hamilton Organics here. I just got a couple of buckets from the catering company that I collect food waste from. They are labeled “S-lime powder”. I’m still waiting to here back from the executive chef about what that is exactly before adding it to my bins. Do you have any idea what that is? I was thinking maybe it’s food grade type S lime, but I’m not exactly sure what that is made of or if it would be safe for the worms. I too add egg shell powder and a special mixture that I created that includes two types of rock dust and oyster shell flour. They are healthy as can be. I don’t want to add something that may be harmful. Thanks in advance for your thoughts.

    • Dave
    • October 18, 2015

    We had some leftover from a bulk hydrated lime silo, and had been using it around trees. It is food grade, very fine white fluffy lime.

    Today we spread some in the top of a worm bin. It killed every worm that it touched. Even a tiny amount caused them to writhe about vigorously for 5 seconds, goo coming out both ends, then die.

    So, from our experience, no definitely don’t use hydrated lime in worm bins.

    • Mel
    • June 27, 2021

    Quick query re lime please!
    I am in spain and bought something called cal viva – quick lime which says it is calcium oxide CaO. When you add water it get extremely hot but I wanted to know if – once it has completely cooled down – it wiuld be suitable for my wormery? Currently i have s vast number of tiny thready white worms which i read was a sugn of over acidity. Unfortunately I was giving them my friends remains from her daily juicing but did not realise that she puts lemon in it! If this wuick lime is not suitable can you advise me how much egg shell is recommended to use and how often? My wormery is only small – each level is probably 5 capacity. Many thanks!

    • Bentley
    • September 1, 2021

    Hi Mel
    Sorry for the delay responding. I would say ABSOLUTELY NOT to anything called “lime” that is not ag lime or dolomitic lime. What you are referring to is almost certainly very caustic stuff – definitely not the same thing at all. You want CaCO3 or something like CaMg(CO3)2. This highlights one of the major problems with common names like “lime” that can apply to a wide range of different things.

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