Best way to extract calcium carbonate from egg shells?

4 years ago

This is actually a chemistry question more than anything else—specifically chemistry in the context of plants, soil etc. I suppose there may be a few in the hydroponics area that might have the knowledge & answers but I'll start here.

I tend to repurpose/recycle everything. We eat a lot of eggs so I save, dry, grind up and try to make use of a lot of egg shells. Unfortunately, over the years reading, research has revealed that every commonly suggested garden related use (adding calcium, lowering soil pH, repelling snails, etc.) for them has proven false. I’m aware it has repeatedly been discussed here and and elsewhere that egg shells can/should be added directly to the soil or to the compost pile. This can be done, causing no harm, but it seems neither will any benefit be received. I won't go into all the details here (though this covers some), it can be easily found online, but apparently they don't decompose much or very fast, they don't break down but simply get smaller and smaller. I was surprised to find one article about how in one case, chicken & duck eggshells were found to not have decomposed very much after being in the soil 165 years!

Based on whatever I've read, all the research I've done I’ve come to the conclusion that the only way they can be utilized—specifically their calcium carbonate, is to extract it by applying to them something as acidic as they are alkali. I used to mix them with old wet coffee grounds & let them stew awhile but it's unlikely they are nearly acidic enough to make any significant change.

I suspect any common acid (acetic, citric, hydrochloric, lactic, malic, oxalic, phosphoric, sulfuric, etc.) of the right type, amount and concentration would be sufficient to release calcium carbonate but might or could also introduce any number of other unwanted, harmful elements as well.

A frequently recommended method to lower soil pH is the addition of elemental sulfur. Thus far, I've been unable to determine what the actual pH of elemental sulfur is. So, I suppose my questions are:

• What is the pH of elemental sulfur?

• Is elemental sulfur sufficiently acidic that it could be mixed with water and powdered egg shells to render something near neutral, that would add to the soil calcium carbonate, sulfur, possibly other beneficial nutrients but nothing harmful?

• If elemental sulfur is not sufficiently acidic, could the above be accomplished using sulfuric acid?

• …or phosphoric acid instead rendering calcium carbonate, phosphorus (and possibly other beneficial nutrients but nothing harmful)?

• If “no” to all of the above what are your suggestions on how this could be accomplished?

Thanks

Featured Answer

toxcrusadr

4 years ago

I'll take a crack at these questions. In general, though, this is something you don't have to fret over.

What is the pH of elemental sulfur?

Sulfur is not an acid by itself. It has to be processed by soil bacteria into sulfuric acid before pH is affected. Given sufficient time (months) it does lower pH and would presumably result in increased dissolved Ca, Mg (etc.) from dissolution of carbonates.

• If elemental sulfur is not sufficiently acidic, could the above be accomplished using sulfuric acid?

• …or phosphoric acid instead rendering calcium carbonate, phosphorus (and possibly other beneficial nutrients but nothing harmful)?

Yes, but the fact is that no one does this, and there are good reasons why. First, you'd be adding a lot of sulfate, which although it does not directly affect pH, would be increasing the total salinity, which can affect the rest of the chemical web. BTW calcium phosphate (which would be the product of calcium carbonate + phosphoric acid) is a highly insoluble salt which would be contrary to the stated goal of increasing dissolved calcium using acids. Also you can get too much P in the soil this way.

• If “no” to all of the above what are your suggestions on how this could be accomplished

As I alluded to above, soil chemistry is a web, with everything interacting. There is a reason calcium carbonate doesn't dissolve quickly in soil, whether in egg form or powdered lime for example. It's very insoluble except in acid (as you seem to know already). Yet this is not a problem - the system draws whatever it needs as long as the other elements are in place.

You could pour on a water soluble calcium salt, like calcium chloride, in liquid form, but it would not stay in dissolved form. The calcium would react with the soup of minerals in the soil pore water, and precipitate as Ca carbonate, sulfate etc. until equilibrium was again reached. Probably screwing up other things the the process. If you had a serious deficiency of dissolved Ca, it would probably help, if not overdone.

In fact, there is no real need to try and rush the process like this for 99% of gardens. Adding lots of organic matter will help provide not only biological activity, but also ion exchange sites, both of which increase the availability of minerals. I would focus on that and don't worry about how to get calcite to dissolve. Let the microherd take care of that part of the balance for you. I added compost for 20 years and got available Ca and Mg at very high levels - in fact I don't even compost my eggshells anymore so as not to get into 'excessive' territory.

I too am curious if you actually know your available Ca is low by testing?

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