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Eggshells in a TLUD?  RSS feed

 
Gilbert Fritz
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Location: Denver, CO
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We eat a lot of eggs, so we have a lot of eggshells to get rid of. Eggshells don't break down in the soil/ compost; there are tests to prove this. Furthermore, they can attract critters. And we have no chickens to feed them to. However, if they are charred, I've heard that they do break down; the calcium and phosphorus becomes available faster. And at that point the critters would not be interested.

So what if I mixed them into the wood in my TLUD biochar stove? Would they just char nicely with everything else? Would the calcium and phosphorus be "quick" like quicklime when done? Or not? (In other words, would I have to be careful when I doused it with water?) What about bones? I know people char them; are they reactive to water when done?
 
John Elliott
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Bones, yes, toss them in the TLUD and they will come out as black bone-char.  A good reserve of phosphorous when added to soil. 

Eggshells, being very thin calcium carbonate, are going to oxidize to calcium oxide (lime), and then when you add water, you get slaked lime (calcium hydroxide, Ca(OH)2).  It might be detrimental to very alkaline soil, but for soil that has any organic matter at all, it's not going to matter.  I take it we are talking about a couple dozen eggshells added to the burn, not several hundred.  If in doubt, save the eggshell biochar batches to use on tomato plants, they are always hungry for calcium (which prevents blossom end rot).  
 
Craig Overend
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Location: Melbourne, Australia
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I'm terrible at chemisty, however to produce that calcium oxide John mentions, a study I've browsed showed that to yield well eggshells need to reach over 800C (1472F) which is pretty close to the melting point of calcium at 842C (1547F). Compared to other calcium soil amendments, another study suggested slaked lime was the most effective at reducing soil bulk density. But in a fire would it form calcium carbonate or another compound?

Phosphorus boils at 280.5C (536F), no idea what happens to that. I assume it goes up in smoke unless it bonded into something with a higher boiling point?
 
Casie Becker
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I keep thinking about this, but the one time I had blossom end rot, I sprinkled powdered egg shells around the plant roots and immediately stopped having new cases. Poking around it seems that the powdering of the shells was key to my fast results. Eventually even whole egg shells will degrade, given enough time. Mild acids dissolve egg shells and living soil is full of hummic acids.

This does remind me that we should start putting egg shells aside for just this reason, in case we have another very rainy spring.  I found a chart at one point that was supposed to tell you what soil deficiencies your weeds would indicate and the only consistent one was calcium. Seems very strange for a limestone based soil.

 
John Elliott
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Craig Overend wrote:
Phosphorus boils at 280.5C (536F), no idea what happens to that. I assume it goes up in smoke unless it bonded into something with a higher boiling point?


That would be elemental phosphorus.  In living (and dead) tissues, phosphorus is present mostly as phosphate (PO4---),  and when it burns, it ends up as phosphorus pentoxide (P2O5).  This pentoxide then reacts with water to revert back to phosphate, keeping the whole phosphorus cycle turning. 

How hot you have to burn calcium carbonate to get lime is an interesting problem.  Cement kilns really crank up the heat to get a good conversion.  But the original settlers of Georgia and the Carolinas got decent results by heaping up seashells and wood and lighting it up.  Decent here meaning that they could use their burned seashells to make tabby, which although not as strong as cement, has lasted in some structures to the present day.

As far as plants are concerned, it doesn't matter much to them if it is CaO,  Ca(OH)2, or CaCO3, rain and humic acids are going to make it bioavailable Ca++ ion for them.
 
Gilbert Fritz
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So the calcium phosphate in eggshells will still be there after I quench the burn? Or will it still be there in another form?
 
John Elliott
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Gilbert Fritz wrote:So the calcium phosphate in eggshells will still be there after I quench the burn? Or will it still be there in another form?


Calcium and phosphorous aren't volatile, so they are left after the burn is quenched.  If you soak your biochar/ash mixture afterwards, some of the ions will migrate onto the biochar matrix, which is a beneficial effect.
 
William Bronson
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Just curious,if you dissolve egg shells in vinegar,would the  acid be neutralized and would the calcium and phosphorus be available to the plants?
 
Casie Becker
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I don't know, but even if it isn't neutralized, the soils here are very alkaline.  A little acid won't hurt
 
John Elliott
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William Bronson wrote: Just curious,if you dissolve egg shells in vinegar,would the  acid be neutralized and would the calcium and phosphorus be available to the plants?


Yes, eggshells will neutralize vinegar and the calcium will still be bioavailable.  I should point out that eggshells have very little phosphorus; bones are what contain phosphorus. 
 
Gilbert Fritz
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OK, that makes sense. Thanks a lot. I'll post about what happens if I do it.
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