What makes a clay pot get slimy on the outside?

Moccasin(z9aMobileAL)March 4, 2011

I've had a couple of flower pots, terra cotta ones, which will sort of sweat, then get slimy and feel cold.

They have holes in the bottom, the water drains, but it is really gross to feel it. The latest is an orchid that was fine when I bought it. I've had it about two weeks, watered it once, and now it is really slimy.

What am I doing wrong? This is in Mobile AL, a high humidity place.

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dellis326 (Danny)

algae or bacteria would be my guess if they're usually damp.

    Bookmark   March 4, 2011 at 10:55PM
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tapla (mid-MI z5b-6a)

As water passes through the container walls, it evaporates when it hits the air and deposits hydroxide ions and alkaloids on the outside of the pot. These deposits are very basic (a base is the opposite of an acid where pH is concerned). If you know anything about soap-making (saponification) you know that mixing a strong base (like lye) with animal or vegetable fat makes a soap.

We have lipids in our skin that are rich in fats, waxes, and sterols that react with the high pH basic material on the outside of the pots. Other organic molecules that come in contact with this base material react as well. The slimy or soapy feel on fingers is due to saponification of the lipids in our skin and other diverse fatty acids that come in contact with and collect on the pot's outside walls. During the process molecular bonds are broken that release fatty acid salts and glycerol (you know how slimy glycerin is).

The short answer is: it is soap, and starts turning some of the lipids on the surface of your skin into a film of soap when moisture is present and you touch it - thus the slimy feel.


    Bookmark   March 5, 2011 at 11:24AM
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Lordy memrcy, tapla, I'm going to read that over again. And keep it forever.

I'll even get my DH to read it, because he will know exactly what you mean. I am more challenged by chemical stuff, but I can appreciate a serious reply to my question.

I was being serious in wanting to know what was happening.....
because it could be that I could do something differently to avoid being slimed. Or maybe I should buy pots from a different source. But you are right. It does feel like ....SOAP...... I can live with soap......I just hope that it goes away without harming any of my plants.

Thank you VERY much.

    Bookmark   March 5, 2011 at 8:45PM
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tapla (mid-MI z5b-6a)

It won't harm your plants. I have lots of very old terra cotta containers, some with amazing amounts of patina on the outside, & plants do just fine in them.


    Bookmark   March 6, 2011 at 2:11AM
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rhizo_1 (North AL) zone 7

I was going to suggest it is simply a bio-film growing on the porous surface of your pots. I've had this occur countless times over many years. The 'bio' part of the film would be feeding on the substances (and moisture) evaporating through the micropores on the container. I'd guess that this might be one of the gillions of ubiquitous bacteria causing that slimy feel. Not harmful.

    Bookmark   March 6, 2011 at 7:04AM
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I understand the "patina" concept as something good, so I won't be worrying about it as much. I'm very new at this orchid business, and somme of the plants cost too much money to learn by trial and error that I'm doing it all wrong.

I am INTENDING to put in a rain barrel to catch the water for plants in my new Teahouse/plant house. Mobile has a great deal of rainfall per year, our municipal water supply is a bit heavy on the chemicals--which I can smell coming out of the tap after being away from it for several weeks.
I'm wondering if the tap water could make the pots aspirate the salts/chemicals onto the exterior of the terra cotta? I am very fond of a good terra cotta pot, so wish to use them as much as possible. Will the stuff collecting on the outside that turns white be an indicator of changing to filtered water, or does it reach a point you can SEE any harmful effects in TOO MUCH buildup, to the plants contained in the pot?

I'm about to repot my monstera as I move it to its summer quarters, maybe for the last time, so I want to make sure it has its best shot at greatness. Lovely lovely plant.

Hope I've expressed this clearly. Tapla was so succinct in his explanation of slime.

    Bookmark   March 6, 2011 at 1:34PM
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tapla (mid-MI z5b-6a)

Dissolved solids in your tap water are a part of the accumulation on the pot's exterior. Water from the soil solution moves through the walls of the container, carrying dissolved solids from tap water and fertilizer solutions, as well as naturally occurring organic salts with it. When the water reaches the pot's exterior it evaporates and leaves behind everything that was dissolved in it. The only way these solutes could make it back into the container is if the flow of water was reversed. This would require that the solids on the pot exterior go back into solution and then migrate back into the soil solution. In order for that to happen, something needs to be seriously out of whack - like a pot that is continually wet outside and a dry plant inside the pot.

The last thing you need to worry about are the salts that have already precipitated out of solution. Those are salts that are no longer a threat to your plants. Obviously, if you can see them they're not in the soil solution, though visible accumulations on the soil surface are good indicators that you should have a closer look at your soil choice and/or your watering habits and adjust as required. The ability to flush the soil each time you water is important to the best interest of your plants, and usually eliminates almost all of the salt accumulation on the soil surface, though it doesn't do much to slow deposits on the exterior pot walls.

Basically, the water moves only one way - from inside to outside. Once the salts have collected on the exterior of the pot, they present no risk of elevated EC/TDS (roughly salt levels) of the soil solution. I have many pots of all sizes with thick crusts that you can flake off the outside of the terra cotta. Before repotting into these containers, I wash the inside with a bleach solution to kill anything harmful that's biotic, and don't even concern myself with the outside - I love the old, weathered look of terra cotta and the plants can't tell if the pots are 10 days fresh or 10 years old.


    Bookmark   March 6, 2011 at 4:18PM
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