Cactus and Succulents forum→Something I don't understand about pumice

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Name: Ross
Lancashire, England, UK
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RPK82
Jun 4, 2020 8:17 AM CST
I hear a lot how it's a great idea to add it to soil for succulents, because it causes spaces in the soil for the roots to breathe, and holds onto nutrients and water.

If it holds onto water doesn't that mean that the soil will be wetter for longer? I thought that would be a bad thing for succulents and would make it more likely for them to suffer from over watering?
Name: Baja
Baja California (Zone 11b)
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Baja_Costero
Jun 4, 2020 9:33 AM CST

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Compared to organic material, pumice can hold much less water. It may hold in the range of 30-50% water by weight, compared to cocofiber for example, which can hold 5 times its weight in water, and its starting weight is pretty low because it's mostly air. Pumice serves a role in breaking up the organic part of the soil, and allowing water to drain efficiently, and creating little air pockets outside the pumice.
Name: Daisy I
Reno, Nv (Zone 6b)
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DaisyI
Jun 4, 2020 3:25 PM CST
The only water in pumice is in the little holes. Rocks don't absorb water.
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Name: Baja
Baja California (Zone 11b)
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Baja_Costero
Jun 4, 2020 3:28 PM CST

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Adsorption or absorption gives pretty much the same outcome in a soil mix. In pumice the air and water trade places when it gets wet, as Daisy has indicated.

Perlite also adsorbs water. The water sticks to the surface of the particles and does not enter them, because the airy part is closed. So they float, unlike pumice, which mostly sinks when processed to a fine gravel. Because the water-holding capacity of perlite is related to surface area more than volume, the same amount of perlite will hold more water if it's a finer grain size.

https://www.perlite.org/the-wa...
[Last edited by Baja_Costero - Jun 4, 2020 3:59 PM (+)]
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Name: Frenchy
Falls Church, VA (Zone 7b)
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Frenchy21
Jun 4, 2020 10:15 PM CST
I previously bought 1/8 size pumice but I think it's too small - what size do you recommend for mixing into soil and for using as a top cover?
Virginia, USA (Zone 6b)
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SedaS
Jun 5, 2020 5:54 PM CST
Frenchy, I like the 3/16 pumice for smaller pots (like 4 inch and smaller) and 3/8 for larger. I tried the 1/8 previously and had a lot of problems with the pumice compacting into big chunks in the pot. It was like cement. Not sure why that happened. Maybe it was a bad batch or wasn't rinsed well enough?

I've also moved toward using pure pumice for succulents (or pumice and Reptibark for tropicals) rather than mixing it with soil since the soil always ends up collecting at the bottom of the pot.
Name: Baja
Baja California (Zone 11b)
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Baja_Costero
Jun 5, 2020 6:07 PM CST

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I use pumice screened to within 0.1-0.25 inches, and save the pieces just larger than that for top dressing. When I start seeds I often use pumice screened to about 0.1 inch. For bigger plants I have used pieces larger than my usual cutoff in the soil.

I realize it's hard to find pumice in a lot of the country, so you kind of have to work with what you can get, but my ideal size would probably be about 0.2 inches for most succulents. Less than 0.1 inch may be too fine for some situations. Always consider washing or screening the material if it seems to contain a lot of fines or dust (and wear a mask while handling it if that is the case).

A note about pumice in soil mixes: pumice (the kind processed and sold for use as a soil amendment) changes color when it holds water. It's usually bright white when dry, slightly gray when moist. You will find pumice easier to incorporate into your soil mix if it is moist, not dry, when you add it. It "sticks" better to the soil around it, and the mix ends up more uniform as a result. The pumice I usually buy is premium and it comes already moist in the bag, but when I've used material from other sources I have found it helpful to prewet the rock before adding it to the soil. Not dripping wet, just moist, enough moisture content to make it gray.
[Last edited by Baja_Costero - Jun 5, 2020 7:53 PM (+)]
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Name: Frenchy
Falls Church, VA (Zone 7b)
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Frenchy21
Jun 5, 2020 11:23 PM CST
Thank you SedaS and Baja for the information. I will try the 3/8' size and screen it before using it. I have masks so that won't be a problem. Big Grin
Name: Ross
Lancashire, England, UK
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RPK82
Jun 6, 2020 10:28 AM CST
Cool, i was curious about the size.

Would it be best to buy just pumice, or some kind of a mix (I've seen some of these on ebay) like pumice, pink granite and alpine grit, or pumice and lava rock, etc?
Name: Baja
Baja California (Zone 11b)
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Baja_Costero
Jun 6, 2020 10:48 AM CST

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I don't mix the rock in my soil, unless I have some leftovers I need to dispose of.

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