Thursday, October 13, 2011

My recipe for fast-draining potting mix

I find that most commercially available cacti and succulent mixes either contain peat (which is almost impossible to rewet after it has dried out) or too much organic matter, resulting in soil that stays wet too long after watering. That, in turn, could lead to rot, especially in combination with colder weather. The only brand I feel comfortable using unamended is Black Gold Cactus Mix; the formulation for California contains 40-50% pumice, which guarantees excellent drainage.

For a while now I’ve been making my own succulent soil mix. It’s cheaper than Black Gold Cactus Mix, it allows me to control all the ingredients and ratios, and it’s fun in a geeky sort of way.

I use only three ingredients:

1 part coir
1 part commercial potting soil
2 parts pumice

Coir is the coarse fiber from the outer husk of coconuts. Check out this earlier post about coir. I like it because it loosens up the mix while adding a bit of water retention. In contrast to peat, coir rewets easily and doesn’t compress as readily.

Coir is available online in compressed bricks, which need to be rehydrated. I simply put the brick you see below in a Tubtrug filled with water, topped it with a few rocks to keep it submerged, and let it sit for a couple of days.

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Compress coir brick
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Rehydrated coir

The rehydrated coir went in our trusty garden cart.

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Garden cart with coir…

To the coir I added the same amount of potting soil (by volume, not weight). I’m not that picky about which kind or brand except that I make sure it’s free of peat. As I mentioned earlier, peat is hard to rewet after it has dried out, and when wet it holds moisture longer than is desirable during colder times of the year.

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…and an equivalent amount of potting soil

Finally I added the ingredient that makes up 50% of my mix: pumice. Pumice is lightweight volcanic rock that, for horticultural use, is crushed or ground into small particles. From what I’ve gathered, pumice is primarily available in the Western U.S. In some locations, pumice is sold in bulk in garden centers or rock yards. Where I live, the cheapest source of pumice is a product called Dry Stall, sold in feed stores as horse bedding material. I get mine at Higby’s Country Feed in Dixon, CA for $9.99 for a 40 pound bag.

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Pumice sold in feed stores under the brand name Dry Stall
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Dry Stall is the perfect size for my purpose
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Pumice added to coir and potting soil…
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…and mixed in
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The end result is a very loose, airy mix that drains quickly

If you can’t find pumice, there are a number of substitutes that work just as well:

  • perlite or vermiculite (both are sold just about anywhere although finding larger, economically priced bags may not always be easy; check larger nurseries—they may sell you perlite or vermiculite at cost)
  • scoria (small, crushed lava rock)
  • gravel (the smallest size you can find)
  • decomposed granite
  • calcined clay (often sold as cat litter; just make sure you get the right product—you don’t want to get something that turns to mush or has added fragrances or disinfectants)
  • coarse diatomaceous earth (sold in the U.S. by Napa Auto Parts as multi-purpose absorbent; be sure to follow all precautions, such as wearing a respirator if so advised)

You can vary the amount of each of the three main ingredients to create a mix that drains even faster (for example, for extremely xeric cacti) or that holds water longer (for example, for plants from humid environments that like more moisture). You can even add slow-release fertilizer for an even more complete mix.

You are the master of your soil and have ultimate control.

Doesn’t that feel good?

6 comments:

  1. Excellent timing -- I have some cactus that need to be potted! A couple of comments...

    - I can confirm that pumice is not readily available in St. Louis. I've never seen it anywhere. Perlite is what I can find here.

    - diatomaceous earth is somewhat dangerous in that the dust should *not* be breathed. Isn't it too fine for the mix?

    - Why not use sand in the mix?

    - I was going to ask how you found bagged potting mix that didn't contain peat, but then realized you said potting *soil*.

    Great post!

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  2. Alan, all excellent points. It's great to hear from gardeners in other parts of the country to find out what their experience with these ingredients is.

    The Napa Auto Parts oil absorbent is actually very coarse but precautions should still be taken. I'll add a note to that effect.

    Sand stays moist much longer than people think and can contribute to rot, especially in the winter when it's colder and plants don't consume as much water. Very coarse sand would be OK, but all I ever see around here is the find playground sand. That's a definite no-no.

    Many bagged potting mixes sold here (especially the cheap ones) actually don't contain peat. Mostly it's "composted forest products." Peat is typically found in more expensive specialty mixes. I guess that's another regional difference.

    There's a lot of information on succulent soil mixes in the various gardening forums, and the consensus is that as long as your drainage component (pumice, perlite, gravel, etc.) is at least 50% of the mix, you're OK.

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  3. Interesting mix Gerhard, and a very good one too! Ticks all the boxes to promote healthy succulent growth. Pumice is alot more difficult to get hold of here so I use Perlite most of the time.

    I agree that everyone has their own individual take on the ideal mix (master of your soil as you've said) :)

    Speaking of vermiculite I find that it holds on to more moisture than others. Perlite is much better for drainage and circulation.

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  4. Great post Gerhard! Sounds like a good mix. Ok so this is what I do. When I first started and had to put the 4 tiered cement block planter around my whole back fence I needed a lot of dirt. I contacted a rock and gravel company and ordered a bunch of their highest quality dirt/potting mix. We tossed it in planted my little plants then put a layer of yellow rock over the top. I think because my planters are so high it worked. My succulents are huge, over the fence. Then when it came to do my front planters I had a much smaller area. I had learned more and found good deals on the palm/cactus mix and used that to mix with (some) of the regular dirt I left. They are doing well. Stan and my son just finished the planter on the other side of the front of my house. This time I wanted to be even smarter and thriftier but I could be in trouble. We will see. First of all I looked at Debra's books and looked at soil mixtures. I think I really only payed the most attention to one of them because now when I look at the other she says (what you say) that in colder climates your mixture should have less peat. Wellllll, I did go and get 4 huge 40 lb bags of Dry Stall at a Tack and Feed store here in Roseville. But the potting mix I found a great deal on was a large Supersoil variety that has compost and PEAT. Darn! Now we did not use all of our Dry Stall but I have already planted quite a few plants. I wonder if I just add more Dry Stall all around them if I'll be ok. I don't want to dig up the plants and shock them anymore than they already are. Geez!

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  5. Candy, you should be fine because you mixed in so much pumice. It's not like Supersoil is all peat, it's just a part of it. Most of it is your run-of-the-mill "processed forest product," which is fairly fast draining (that's why they add peat in the first place). You can always put some more pumice on top to keep the crown of the plants relatively dry.

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  6. Yea! Good to hear. I did add another 20 pounds of dry stall. We did not add all 4 bags and I still have 1 bag left. I have been having so much fun planting this planter I did not take pictures. But I will take after pics. You should have seen me yesterday and today. So sweaty and almost completely covered in dirt that stuck to me. In my shoes and gloves. So fun, felt like a little kid.

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