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.

Compressed coconut coir brick

Rehydrated coir

The rehydrated coir went in our trusty garden cart.

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.

…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.

Pumice sold in feed stores under the brand name Dry Stall

Dry Stall is the perfect size for my purpose

Pumice added to coir and potting soil…

…and mixed in

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?