Thanks for the article and everyone's input on what soil works for them. As stated, we all have different climates, species, and care schedules so there is no one perfect cactus soil. After years of tinkering this basic mix works for us in the Upper Midwest with the pots pretty much left on their own except for winter. Drains quickly enough for periods of extended rain, enough moisture retention to go long periods between rain in high temperatures, some nutrient holding capacity, and heavy enough to sit tight in 60mph wind.
Previous mixes that had much fine rock and very coarse sand retained too much moisture for us. Found that an aggregate of fairly uniform size and not too angular was best to preserve air pockets between the pieces. Located sources of pea gravel with the right type and size of rock, and then used an old cooker pot to separate into two more uniform sizes - the smaller being 1/4" and a bit smaller (but no fines under 1/8") and the larger ranging from 1/4" to 3/8". These are used in different mixes depending upon the goals. Each 42 pound bag yielded about 26# of the small and 16# of the large.
- sorting gravel
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This shows the mix using the smaller gravel. Ideally the pumice would be the same size, but we buy in bulk and it is all 3/8". We used to use 50% or more pumice but thought it might stay too wet during long periods of heavy rain. This mix is:
pumice - 2 parts
1/4" gravel - 4 parts
hairy coir - 1 part (The more fibrous not pithy type)
potting soil - 1 part (Found Black Gold to hold up better than most)
Counting the mineral portion of this potting soil, the mix is almost 80% mineral by volume.
In this mix, the lower container showing the larger gravel is not used.
- soil mix components
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Some time release fertilizer is mixed in depending upon the season. A pinch of Epsom salts for the coir. Occasionally use our tap water to supply calcium that the coir could bind.
Here is what it looks like freshly mixed. Some of the finer material is washed away through the pots over time.
- cactus potting mix
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This is the mix that works well in our outside pots using the 3 9/16" x 4 1/2", .715 liter Anderson bands with mostly closed bottoms. The standard "bottom cross" bands cannot retain a gritty mix.
This is just another idea that could be useful to some and worthless to others, and is really just a small adjustment to the standard theme.
Iowa / South Dakota border area. USDA zone 4b/5a Heat zone 6-7