Most cacti and succulents require a free-draining compost. How you go about providing this is not precise and while many enthusiasts will swear that their recipe is the only one you should use, in reality cacti and succulents are very tolerant.
If you are a novice and are growing the easier species, it’s safe to use the same compost for all your plants. If you only have a few plants I would encourage you to try commercially available cactus compost rather than mixing your own. However, add about 20% by volume of potting grit as most ready-mixed composts lack these larger mineral particles. You would be surprised at how poor the soil is in habitat but cacti and succulents thrive.
If you decide to mix your own, I suggest you use the same recipe for cacti and succulents. The formula recommended by the BCSS uses a loam-based compost as the starting point (in the UK most growers use John Innes No 2 or 3) and the rest should be a mix of horticultural sand and grit, or other things such as clay granules and pumice. I use a 50/50 mix; that is, 50% loam-based compost and 50% mineral substrate.
Don’t use general potting compost as this has a high proportion of fibrous material which is unnecessary. It will also contain too much fertilizer and may also have been made from recycles waste (see below).
LOAM-BASED COMPOST
John Innes composts are supposed to be blended to a formula set down by the John Innes Institute but you will quickly learn that the different brands vary considerably in appearance and composition. Peat-based composts have largely disappeared, except for commercial use, but the replacements, such as coir, are not very good.
I have started using Silvagrow John Innes No 3 which seems to be of good quality. Previously, I have experimented with sterilized topsoil on the basis that I have heard that some growers collect molehills to use in their compost.
While I’ve tended to use the same John Innes-based mix for both cacti and succulents I am starting to use a loam mix for cacti but what suits a fast-growing Aloe does not necessarily suit a slow-growing Ariocarpus.
Composts to AVOID are ones made from recycled garden waste from municipal waste facilities. This is usually marked as such on the bag but such composts are a complete disaster for sensitive cacti and succulents due to the herbicide residues present. The scandal of recycled materials being put into general commercial composts has largely died down after the issue was exposed, but it’s still best to go with a trusted brand.
Always sieve the compost with a 10mm garden sieve before mixing it. You’ll be surprised as how much debris you find in some brands. the aforementioned Sylvagrow is incredibly clean, whereas you will find large tufts of coir in others, and I even found cardboard in one brand.
MINERAL SUBSTRATES
Sand and grit
The usual advice is to use horticultural sand and not builder’s sand which is intended for cement mortar, but I know people who have used the latter without a problem. Some growers omit sand and just go with grit for the mineral content because they say that sand can bind the compost, making it less crumbly.
Horticultural grit comes in various grades and geological sources. A larger grade of grit (3-6mm), such as alpine grit, is perfect for creating air spaces within the compost. You can also buy ‘grit sand’ which is a mix of both sand and grit.
I would not bother with something like ground rock because while it may provide additional minerals I do not believe it will make your plants grow any better, despite the claims on the bag.
Clay granules and other absorbent materials
Grit does not absorb water but you can add mineral products that do. These can help retain moisture without making the compost too wet. Back in the 1970s when I first grew cacti and succulents, one of the recommended ingredients of compost was ‘crushed brick’. I could never figure out how you were supposed to crush a brick but the clay substrates available today are much more convenient.
The most commonly used clay product in the UK is non-clumping cat litter, a form of what’s called molar clay. The one often recommended is made for Tescos and is a light terracotta colour but there are other brands, including from the large pet store chains. The important feature is that is must be non-clumping and it’s best to sieve it to get out the dust. There is an equivalent molar clay product sold for oil spills which is apparently a good alternative but check that it is the right kind of clay.
PHOTO TO COME: Cat litter, Seramis, vermiculite, perlite, pumice
For seed compost I use a very fine (1-2 mm) form of molar clay called Biosorb (not to be confused with the medical dressing, or the water-absorbing horse bedding!) which is sold as a lawn improver.
Horticultural clay granules intended for hydroponic cultivation are more expensive (eg, Seramis® and Akadama®). If you want to try some it is worth shopping around online as prices are very variable. I now only use Seramis for rescuing sick plants and for propagating cuttings.
Another similar absorbent products is pumice, the pale-coloured horticultural form, which is not too expensive if you buy a few large bag (try Specialist Aggregates).
Perlite is a type of volcanic glass and is a non-renewable resource. It is white and very light-weight. I use it instead of grit in hanging pots for epiphytic cacti as it makes the compost much lighter and help to keep it moist. Avoid breathing in the dust.
Vermiculite is a silica-based material used in various industries, including horticulture. I don’t put it in compost but do use the fine grade for seed-raising. There are concerns about its safety, particularly through long-term exposure.
Limestone
A small number of cacti live in limestone soil and may benefit from the addition of calcium-rich material such as crushed limestone or even crushed oyster shells such as that sold as a bird food supplement. However, these are the more specialist plants so don’t worry about this.
WHAT PROPORTIONS?
For cacti and succulents I use 50/50 compost/mineral substrate but you can vary it if you wish. For the mineral substrate I use 1 part alpine grit, 1 part fine grit, and 2 parts horticultural pumice, but this has varied over the years.
For epiphytic cacti that are to go in hanging pots, I use a bracken-containing wool compost with perlite and vermiculite. One brand of compost is made by Dalefoot Composts and is readily available from garden centres in the UK.
As you can tell, compost making is not a precise science!
MINERAL-BASED COMPOST
This approach requires a bit of experimentation but it’s one used by many fine growers in the BCSS. Pure pumice or perlite with regular feeds with a balance fertilizer (see section on Fertilizer) can produce remarkable results. I know from my use of Seramis to rescue sick plants that you get a good root system and the plant will flourish.
Previously I have chickened out at putting plants into a pure mineral mix but did plant some seedlings in pumice last year and they seem to be fine. I am going to start planting established plants in pumice to see how they do. This will require careful monitoring of watering to ensure appropriate feeding of the plants. I will report back in due course!
DOES THE COMPOST MAKE A DIFFERENCE?
There has been little robust scientific study of growing media for cacti and succulents; perhaps they are too slow growing to test this properly. I will confess to being swayed by nothing more than personal preference and that has little scientific basis.
RECYCLING OLD COMPOST
I never reuse old compost although some growers do. Local authority waste facilities in the UK now charge for disposal of soil so if you can’t put it on the garden compost heap I suggest you save it up until you have a large bag, grit your teeth (apologies for the pun) and hand over some money to get rid of it.
Windowsill, Conservatory or Greenhouse?
Growing outdoors
Watering
Compost
Pots & containers
Top dressing
Potting up
Fertilizer
When things go wrong