Cactus Compost

[Rob Kitching]

I recently received some cacti from a reputable UK nursery with cultural instructions which said "On receiving your plants they should be potted up in any John Innes compost, or any peat based pot plant compost........you can add extra drainage material in the form of horticultural grit or sharp sand, but this is not essential". Well, I have found that extra drainage is always essential, and in my ten years of growing cacti I have had more disasters using multipurpose peat potting compost than anything else. Early on I came to the conclusion that the compost used by some nurseries is not the same as that which they recommend to their customers. Some cacti do like a peat based compost, but many do not. When ordering plants from a nursery, the one piece of information, apart from the correct name, that I always wish for is the growing medium, pH and constituancy of the compost which they have been grown in, especially the rarer species.
I don't have examples of all the gerera in my greenhouse, but I have, over the years experimented with various compost mixes and come to the following conclusions:
1. If in any doubt about a suitable compost for a cactus, use a good JI no.2 (soil based compost) with added drainage. Don't go anywhere near multipurpose peat compost (organic).
2 The few genera I have found that do like an ericacious (lime free) peat based compost are: Sulcorebutia, Gymnocalycium and Matucana. I have over fifty Sulcorebutia, and all these I have growing in a peat compost. These will grow in JI but they seem to prefer the peat. Some of the non taprooted Mammillaria also seem to prefer peat compost.
3. For the slower growing types like Ariocarpus, Aztezium, Strombocactus, Encephalocarpus etc. I use Seramis.
4. All the others, Rebutia, Echinopsis, Lobivia, Eriocyse, slow growing taprooted Mammillaria, Escobaria, Notocactus, Echinocereus and particularly the lovely Aylostera and Chamaelobivia hybrids must have the JI no.2. Some of these will survive in a peat compost, but keeping them going for long, in my experience, does not work. Any comments would be most welcome, but these guidelines have worked well for me over the past few years.

[Liz M]

That's a really useful piece of information, something a lot of people will be grateful for. Thanks.

[Jim_Mercer]

By using a John Innes compost you are not avoiding peat and ground limestone is also part of the mix...

"During manufacture of the composts the pH has to be balanced and nutrients added. Peat is an essential part of JI composts, giving them structure and improved available water-holding capacities. Peat, however, is acidic, so ground limestone is also added to raise the pH to around 6.5."

http://www.johninnes.info/technical.htm

[ralphrmartin]

Well, I've been growing cacti for over 40 years, and my conclusion is they will grow in almost any compost - from peat based to pure mineral. The compost more or less is there to hold them up and to get the water and food to the roots.

Some compost is better at buffering the pH than others, and some holds water longer than others, and these are relevant points to take into consideration. But I think most people get far too worried about what's in the compost. I feed my plants every watering.

[gerald]

I recently spoke to a professional grower about the same issue, and the response was that young, healthy, fast-growing plants full of the joys of youth are usually in small pots which dry quickly, and besides that are so vigorous that they'll pretty much grow in anything. It's the older, more cantankerous plants that are fussy and more difficult.

A bit like the human species?

[DaveW]

Like Ralph I think composts go in fashions and what is a no-no one decade is often flavour of the month the next.

I remember Ron Ginns saying in the 1960's that cacti should only be grown in mineral soil, where all around him people were growing plants much faster than he did in modified JI composts which included peat. Now peat is the no-no and the fad is mineral soil. The fact is plants in pots do not behave like plants in open ground, so habitat soils don't always have any relevance to potted plants. Plants grow where they have to in habitat, often simply where their seeds fall unless the ground is too toxic for them, not always in optimum conditions for their growth. It's always a case of what works for you in your collection, but your magic recipe may not work for others who have greenhouses with different microclimates.

The Continental nurserymen have proved they can grow cacti to quite large sizes in largely peat based composts and much faster than most of us can, but we struggle with them because they are hard to re-wet after they dry out. Maybe they don't ever let them dry out?. Often it comes down to different growing methods suiting different composts.

So far since the Society started we have had recommended Endean's baked clay, Buxbaum's broken brick, peat (sphagnum-moss and sedge), coir and cat litter. I wonder what the next compost fad along the line will be, plastic granules probably.

I remember when the University of California came up with peat based composts to replace JI ones and they were published in one of the early American Journals for our plants, formula's Levington more or less copied in the UK that people were then growing cacti in.

Here was the UC Succulent Compost:-

"University of California Succulent Mix
•1 part ground sphagnum peat
•1 part redwood compost
•1 part horticultural sand
•2 parts volcanic pumice
•for every one cubic foot of mix add:
◦5 tablespoons dolomite limestone
◦2 tablespoons gypsum
◦3 tablespoons superphosphate
◦1 teaspoon potassium nitrate
◦1/2 teaspoon potassium sulphate

I think Levington's Cactus Compost is still peat based. if plants did not grow in it OK it surely would not still sell?

Now if you mention peat in many cactus circles you can see the shock on the faces around you, yet strangely people in the past were growing plants just as well as we do now! The fact is we are all never satisfied with what we are using at the moment and always searching for that better, but usually unobtainable, new compost that grows the plants twice as quick and even better than we can grow them now.

[Josh]

1. You mention producers, brands, but what if I'm in another country?
2. What about advice from around the Web - particular to certain species?
I have a bunch of tiny mammies, also several selenicerei - of a tiny size, too. Can they thrive in the same pots?

[iann]

I think Levington's Cactus Compost is still peat based. if plants did not grow in it OK it surely would not still sell?

A spurious argument if ever there was one. I can't even begin to count the number of people, myself included who've tried growing cacti (and Lithops even more so!) in the commercial bagged cactus soils and failed. That doesn't stop the next person picking the bag off the shelf that says "cactus" on it. It just discourages a lot of those who try and fail. There are plenty of people using peat-based composts, usually heavily modified from what comes in the bag, but also plenty of even very experienced growers who claim certain plants are impossible on their own roots. Sometimes they just call certain plants rot-prone, blame the climate, and continue to grow the ones that like or at least tolerate a nice organic soil. There's nothing faddish about recognising that what comes from B&Q in the bag with a picture of a cactus on the front is entirely unsuitable as is for growing any of the desert cacti that are the mainstay of the hobby.

[Liz M]

Hi Ian, I think your argument sums up the whole discussion. In my opinion, stick with John Innes and something non-organic to open up the compost, like grit, cat litter, perlite, vermiculite or pumice and grow your plants in that. Very few of the plants we grow, naturally grow in peat, so why expect them to thrive in it?

[ceacole]

Hi guys,
As a beginner I have found this discussion really useful.
I have read dozens of different sources that all have a different
Twist on compost. It is good to read about peoples preferences
Based on years of experience rather than fashion.

Thanks