How to make the perfect substrate for cacti and succulents. Is peat really a cactus killer?

Due esemplari di Ferocactus in Baja California

Do I use lapillus or just pumice? Is peat really a cactus-killing devil, as we hear people say? But then why nurserymen cultivate their plants nearly 100% with peat, and their plants don’t die in the greenhouse? The question about materials and elements that end up in compost for cacti and succulents is a limitless one. Firstly because the variables are endless and range from growing regimes, environmental factors, latitude, to plant type (for example, there are differences in substrate requirements between a caudiciform and a cactus). Then, because the same elements, as like the field soil, can vary immensely between them – for example, based on the area where it is picked up: it’s evident that the loam of the Po Valley, where I live, cannot have the same chemical characteristics as that one existing in Bolivia, for example.

So, how can we orient among the many elements and materials we can find – some easily, others less – to mix them and make good compost? A first answer, perhaps obvious but reasonable, is to do experience and direct observation. In a word: experimentation. Another one, trivial but overlooked, is knowledge – the knowledge of the single “ingredients” properties that create the substrate and of the individual plant’s needs.

In the following article, you will find an overview of some elements that over the years I’ve used in my mixtures (in other words, substrates for cacti and succulents): some I’ve studied through the comparison with friends growers or reading books written by reliable cactus lovers; others unconsciously (or should I say criminally?) assembled to test the level of tolerance of this or that plant.

Field soil (clay)
Terra di campo con aggiunta di sabbia fine
Field soil with sand

From a logical point of view, it should be the first choice. I mean: where do plants live, where their roots are? Right, of course, but there’re soil and soil. There’s the “fat” garden soil, and the lean soil that you see in some movies, where when the wind blows on it raises dust everywhere. It can be more or less calcareous, more or less sandy… For the type of cultivation that I try to carry out, the field soil (based on clay) is still the starting point, the base on which to build the substrate. Over the years I’ve tried several, always from the Po Valley, where I live. Usually, it’s fertile clayey soil, rich in elements, quite calcareous, often lacking in aggregates, just a few stones that you find from time to time. The loam I use is not very calcareous (pH 5.5-6) and very clayey: if you water it, it turns into mud, which hardens once it dries to the point that you need a hammer to break it up. But it dries quickly, and this is a crucial factor for cactus growers.

Please note that it’s absolutely better to take the soil from non-cultivated fields or at least at “rest” for a long time, to avoid as much as possible the presence of chemicals or fertilizers in the substrate. Since the soil I can get here in North Italy is mainly clayey and very fine, I usually mix it with fine sand (obtained by sieving river gravel, which along the Po river is practically available everywhere), in a ratio of 5 parts of soil and 1 of sand. You can use common field soil and in this sense, having farmer friends who do not use chemical pesticides, this helps a lot, unless you decide to go to the fields at night, risking to be caught by the farmers!

Be careful, though: field soil almost always contains bacteria and parasites that, in some cases, can be deleterious to plants. It’s good to experiment with specific soil samples on a few plants and see how they react over a couple of years, before using that same soil for all your plants!

River gravel
Ghiaia di fiume setacciata
River gravel

I’m lucky because close to the Po River (Lombardy, northern Italy), there are pits for sand and gravel. Getting a few buckets of it for free is not a problem. Instead, if you are forced to some building materials dealer, the cost can be in the order of a few cents per bucket. I use the so-called “big sand” that goes from 0 to 6 millimetres: I sift it to divide the fine part from the coarse one, and so I get two perfect aggregates for my substrates. I use the fine sand to lengthen the field soil and make it more “inconsistent”, less compact, as I wrote above; the coarse part ends up in the mixture in percentages generally not less than 30%. It has an excellent draining effect and, since it is heavier, it contrasts with pumice, a very light material.

Pumice
Pomice
Pumice

This is the wild card of the cactus lover. A lot or a little, it’s always good. It’s a porous material and it holds water well and releases it slowly, ensuring that the soil doesn’t dry out too quickly. I use it in grain size of 3 to 6 millimetres, pH between 7 and 8, so lightly basic (I compensate it with acidic blond peat). I have a nurseryman friend who gets me 10-pound bags of it, but you can readily found it on the Internet from several suppliers and in many nurseries.

What they say about pumice, as far as I’ve experienced, it’s true: it’s perfect for rooting even the most stubborn plants, and you can even use it pure. I have known nurserymen in Eastern Europe who grow their cacti in pumice only. The root system branches out considerably, thickening because of the “spongy” properties of pumice, which helps the production of absorbent hairs on the roots. Except for rooting plants with compromised roots, however, I never use pure pumice: it seems to push growth too much, both in terms of speed and plant volume.

Peat
Torba bionda setacciata
Blond peat

Some growers get hives just mentioning it. Personally, I am not one of them, and I believe it’s useful that 10% of good quality peat (even 20% or 30% for Gymnocalycium and Parodia) is in the compost. It guarantees a valid organic contribution and if used in low percentages does not contribute at all to the water retention of the substrate. I use blond peat, a bit more acidic than other types, I sift it and keep only the fine part, discarding lumps and filaments. It is feared because, when it is in high percentages in the compost, it takes too long to dry. Paradoxically, when it’s dry (for example because it has been for months in open containers in the greenhouse), it has a hard time rehydrating, and it takes several waterings before it becomes waterlogged again. For these reasons, it is not recommended to use ready-made commercial potting soils for cacti and succulents: they are almost always simple peat with a little sand or pumice added.

Fine sand
Sabbia fine
Fine sand

I get it from sieving gravel and use it sparingly to lengthen field soil. Silica sand is ideal. Sometimes, more for testing than anything else, I’ve added it to compost in percentages of 10-20%. I haven’t had any significant losses but no striking results either. I think the little sand I add to the field soil is enough. If present in the substrate in large quantities, the fine sand will aggregate and hold water, having a sort of “shoreline” effect at sea. To be avoided.

Pelite (not perlite!)
Pelite
Pelite

It is a rock that originates from clay-based muddy sediments. It occurs in brown-coloured flakes that are easily broken by fingers. I’ve learned of the existence of this material just in the last five years, thanks to a grower friend. I have obtained some for using it in some substrates to lower the percentage of pumice. I don’t have enough experience with this material yet, but so far I’ve seen that the substrates I’ve used, it works out perfectly: they dry quickly, and the pelite flakes make the mixture melted.

Be careful: “pelite” is not to be confused with “perlite”, a very crumbly white volcanic rock often used in nurseries. I’ve tried it a few times, but I don’t like it: it’s too light, and it spreads everywhere.

Natural chalk
Gesso agricolo
Natural chalck

In recent years I have used the so-called “agricultural” chalk (natural chalck), in addition to other materials, for some genera that in nature have their roots in this rock: Aztekium, Turbinicapus lophophoroides, Turbinicarpus hoferi, Geohintonia, Strombocactus. I also used it, as a test, for some Epithelantha, which I don’t think lives in chalk in nature. I have tested it at 20% mixed with aggregates and field soil, and so far it has given good results. In agriculture, it is often used as a soil amendment to lower the pH of soil that is too alkaline.

Marl
Marna setacciata
Marl

This is the manna of “wild growing” cactus lovers. The use of this material was introduced in Italy by Andrea Cattabriga, who describes it on his website “Mondocactus“: “Marl is a disintegrated terrigenous rock, which decomposes to form an earthy matrix, mainly composed of clay made compact by the infiltration of various minerals, especially calcium carbonate, dolomite or, more rarely, silica“.
Initially used mainly for Ariocarpus and other plants considered rare or slow-growing, such as Encephalocarpus strobiliformis, it then ended up in the substrates of many others genera, such as Mammillaria, Astrophytum, Turbinicarpus and even Ferocactus. Some people tend to downplay the properties of marl, others would like it even for the ficus in the office.

I’ve been using it for several years now just thanks to Cattabriga, who made me try it some time ago. And imagine if, after seeing his plants, I wasn’t tempted by this greyish soil, a fantastic material from the point of view of aesthetics and an excellent one for growing particularly slow cacti. I use it in percentages ranging from 50% to 70%. Sometimes I’ve even used it pure. I’ve tested it with many genera, including Copiapoa and Ferocactus, and I must say that I’ve been disappointed only with some single specimens. It slows growth, particularly when used in high dosages in the substrate, so best to go easy on young plants. I’ve had several E. horizonthalonius stalled for two years. My fault: they were two or three-year-old plants, and I put them in a compost of 70% marl. Repotted in field soil, aggregate and 20% marl, they started again immediately and within a year doubled in volume.

Warning: as with field soil, there are several varieties of marl, each with its own chemical composition and properties. In this case, too, it is necessary to experiment directly to understand if the marl we have found is good or not for our plants.

Lapillus
Miscela standard (pomice lapillo torba)
Lapillus and pumice

This is the heavy element of the pumice/lapillus/peat triad. I’ve used this compost for years, probably still the most popular among cactus and succulent growers. When it’s supported by the right exposure, proper fertilization, and a good watering regimen, it works. It pushes the plants a little too hard. At least for my taste.  Apart from this, I still use this substrate with some of my seeded succulents, just to speed up growth a bit in the early years.

In all other composts, however, I’ve abolished lapilli. It tends to absorb water, like pumice and holds it for too long. If you use lapillus in combination with high doses of peat, you have a potentially deadly mix for succulents: when I used it for the surface layer around the plant as well, it dried very slowly, and in a few cases, I think it was responsible for collar rot. No doubt, in more than one case the moisture from the surface lapilli contributed to several plants staining at the base.

Expanded Clay
Argilla espansa
Expanded clay

Many growers use it to create a draining layer at the bottom. I did this during the first few years of growing: I was terrified of possible rot and thought that the roots should have as little soil as possible, a nonsensical belief. Today I hardly use it anymore. I only put a thin layer at the bottom of the pot when I’ve large plants with no roots and want to root them in pumice. In those cases, I’ve to use large pots to hold the plants and prefer to lower the depth with coarse drainage.

Quartzite
Quarzite
Quartzite

It’s a pure inert. Great for drainage, I use it in conjunction with gravel as a heavy compost element. The only problem is that it costs money because in the place I live you can only find it as a material for aquarium enthusiasts. It’s a helpful element for seeding: it’s in a small fraction, and I use it as a draining agent in the compost and as a surface layer to help the seedlings “come up straight”.

Stabilized mixture
Stabilizzato (roccia calcarea)
Stabilized mixture

It’s a rock of a calcareous nature generally used to create walkways, lay-bys, and transit areas in gardens and parks. I got a bucket of it from a building materials dealer to test it precisely because it is limestone material. I added it to the compost that I then used with some Astrophytum, a couple of Weingartia and some Ferocactus from my sowing. I feared deleterious effects because of the nature of the material in combination with the water I use, very calcareous, but five years after the experiment, I have not noticed any abnormalities. In rare cases I have used it as a surface layer: with watering, it loses some of its snow-white to turn yellowish.

Earthworm Humus
Humus di lombrico
Earthworm humus

I’ve been testing this for at least six or seven years but haven’t seen any disturbing results yet. It’s an organic component (derived from cow manure), and it seems to me that it can easily replace peat. I always use it in low doses (10% or 20% in the final compost). I’ve also done some tests with highly organic substrates: one-third humus, one-third field soil, one-third gravel. I wanted to interpret in my own way the advice of Giuseppe Lodi (Italian pioneer in the cultivation of succulent plants), who in his book suggested to use one third leaf mould (for the organic part), one third field soil and one-third sand or gravel. I’ve been trying my own version of this substrate for five years now, and so far I’m quite happy with it.

Zeolite and Akadama

Finally, a couple of materials that I’ve tried out of curiosity years ago but I will not use anymore, Zeolite and Akadama. The former is a mineral material similar to pumice, while the latter is a clay-based material. Both were pushed by some growers (Akadama in particular by the Japanese) who described them as miraculous. I’ve used them in different dosages, always combined with other materials such as field soil, lapilli, gravel. In many cases, especially with Akadama, I’ve had plants that have completely lost their roots after only one year in the new compost. So, for this reason, I’ve definitely abandoned these materials, moreover expensive.

Substrates

The various materials I’ve listed can be mixed in an infinite number of ways to create a real substrate for cacti and succulents. At this link, you will find the various substrates I’ve made and experienced over the years.

My Youtube video

In this video on the Youtube channel of Il Fiore tra le spine, you can see some of the materials described in this article and some of the substrates that you can make with these materials/elements. If you like the video, subscribe to the channel!

You may also be interested in “How to repot cactus“, or “Watering cactus and succulent plants“.

© The texts, videos, photos and graphic elaborations of the site “Il fiore tra le spine” are original material and are covered by copyright. It’s forbidden to reproduce them in any way.

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