Substrate, loam, soil mix: which is the best soil for cactus and succulent plants?

Mammillaria pectinifera

I may look like a mordant, but I want to clear the field of misunderstandings and false myths: the right soil for cacti does not exist. There are many types of soil (or composts, substrates, mixtures, the question does not change) and there are genera that prefer certain substances and others that require more. Established this and removed one of the first Faq (Frequently Asked Questions) by cacti enthusiasts to the first arms – “Which is the best soil for my cactus?” – it can be said on the contrary that on the one hand there are the characteristics that a good soil for cactus must necessarily have; on the other hand the needs of the single plants.

The question was simple and the related answer was given by italian cacti expert Giuseppe Lodi, who, after observing “the butts of roots of certain imports” and having noticed how these were encrusted with clay loam, suggested a base soil absolutely natural and versatile: “You can start from a mixture of common clay loam (field or garden), coarse sand and leaf soil, in equal parts. Of these three components none of them can be enough, alone” (Giuseppe Lodi, “Le mie piante grasse” – Edagricole).

Except perhaps for the difficult availability of the loam of leaves (be careful to go for the woods and get bags of decomposed foliage: there are fines for collections of this kind), the recipe provided by the Italian pioneer in the cultivation of cacti and succulents was more than sensible, as well as experienced. Considering the difficulty of finding the loam of leaves (Lodi suggested leaves of beech or chestnut), that moreover must be well decomposed (and it may contain fungi and bacteria dangerous for the plants), this element can be replaced by good quality peat, sieved fine, without lumps and filaments. Too bad that over the years we have forgotten Lodi advice to focus everything on what for many is still the standard substrate of peat, lapillus and pumice in equal parts, standard enough to fit any kind and species of succulent.

In the following article now we see which is the best substrate for growing cacti and succulent plants based on my experience over the years with proofs and experiments on various mixtures.

Field loam and nutrients
Ferocactus chrysacanthus, una mia semina
Ferocactus chrysacanthus, my seedling in marl soil

In recent times many growers have experienced a bit of everything, in many cases returning, in any way, to the initial suggestion. Basically, it is a back to the ground question. On the other hand, why not start from here, since most of the cacti, in nature, have their roots in common clay and inert soil? Of course, there is soil and soil, and it will be hard that a field in North Italy has the same composition like that of, for example, an area of Arizona where those beautiful Ferocactus thrive. However, already by saying clay soil (possibly slightly calcareous; just do a pH test with the classic leaflets), one narrows the field. With some experiments with loams from different areas, then, you can understand whether the one we have is suitable or not.

The other two components indicated by Lodi provide us with other precious indications: the loam of leaves (or, jumping nowadays, the peat of good quality) is rich in nutrients for the plants and represents the organic component of the substrate, while the coarse sand, inert almost pure, denotes the draining element.

Here it closes the circle: the three elements mentioned mixed together lead to a substrate that will inevitably be dissolved, draining and enriched by a healthy amount of nutrients. In other words, it will have the fundamental characteristics for xerophiles (plants that have adapted to arid environments).
Moreover, thanks to the clayey soil and the coarse sand, the substratum will tend to dry up quickly, imitating what happens in habitat, where, to torrential rains follow more or less long periods of drought, during which the ground completely dries up, to produce the classic spider-web fractures.

Here you can find an article on watering cacti and succulent plants.

The traditional mix
Miscela standard (pomice lapillo torba)
Traditional mix with peat, pumice and lapillus

Why insist with the triad peat/lapillus/pumice in equal parts then? Because these materials are easily available, inexpensive, driven by beliefs now rooted (pardon the pun) in the heads of many fans. It is said: the cacti fear the water (another conviction on which it has been wrongly embroidered to excess), therefore little peat, which holds the damp, and many inerts. Then, pumice and lapillus are not totally inert (pumice, for example, contains several elements such as silica, iron, manganese) and, because of the porous structure, absorb and retain water. For this reason, the pumice is excellent because it releases it a little at a time, while the lapillus travels on much longer times and in perspective can be dangerous.

Mind you: the standard soil based on peat/lapillus/pumice is not wrong; if accompanied by careful cultivation in terms of light, watering, fertilization, it works very well for most succulents. The problem is that with its 30 and more per cent of peat it retains moisture longer than poorer substrates, and helps to push the growth of plants, which in nature, also under the different substrate, have slower rhythms. Since I try to ensure that my plants resemble as much as possible to those of habitat, I prefer other soils, restricting the use of the standard to young plants that I want to grow a little more quickly at least during the first two/three years.

My basic substrate
Composta con terra di campo e inerti
Substrate with clay and inerts

Over the years, with the advice of my friend growers and through the experimentation on more specimens from the same sowing, I have selected a substratum that mixes natural elements with elements not present in the habitat. I use it for most of my cacti: it is loose, draining and sufficiently nourishing. It does not force growth and dries in no time. What is the recipe?

– 20% of calcareous clay field soil (with a part of fine sand)
– 10% of fine sieved blonde peat (alternatively I used earthworm humus)
– 40% of pumice (3 to 6 mm)
– 30% of river gravel (sometimes with a quartzite part)

It is a substrate that collects Lodi suggestions, adapting them: there is the field soil, the (little) peat for the organic element, and the inert, that is the river gravel. This is my standard soil, I use it for genera like Echinocactus, Ferocactus, Mammillaria, Astrophytum, Parodia, Opuntia, Tephrocactus, Thelocactus, Ancistrocactus, Echinocereus, Neoporteria, Escobaria, Coryphantha and many others.

The marl
Scarpata di marna
Marl in habitat

At this point, I’ve to talk about marl. I don’t go into detail here (I’ll do it in another post), but this element, also of clay origin, for several years is intriguing many aficionados (first of all, thanks to the research and plants of Italian grower Andrea Cattabriga). However, at the same time, other growers are censuring it.
I find it very suitable for many genera, especially those with taproot roots, like Ariocarpus, or slow-growing, like Pelecyphora, some white Mammillaria, some Turbinicarpus, Epithelantha, etc. The marl tends to slow down the growth, from what I have been able to verify, and for this reason, it is excellent for keeping compact the shape of globose cacti. I have experimented it in different percentages, and I have seen that on some species and on young plants if used in high doses (more than 50%) the development slows too much.

The compromise I reached is this:
– 50% of marl
– 10% of fine sieved blonde peat
– 20% of pumice
– 20% of river gravel

Richer substrates
Aztekium ritteri in gypsum

For some genera of cacti, I use more targeted soils, trying to satisfy the needs of the plants. To know the various needs of our cacti you need to learn about texts or websites reliable or compare with other enthusiasts, although the best thing is as always the experience. Gymnocalycium, for example, generally want richer soil (although I have several in my standard soil and they are fine). For this genre, like Parodia/Notocactus, I handle a variant of my standard compound, to which, in fact, I increase the percentage of peat:

– 20% of clay soil (with sand)
– 20% of peat (acidic) or earthworm humus
– 30% of pumice
– 30% of river gravel

On several sowings of Echinocactus, Ferocactus, Gymnocalycium, Mammillaria, Thelocactus, I also tried a very simple loam, still inspired by Giuseppe Lodi: earthworm humus, river gravel, field soil in equal parts. I’ve been using it for a couple of years, and it seems to be getting good results so far.

Moulds for succulents

For Agaves, Aloes and succulent plants like Crassula and Echeveria, I use a quite rich soil:

– 30% of field land
– 20% of peat
– 50% of various inert matter (depending on what I have available)

Over the years, I have been able to see that this type of mix meets the needs of succulent plants, not cacti, favouring the growth and retaining the humidity a little longer than the soils I use for cacti.

Mix with plastern (gypsum)
Epithelantha micromeris in gesso
Epithelantha micromeris: my seedlings in gypsum

For the plants that live in nature on plaster (Aztekium, Geohintonia, Turbinicarpus hoferi and lophophoroides, for example), for some years, I use this mix:

– 40% of marl
– 20% of agricultural gypsum
– 20% of pumice
– 20% of river gravel

Even this type of mix, when used with the right plants, gives excellent results in terms of balanced growth and drainage.

Marl and clay mixed together

Finally, for some plants such as Copiapoa, Turbinicarpus, Thelocactus Epithelantha, I also experienced a middle way between marl and standard soil. I tried to reduce the slowdown given by the marl mixing it with clay field soil:

– 20% of marl
– 20% of clay soil (with fine sand)
– 10% of sifted blonde peat
– 20% of pumice
– 30% of gravel of various sizes

Observe the reactions of plants

The possibilities, considering materials and elements available (some easy to find, others less…) are endless. Also, in this case, from my point of view, the rule of direct observation on specimens obtained from the same sowing is valid for testing the differences in the growth.
For those who wonder it after seeing some photos on this site: usually, when the plant is potted, I put a surface layer about one centimetre high on top of the compost. I use, for the superficial layer, pure clay, clay mixed with sand, clay with quartzite, marl in purity, marl and quartzite. Other times I just leave the compost on view. Anyway, it depends on the taste, without any pretension to visually reproduce the habitat.

Substrate and proper growth

Finally, let us not forget that the substrate, like other elements such as exposure, fertilization and irrigation regime, hugely influences the development of plants, starting from the strength of the thorns.
Here you can find an example of targeted repotting, i.e. plants from identical seeding repotted in different soils to evaluate the influence of the substrate on growth.

My video

Texts and photos covered by copyright. Reproduction prohibited.

 

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