Marl as a substrate for cacti: is it really so good? How to use it and with which plants does it work

The use of marl as a substrate component for the cacti’s cultivation has been widespread for years, especially in Italy. Thanks to the studies and research conducted by my friend Andrea Cattabriga – grower, researcher and succulent expert at the international level. But what are we talking about, when we discuss marl? Quite simply, a greyish and highly friable rock, to the point of breaking up into flakes until it becomes powder. It’s used to create substrates for the cultivation of many cacti and some succulent plants when combined in some dosages with other materials such as quartzite, pumice, sand, gravel, lapilli, peat, field soil (clay).

In this article we explore the benefits of using marl in cactus cultivation, we see how to make a valid marl-based substrate, and we try to understand, above all, with which kinds of cacti this material can work and with which ones it should be avoided. (…)

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

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.

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

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).

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