Do you know that all the cacti are succulents but not all succulents are cacti? Both cacti and succulents store water in their stem or foliage, but the cacti also produce areoles, tiny bumps on the outside of the plant from which the spines develop. With the proper treatment, these desert native plants can be just as happy to grow on a shelf within your home.
Cactus requires little care — it can even withstand some neglect, as some varieties only need to be watered every two or three months. This brings us to the topic of soil for cacti — what kind of soil do cacti need?
What Kind of Soil do Cacti Need?
Cacti need a well water-permeable substrate. Wetness or prolonged damp stickiness leads to rotting and loss of cacti in cultivation. Therefore, very loamy to clay as well as extremely humic substrates (e.g. ordinary potting soil) are unsuitable for all cacti in cultivation, as these can hold water for several days due to the very fine and high humus content.
Sure, you can find some cacti species in loamy soil in their natural habitats, but these are located in completely different conditions (e.g. local climate and terrain conditions) than in your cultivation pot or greenhouse.
Commercial Cactus Soil
In retail (hardware stores, garden centers, etc.) there are ready-mixed “cactus soil” from numerous well-known manufacturers, even under the slogan “For all cacti and succulents”. These substrates essentially consist of organic material (often compost or peat), to which sand and fine gravel have been mixed in different proportions. Such substrates are quite suitable for some relatively ‘undemanding’ cactus species (e.g. the genus Echinopsis or Opuntia). But they are by no means suitable for all cactus species.
In addition to a comparatively high humus content which keeps water in the cultivation pot for a long time, the ready-mixed cactus soil have a low pH value. However, many cactus species thrive optimally in purely mineral substrates, which comes closest to their natural location. These can be substrates with lime content (pH >7) or without lime content (pH <7). In literature and in media, a consistently low pH is often recommended for the substrate of cacti in cultivation. However, this only applies to a part of the cactus species.
Calcareous, Humus-Free Substrate
This substrate is suitable for all non-frost-resistant and non-hardy cacti in cultivation behind glass or in the greenhouse or conservatory, which grow at the home location in calcareous soils from the weathering of carbonate rocks. The natural habitats of these plants extend from the southern USA to the northeastern part of Mexico. These include, for example, the genera Ariocarpus, Astrophytum, Ancistrocactus, Aztekium, Epithelantha, Ferocactus, Glandulicactus, Lophophora, Leuchtenbergia, Neolloydia, Strombocactus, Thelocactus, Turbinicarpus, Cumarinia, Echinocactus, Echinocereus, Mammillaria, Ortegocactus, Thelocactus, and Thelocactus.
Widespread in the areas mentioned are slightly alkaline, nitrogen-poor semi-desert soils with pH values between 8.0 and 8.5. In hot and dry climates, these soils, which are often rich in stones, have comparatively low levels of organic matter in the shallow topsoil. They are usually well below 1%. The organic substance comes largely from the accompanying grass and shrub vegetation (e.g. Fouquieria splendens, Castela texana, Bumelia celastrina, Bouteloua trifida or Monanthochloe littoralis). On the stony slopes of hills and mountains, on erosion sites such as debris or river deposits, there are usually only poorly developed raw soils with weakly pronounced, very humus-poor top soil.
In cultivation, the natural soil properties can be satisfactorily mimicked by adding calcareous raw loess (grain size between 0.002-0.063 mm). In addition to loess, the substrate should contain pumice and/or other purely mineral, lime-free components in different grain sizes (>0.063 mm). Mix pumice and other mineral components and loess in a ratio of 10:1. Too high proportions of loess can cause water to remain longer in the pot due to the fine grain content, which can lead to rotting.
Loess contains clay minerals (including montmorillonite and illite), which are important for the exchange of nutrients. Despite its low clay mineral content compared to clay, loess has a relatively high exchange capacity for nutrients. The pH values (pH in H2O) of the Central European loess are on average 8.1 due to the lime content, which is quite similar to the conditions at the natural cactus habitats and their soils. The pH value also explains the relatively high exchange capacity of loess, since this increases with increasing pH value.
Humus-Free Substrate with Low pH
This substrate is suitable for all non-frost-resistant and non-hardy cacti in cultivation behind glass or in the greenhouse or conservatory. Cacti for this substrate are plants that grow in soils from weathered products of ‘acidic’ rocks. These can be volcanic rocks, metamorphic rocks or sedimentary rocks. For example, cacti of the genera Copiapoa, Browningia or Eulychnia.
The substrate should contain a low clay content (clay minerals for nutrient exchange) and should be permeable to water by the addition of quartz sand and other mineral components. The latter can be perlite, brick chips, pumice or various other purely mineral, lime-free substances. Despite adding clay, the substrate should not clump or harden in the dry state. The mixture is optimal if the finished mixture briefly retains its shape after being pressed together in your hand, but then loosely falls apart. The mixing ratio of coarser mineral components to clay should also be about 10:1 here.
Low pH Humus-Containing Substrate
Such a substrate is suitable for cactus species that thrive well under glass with slightly higher humus content. These include the genera Cereus, Chamaecereus, Cleistocactus, Denmoza, Echinopsis, Gymnocalycium, Lobivia, Marginatocereus, Melocactus, Myrtillocactus, Neobuxbaumia, Neolloydia, Opuntia, Parodia, Rebutia, Stetsonia, Sulcorebutia, Trichocereus, and Weingara.
The substrate mixture should consist of approximately 80% purely mineral components and approximately 20% organic material. The mineral content can be made up of a mixture of lime-free sand, fine gravel, perlite, brick chips, seramis, pumice and, for example, crushed expanded clay (substrate for hydroponics). The organic content can consist of sifted compost (cost-effective) and/or conventional potting soil or garden soil. Peat as an additive also enables a low pH, but should not be used to protect the bogs.
Such a substrate is also suitable for hardy cacti outdoors. For example, Echinocereus triglochidiatus, Echinocereus caespitosus, Echinocereus viridiflorus, Echinocereus coccineus, Escobaria vivipara, Escobaria missouriensis, Opuntia imbricata or Opuntia polyacantha.
Examples of Low-Cost or Free Sources of Substrate Components
- Smashed or broken unused clay pots (= brick chips)
- Smash used and cleaned expanded clay
- Sand, gravel, pumice and loess from mining areas (ask politely and observe regulations!)
- Collect sand, gravel and other mineral components from the natural environment
- Own compost, sieved
- Compost from a composting plant
- Inexpensive potting soil
Substrate for Leaf Cactus
Leaf cacti mostly live epiphytically (sometimes also lithophytically on rock and terrestrially on the ground). Epiphytes (from ancient Greek epi = on and phyton = plant) are plants that grow e.g. on trees and their branch forks. Over time, sparse organic material and dusts accumulate in them, which serve as a substrate for the plants.
The collective term “leaf cacti” refers to a number of cacti of various genera, including Disocactus, Epiphyllum, Hatiora (the Easter cactus, a hybrid of Hatiora gaertneri and Hatiora rosea), Lepismium, Rhipsalis, Selenicereus (e.g. Queen of the Night), Zygocactus and Schlumbergera (the Christmas Cactus).
Most leaf cacti also need a water-permeable substrate in cultivation to prevent rot. The substrate for epiphytic cacti should therefore, similar to an orchid substrate, be extremely loose and permeable to the water, contain humic components and no lime, but still offer the plant sufficient stability in the pot. The mixture can consist of one third of coarse organic material (e.g. pine bark or mulch of pine bark from the hardware store), one third of commercially available, humus-rich soil (potting or planting soil) and one third of inorganic material (e.g. perlite, seramis, pumice or broken expanded clay).
To make it easier, here are the proportions:
- 1/3 coarse organic material (pine bark or mulch from pine bark)
- 1/3 humus-rich soil (potting or planting soil)
- 1/3 inorganic material (perlite, seramis, pumice or broken expanded clay)