Materials for an inorganic cactus and succulent mix

Since I started my advocacy on using mineral mixes for cacti and other succulents, I have been getting swamped with inquiries regarding the types of rocks to be used, the ratio in relation with the other materials, and other nuances. In this post I will be getting more specific and discuss why some materials are not really suitable for use in this regard and why some ingredients are better than the others. But first let's talk about the pots.

Pots

Perhaps no other branch in horticulture has experimented with so many different kinds of containers than in the succulent sector. The often fantastic shapes of succulent plants lend themselves very well to such containers, many of which can be downright outrageous or even more beautiful than the plants they hold. Whichever container you use, it would be for the best interest of your plant if you choose one that is one or two steps larger than what you would normally consider for the size of the plant. I have said this before and will do so again: an over-potted plant has more extensive root runs and the larger volume of media available around the roots provide a better buffer against lapses in cultivation or sudden swings in weather conditions. So the next time you are on the look-out for new pots, put your plants' needs into consideration first, certainly above your desire to make your plants look 'cute'. And, refurbished containers are nice, but do not forget to put drainage holes under them. Common sense dictates this.

Crocks

Crocks are used for promoting drainage and are placed at the bottom of the pot. One can use practically any material as crocks: broken pots, gravel, large pieces of pumice, even chunks of Styrofoam. Grain size if preferably from 2.5 to 5 cm (1 to 2 inches) but I do not worry myself that much with particle size and I can even include sizes smaller or tad larger than the prescribed sizes, depending on what is at hand.

The medium: some considerations

This is a topic that has stirred great debates and always of great interest, mainly because ones' success with these plants is entirely dependent on media. In my posts I have always asked people to reconsider their choice of using a highly organic soil, due to one very simple fact: most succulents, cacti included because these are also succulents, evolved on soils with little, or wholly devoid, of organic matter. However, I am not a purist either to espouse the total elimination of organics- it is of value, but instead of using it as the main ingredient and then adding the inorganics as mere components to encourage drainage, one perhaps should ask this: would organics be needed at all for this plant? This shift in mindset places more importance on the role of inorganics as more than just drainage promoters but the source of a plant's nutrition, with the organics playing a very minor part in a soil's composition, if at all.

Yes, you read that right. inorganics are more than inert. These materials, in all their entirety, are the nutrients. And for as long as there is that lump of rock resting next to your cactus, you can be sure that it's leaching nutrients for your plant's needs- assuming you are using a right choice of rock. The same cannot be said for organics: organic matter eventually decomposes, which would require changing the medium, or its souring can and will spell disaster for your plants if not replaced in time.

What then are the qualities you need to find in an inorganic material? I believe that the first consideration should be its roughness. Smooth rocks have less surface area and furthermore are less likely to be inhabited by colonizing endophytic bacteria. These bacteria slowly degrades the inorganics which render its nutrients to be available for your plants. Of course you can use smooth rocks and pebbles, but doing so will not maximize the potential of such bacteria. They're just there, as nutritious as junk food. A second consideration is the stability of its pH. Some, like limestone, greatly increases the alkalinity of the medium over a short period of time. Some sedimentary rocks, with the notable exception of sandstone, also have fluctuating pH values. If you are unsure of the rock present before you, don't use it. The last consideration is its structural integrity. Some 'rocks' are soft are crumbles easily when held. I have used these types but in small quantities and under very careful handling. I certainly will not recommend using these types until one has gained enough knowledge and experience.

Generally speaking, all rocks suitable for aquaria, with the exception of limestone, which is used for Rift Lake cichlids, can also be used for succulent mixes.

Dragon rock, used extensively in Iwagumi aquascapes, should look particularly dramatic on well-thought dish gardens.

Above and below: some rocks from rivers are quite rough and usable for succulent mixes, in small proportions.

Shown above is an example of sedimentary rock, with its accumulation of sediments. Proceed with caution when using these rocks for plants.

Rocks are loosely grouped into felsic and mafic types. Felsic rocks are light in coloration while mafics are darker; serpentine rocks are so-called because of their dark green color. When using rocks for succulent mixes, a ratio containing more felsic rocks is preferable than one dominated by mafics. Now let's talk about the components.*

*I am not a geologist and would not pretend to be one. Should any mistake or misrepresentation have been committed, please feel free to point them out.

Sand

The most often recommended sand type within horticultural circles is river sand. This is the same type that is quarried for general construction use (the so-called 'construction sand'  ). As much as I can, I stay away from using this type of sand because it eventually becomes impacted and it actually stays moist for longer periods than one would want in a succulent mix due to capillary action; this combination of properties can invariably rot the roots. Furthermore, river sand attracts ants such as the tropical fire ant (Solenopsis geminata, known in Tagalog as 'langgam na pula' which translates to 'red ant') and a variety of black ants, some of whom bite and sting and/or carry with them ubiquitous pests such as mealybugs and aphids. Combine river sand with an organic mix and you have just created the perfect recipe for establishing a colony of ants. You can use this sand type if no other similar material is at hand, but I suggest using it for not more than 10% of the entire mix.

Beach sand is sometimes used, which is composed most commonly of silicon dioxide in the form of quartz. Compared to river sand, beach sand dries quicker but still will not recommend using it to anyone. Because these are deposited on seas shores, beach sand naturally contain salt, which of course will have dire effects on non-halophytic plants. Repeated washing and soaking can rid the sand of salt, but I do not see quartz as a necessary component in succulent mixes anyway- except maybe as an ingredient for Lithops and other mesembs. When purchasing quartz, ask the seller about its provenance, and wash it repeatedly

Beach sand often have rounded sides, as a result of continuous friction of one grain against the other due to wave action. This is beach sand I have been using for years for aquarium purposes, but never for cacti and succulents. 

Pumice sand is sometimes available as a construction material, under the name 'white sand'. In terms of water retention, pumice sand falls midway between river sand and beach sand, and is actually quite lightweight- an important consideration when you have numerous plants in a shelf. I personally use this material in my mixes; for cuttings it can make up for as much as 50% of the entire mix, but for larger plants, I cut down its percentage to around 20 or 30%, and there are mixes in which I do not even use it. If your individual rock sizes are rather large, at about 1.5 cm and above and homogenous, then there would be plenty of spaces in between, and although that is good from an aeration point of view, the same advantage can quickly escalate to the excessive and prolonged drying of the roots, which of course can kill your plants. If you are using particles of heterogenous sizes, then there is no need to use sand.

Rocks

1. Scoria

Also known as volcanic scoria or volcanic rock, which applies to a quite large array of rocks that differ in their properties, scoria is widely available in horticultural shops, even on pet shops. Many succulent hobbyists use this, but I feel this is just a reflection of its wide availability than anything else. If other materials are as easily available, then I doubt scoria will find its way into succulent mixes as frequently as it does.

This material is highly permeable and thus promotes excellent drainage, but the same property also makes it undesirable for our mixes. Granted, succulents require mixes that do not retain too much moisture, but these plants still do need some degree of water around their roots. Scoria denies them that. You can use this material but only as an additive, say, not more than 10% of the total.

Click the link below for an image of the very commonly available red scoria:
http://www.seayearthmovers.co.nz/quarries/quarries-2/scoria-quarry/

2. Coral rocks

Frankly, I do not see why people would want to use this material. People who do are under the impression that incorporating coral rocks in their mixes make their succulents look more 'natural', but except for truly halophytic succulents found in more northerly latitudes as well as some opuntioids growing on cliffs facing the sea, I cannot imagine seeing cacti and succulents, the vast majority of which do not grow near the sea, growing on coral rocks.

Aesthetic considerations aside, coral rocks bring the risk of leaching salts into your mixes. Even when properly washed, its high calcium content will not bid well for our plants, as most succulents are calcophobic. If you are maintaining calcophilic plants like Echeveria chihuahensis, for example, you can opt to use a little limestone or marble chips; for Aztekium, use gypsum. With other choices being readily available, there is no reason to make your plants look like they're growing on a beach. Because they don't.

Coral rocks are actually remains of marine organisms.

3. Smooth pebbles

Smooth pebbles are easily available in pet shops and landscaping supplies, as well as home furnishings and understandably attractive to cactus and succulent hobbyists. But smooth pebbles are, as mentioned above, not easily colonized by bacteria, and your plants' roots cannot easily adhere to these materials either. Of course you can use it as an additive, but it's really just dead weight taking up valuable space for other rocks that can perform far better for your plants.

An even worse type to use are those that have been painted in gaudy colors. Those paint may be laden with toxic compounds that can leach slowly every time you water your plants. Aside from the risk concerns, colored pebbles for your succulents are nothing short of kitsch. Get real and get serious.

4. Construction gravel

The most often available gravel for construction use is sourced from basaltic rocks, less commonly on gabbro. So it would be prudent if we discuss basalt instead.

Basalt is an igneous rock that is volcanic in origin and are among the most common deposits of volcanic material around the world. Technically, it is not a mineral although it does contain minerals, the most dominant of which are olivine, pyroxene, and plagioclase (which contains feldspar). It is a mafic type of rock, and mafic rocks are rich in magnesium and iron, and often dark colored. Apart from magnesium and iron, basalt contains silicon and oxygen. For purposes of succulent mixes, basalt is usable, although keep in mind that in a mineral mix, there should be a higher percentage of felsic rocks (please keep on reading for a discussion on felsic rocks) than do mafics. But if are maintaining cacti from the genus Frailea, you may want to use basalt generously as some of its most popular species hail from basalt outcrops, particular;y in Uruguay. But there is one major hang-up aesthetically: gravel in itself does not lend an authentic look compared to larger blocks of naturally cut basalt. But if you know existing quarry sites, you can drop by for a visit and collect larger pieces for yourself. Additionally, trap rocks are actually basaltic in composition. Have you seen rock formations along hillsides or roadsides that are arranged like staircases or diagonal sandwiches? That's trap rock.

S1, which is available in hardware stores, is more finely crushed basalt. I often use it much like sand especially for small plants and rooting cuttings, but has the advantage of being more open and unattractive to ants.

Gabbro on the other hand is very much like basalt except for its coarser texture. Like basalt, it contains plagioclase. In the trade, it is often mistaken for granite and also used as materials for countertops and tiles.

Vermiculite is naturally hydrated basaltic rock particles and can also be used, but very light and therefore floats on water.

River gravel is an altogether different material but can be used for cacti mixes if an alternative is in short supply or altogether lacking, but make sure that the granules have rough edges.

S1 is crushed aggregate and is widely available in hardware construction supplies.

Basalt typically is fine-grained, but display rough granulations when broken into pieces.

5. Granite

This is perhaps the most useful type of rock for cactus and succulent mixes. It has very little porosity and practically unaffected by extremes of temperatures. Like basalt, granite contains both silicon and oxygen, but both are comparatively in higher percentages in comparison. The magnesium and iron content of granite is paltry compared to basalt, however. Calcium, manganese, sodium, potassium, rubidium, strontium, and titanium is also found on granite. Additionally, alumina is found in granite, together with silica, due to alkali feldspar being one of granite's major mineral component.

Be aware that the term 'granite' is frequently applied on other types of rocks with similarly visible grains. Where to search for granite? Any store that sells natural stones is a good place to start. If a store sells granite countertops or tiles, they will most likely have damaged pieces that they just toss around in their yard. Ask if they would be willing to sell such pieces- most likely they'll be willing to.

Please proceed here to know more about granite:
https://geology.com/rocks/granite.shtml

6. Marl

Marl is a soft, fine grained, sedimentary lime-based deposit that is made up of clay and silt; it is actually mud that has hardened a bit. Due to its clay component, marl has a high cation exchange, which means that it is conducive to plant growth, although clay in the tropics is usually positively charged and therefore can hold different elements than the counterparts in latitudes elsewhere. For a better understanding of what a cation exchange is, please go here: https://www.youtube.com/watch?v=HmEyymGXOfI

However, there are two considerations that have to be discussed with regards to marl. First is that it's crumbly. If you are going to use it, make sure not to crush the particles too much or drainage may be impaired. Marlstone is similar, but as its name suggests, is harder and more stable in its structural integrity. Secondly, because of its lime content, marl is only to be used on plants that are calcophilic, or those that require calcium in their soils. Some of the more challenging cacti fall into this category: Ariocarpus, Aztekium, Geohintonia, Pelecyphora, some Turbinicarpus, as well as a few others. You should NOT use this material on plants that are calcophilic, which includes a vast array of species from a wide assemblage of genera, and if you are going to use marl, then do not use limestone.

So where can you get marl, then? It is not commercially available, but it can sometimes be found on road embankments- marl is usually dark gray with a fine texture, but can also be reddish, depending on the nature of the deposits. It is a material used in the production of cement, so if you know a place where cement producers mine their raw materials, chances are that you'll find marl there.

Marl can differ in color, depending on the nature of the deposits. Photos by Paulo Jerome L. Enguito.

7. Loess

Loess is sedimentary rock deposits that is composed mostly of silica and clay. Loess is rich in strontium and calcium, and also rubidium, titanium, and zirconium. Rubidium's role in plant health is sketchy, though most likely not harmful, while titanium has been shown to increase the chlorophyll content in tomato plants grown hydroponically. I am not aware of any effect zirconium has on plants.

This material is commonly found in nature, often on roadsides and usually described as 'crumbly rock', but is not advised for use in cacti cultivation due to risks of blockage as it can get too fine to be dangerous for plant roots. I do however use this material but only choose the more solidified pieces and at only about 10% of the total mix.

An example of loess deposit. When using this material, choose ones that are not too soft, preferably those that would require a hammer to break into smaller pieces.

8. Laterite

Highly weathered igneous rocks take the form of laterite, which is basically oxidized iron and aluminum. This material has gained importance in aquascape use, but can also be used in horticulture due to its iron content. When using this material for succulents, keep it minimal and preferably only at the uppermost levels of the mix. Being clayey, it runs the risk of impeding drainage when used generously. It is widely available in aquarium shops but can also be found on road embankments in high rainfall areas; its presence is given away by its rusty red coloration.

9. Limestone

Limestone is of sedimentary origin and high in calcium carbonate due to the influence of skeletal and shell remains of marine animals. Limestone is rich in aluminum, calcium, iron, magnesium, potassium, and sodium compounds. As with marl, use this only for calcophilic plants, but it should only be used as an additive or perhaps not more than 30% of the entire mix.

Marble chips are commonly and widely available but cannot be recommended as a substrate ingredient. It is highly unpredictable in its reactions, except for being less porous than limestone and therefore less likely to influence the soil pH.

A piece of limestone.

In its natural state, limestone is often jagged, as a result of eons of erosion due to water percolating on its permeable surface. Photo taken from southern Palawan.

Limestone wall, Samar.

Crushed marble

10. Clay bricks and broken pots

Yes, you can use both, but the former is the better material. Because both are made of clay, expect the same elements to be present. A nice property of both is that these are very porous materials that actually hold water- the exact opposite of scoria. What this means is that even if you are away and cannot of course water your plants, some of the stored moisture in clay bricks is still available for absorption by the roots. You can use either of the two materials in conjunction with pumice/granite/basalt.

11. Pumice

Perhaps the most widely used ingredient for succulent cultivation, pumice is also of volcanic origin like scoria, but lighter in color and in weight. It is characterized by vesicles on its surface as a result of gas bubbles escaping as the material cools down; the vesicles in scoria are noticeably much larger in comparison. Pumice is rich in silica content but low in iron and magnesium.

Perlite is similar in composition, but lighter and has the annoying property of floating on water and clinging to the pot sides.

Pumice is easily determined by the numerous vesicles on its surface, which came as a result of gases escaping as it cools after expulsion as volcanic material.

12. Lava

Lava is another good material, and contains aluminum, calcium, iron, magnesium, potassium, phosphorus, sodium, and titanium, with large amounts of oxygen and silicon. Lava is sometimes available in horticultural shops.

13. Volcanic rock

This is a generalized term used for rocks somewhat similar to scoria for being dark colored, but denser and heavier, and with smaller vesicles. Types with extensive vesicles are used for exfoliation (called 'panghilod' in Tagalog). This rock contains practically the same elements found on lava but much less porous than scoria, making it a desirable ingredient in cactus mixes, and being acidic, can also be used for other succulents. These rocks are usually deposited on river beds and very easy to distinguish due to their vesicles.

A slab of volcanic rock protrudes behind this Anacampseros rufescens.

A type of volcanic rock used for exfoliating skin.

14. Gneiss

Gneiss (pronounced 'nice') is metamorphic rock that is often noticeably foliated, giving it a banded or striped pattern. Interestingly, gneiss is an aggregate of both felsic and mafic rocks. The light portions contain aluminum, oxygen, potassium, silicon, and sodium, while the dark bands carry iron and magnesium. There are a number of South African succulents, especially stapeliads, found on gneiss.

Gneiss is used as a building and finishing material and hence can also be found on home furnishing stores, like granite. However, it can be found on fancy names which can confuse novices. But commercially available gneiss have decorative striping which makes them easy to distinguish.

15. Sandstone

Sandstone is basically sedimentary sand bound by calcite, clay, or silica and has hardened due to pressures from overlying layers. Sandstone can be of various colors depending on the nature of the deposits. Feldspar is a common component in sandstone.

This rock is suitable for use in succulent mixes and is a good material to stabilize and support the roots.

16. Slate/Phyllite

I am not sure what this rock could be, other than it's a foliated metamorphic rock. I am however, leaning towards phyllite due to its luster. Slate is often matted. Quartz and feldspar are minerals that are found on phyllite.

Phyllite slabs are often sold as pavers or sidewalk stones, but are also quite easily acquired from aquarium stores. This is a favorite material of mine as it lends a very natural appearance to the plants.

                                                                                                                                                       
Two plants below on phyllite:
                                                                           

                                                                                                                                                     
Some final words           

I could have discussed other materials too, but this post would then be on the brink of getting tedious. So with that in mind, I tried to present only those materials that are likely to be easily obtained by those interested to try out this approach.

Lastly, with all the information hovering around on what constitutes a good cactus and succulent mix, many of which are conflicting and/or suitable only if you have a greenhouse, I urge you to search for photos and videos of plants in the wild, and be attentive of details. Nature after all, is the best teacher.

Got questions and suggestions? Post them on the comments section below!