The Nursery (Part 9: Soil components)

In this post I will expand upon Part 4 of this series and go into more detail about the various soil components I currently use (to a larger or lesser degree) for my cacti and succulents. I will first discuss organic and inorganic soil, and then go through the different soil components I use to a larger and lesser degree.

Organic or inorganic soil

I prefer an inorganic soil to an organic based one for most of my plants when they are past the seedling stage. There are several reasons for this. One is that most of the species I'm currently growing (and most of those I like the best) are susceptible to overwatering and thus require a very free draining soil which doesn't retain moisture for a long time. This is easier to achieve with an inorganic soil than an organic one. Another is that inorganic soils reduce or eliminate the risk of several pests such as the sciara fly and (I believe) the root mealy bug. A further reason is that an inorganic soil with little or no freely available nitrogen will more easily induce a compact form of growth, compared to organic based soils in which plants more easily stretch.

Yet another reason is that with an inorganic soil it is possible to grow the plants completely without the use of fertiliser and have them look very natural (approximating the "natural" look in habitat), though I am not currently growing any of my plants without fertiliser. If one chooses to grow the plants without fertiliser, one must take care to choose a soil mix that contains all the nutrients the plants need. In the journal Acta Succulenta which is a free online journal available for download from their website Acta Succulenta, there is a very interesting article in the second publication of this year discussing a method of growing cacti they call WIG (Wild Grown). A similar method is discussed at length in a Xerophilia Magazine special edition called "The Rock Eaters", which is also an excellent guide to the use of inorganic soils. It is available for download from their homepage Xerophilia. Personally I find plants grown this way to look more beautiful than plants seen grown in organic soils which tend to become more elongated or bloated - though this also depends on the species.

Example of an inorganic soil.

A very interesting article which appeared on the BBC a few years ago ago reveals how (many or most?) cacti live in symbiosis with bacteria in the soil that break down rocks around the roots allowing them to absorb nutrients that wouldn't otherwise be available. It also shows how the plants incorporate these bacteria into their seeds. This explains how it is possible to achieve excellent results by growing cacti in 100% inorganic soil without using fertiliser.

I grow almost all my current North American species in an inorganic soil. Only about one tray with North American species are potted in an organic based soil in order to compare their growth with similar species grown in an inorganic soil. So far the plants grown in the inorganic soil seem to do better and stay more compact. Since they all share the same level of light and amount and frequency of waterings, as well as the amount of fertiliser given and soil volume, and they're all the same age, I think the only thing of importance that separates them is the soil composition. The ones in an organic based soil (particularly Epithelantha and Mammillaria) all tend to elongate and become more bloated compared to those grown in an inorganic soil.

I do grow some species in an organic based soil because I feel they do better with some organic matter. Chiefly among those are the South American species such as Rebutia, Sulcorebutia, Lobivia and Frailea (and any other South American species not growing in particularly arid environments). I kept my Frailea asterioides in an inorganic soil for some time, but they grew only very slowly. I keep my Discocactus horstii in an inorganic soil, however, and they are all growing very well - though in nature they grow in almost pure quartzite sand and gravel.

Example of an organic soil. This one is based on coir.

What you decide to grow your plants in will depend on your own beliefs and ideas on which soil is the best, which soil components are readily available where you live, the cost of the various soil components, and the types of species you like to grow. There is no answer to which soil is the best and almost every book on cacti will offer different advice. One should also keep in mind that cacti grown in pots are wholly removed from their natural habitat and must be treated accordingly. Adaptations must be made to account for a (probably) much reduced living space for the roots, a different fungal and bacterial flora in the soil, and of course usually a vastly different climate.

Just like seeds will readily germinate in commercial cactus soil mix, so will most species grow happily in the same mixes. They will be more prone to rotting though. If using such a mix I would definitely recommend adding extra grit such as perlite or gravel to increase the drainage.

Soil components

There are a plethora of different soil components available, and the following simply represent the various things I have personally used and have experience with. As I said above, there is no "correct" soil and one grower will have positive experiences with one soil while another will not. 

Peat

The only soil component that I would actively advice against is peat. I discussed my reasons for this in Part 4, but to summarise very shortly: peat takes a long time to dry out and when it has first dried out it is very difficult to re-wet; it is a magnet for the sciara fly, and the root mealy bug too, I believe; it quickly becomes compacted and reduces the amount of air available to the roots; it clings to the roots so that during repotting it is difficult to remove all of it without damaging the roots; it often leads to a less developed root system; it contains a high amount of nitrogen (and is often augmented with fertiliser high in nitrogen) which can lead to root burn and also abnormal growth; and finally it is not very environmentally friendly as the extraction of it destroys natural habitats for many species and can take centuries or millennia to reform, as well as leading to increased greenhouse gas emissions through the release of CO2 and MH4.

The only reason to use peat in my opinion is if you are just starting out with the hobby and only wish to keep a few species in the window sill, or if alternatives are very hard to come by.

Compost

I'm only mentioning this very quickly as it is probably the most commonly used ingredient in organic based soils. I do not use it myself because I have not found a good manufacturer yet. In Norway there is as good as a monopoly when it comes to garden centres, and by far the biggest (and almost only) garden centre does, to the best of my knowledge, not sell composts - at least not any that are useful for growing cacti. Some local garden centres or plant schools may still sell quality composts though, again, I have not really come across any. A good quality compost is probably the best ingredient to use in an organic based soil because it provides most of the nutrients the plants need which reduces the need for fertiliser.

Coir

Coir is fibres extracted from the husks of coconuts. It is a far better alternative than peat for organic soil mixes. Among the advantages are that it is naturally free of bacteria and fungi, it is easily rewettable after drying out, it doesn't compact like peat, it doesn't cling to the roots, and it rarely clumps together (and if so the clumps are very easily crumbled apart). Among the disadvantages is its lack of nutrients which means a greater need for fertiliser, its light weight (which in some cases can be an advantage, though), and the fact that it is poor in calcium and magnesium. A lack of magnesium is a problem for all plants, but a lack of calcium is also especially negative for most cacti. This can be remedied by adding calcium and magnesium through fertiliser or by adding rocks such as dolomite (containing both magnesium and calcium) directly to the soil. Dolomite is an excellent rock to add to the soil for most cacti regardless of whether you use coir or not.

Finally it is also important to check whether the coir you have has had fertiliser added to it. As with peat mixes, I believe fertiliser is often added to bags of coir too.

Coir. A very good substitute for peat.

Leca

Leca (light expanded clay aggregate) is produced artificially by heating clay at very high temperatures. It improves drainage and with it's honeycomb structure it also retains a lot of air. Leca usually comes in sizes too large to be of much use in small to medium size pots. In large or very large pots it would be useful as a soil additive. For smaller pot sizes the main use would be to increase drainage by placing a shallow layer of leca pebbles in the bottom of the pot. It can also be useful as top dressing in larger pots, though it should ideally be washed before use to remove the dust coating the pebbles.

Leca pebbles are usually too large to be of much use in small and medium size pots.

Expanded shale

As with leca, this product has also been fired at high temperatures in order to make it expand. Shale is a naturally finely laminated (fissile) sedimentary rock consisting of clay and silt size particles. When it expands this structure leads to the rock becoming very porous. It has similar soil improving characteristics as leca, only more suitable for small to medium size pots. I have used two sizes, a 4-8 mm grain size and a 1-3 mm grain size. For smaller pots the larger grain size is still a little on the big side, but the smaller grain size is ideal. Similarly to leca it doesn't lose shape or disintegrate. The larger size expanded shale is also useful as a top dressing.

Expanded shale, grain size 4-8 mm.

Expanded shale, grain size 1-3 mm.

Crushed lava

This soil additive is, as the heading says, crushed lava. It is naturally a very porous rock increasing drainage and improving soil texture. It is heavier than artificially expanded materials and so adds a bit of weight to pots (which might otherwise become a bit light if a lot of material like coir or perlite is used). The grain size I have used is 2-8 mm, which is on the large side for small pots. It is also useful as a top dressing, though its sharp edges may cut or scrape the plant as it grows and expands.

Crushed lava, grain size 2-8 mm. 

Pumice

Pumice is a highly porous volcanic rock and a very good soil additive. Much like the materials mentioned above, it improves soil drainage and structure, as well as retaining water and air which is slowly made available for the roots. It comes in many different sizes, though - like the above materials - the larger sizes are less suitable for small and medium pots.

Pumice, grain size 5-15 mm.

Pumice, grain size 2-5 mm.

Zeolite

Zeolite is a very porous aluminosilicate mineral. It occurs naturally but can also be produced artificially. I use a natural form which comes in gravel size. It is widely used as an adsorbant in various industries. It absorbs a lot of water and, due to its molecular structure, can retain various elements. The idea behind using it in the soil is that its capacity to adsorb and retain elements will lead to these becoming available to the roots as the water stored in the mineral is released as the soil dries. However, I am uncertain if it actually works as intended in the soil. To me it seems like the zeolite doesn't really leach these "trapped" nutritional elements as the water drains from it, but rather builds them up to the extant that salts crystallize on the surface. If this is, in fact, the case then its usefulness is very limited since its other chief value of retaining water is achieved by lots of other materials. One other use would be top dressing for certain plants, as its green colour is very nice.

Zeolite, grain size ca. 5-10 mm.

Crushed terracotta

This is another useful material to add to soil. Gravel size it acts to improve drainage and soil structure, as well as absorbing water which is then slowly released into the soil as it dries. In finer sizes it is useful as top dressing and soil component for seedlings. Made from clay, there are also a lot of various minerals available for the roots to extract.

Crushed terracotta, grain size ca. 4-6 mm.

Crushed terracotta, grain size 0-2 mm.

Clay

Clay can be a very useful additive in small components. Because of its very fine grain size it can reduce drainage in the soil and retain too much water if there is too much clay in the soil. It contains a large amount of elements however, and in small amounts it is a useful additive. 

As can be seen on the picture, there are many large clumps of clay. These can be problematic in the soil as they can create areas where more water than desirable is accumulated.

Akadama

Akadama is the name of clay pebbles that occur naturally in Japan. They are widely used in the bonzai industry because of their ability to absorb a lot of water and retain their structure as they dry, and repeat the process over and over. It improves drainage and aeration in the soil, as well as providing a lot of nutrients. It is an expensive product, however.

Akadama clay pebbles, grain size ca. 2-4 mm.

Sand

Sand is a much used soil additive. Added to organic mixes it helps improve drainage and soil structure. However, in inorganic soils I believe it has a tendency to move around and cling together with other sand particles creating zones with more sand, which leads to reduced drainage. In many cases it may be better to use either coarse sand or fine gravel which will do the same job as sand, but not potentially impact negatively on drainage.

Sieved filter sand, grain size 1-2 mm. 

Gravel

Gravel, particularly as crushed rock, is a useful soil additive to improve drainage and structure. It is probably the cheapest inorganic material too (along with sand). If you want to grow in a completely inorganic soil it is perfectly possible to do so by combining gravel from different rocks and minerals in the soil to provide all the nutrients the plants need. For seedlings and in small pots it may be of less value as smaller size material will probably be better, but in medium to large pots it is an excellent additive. It is also excellent as top dressing.

Crushed phyllite, grain size ca. 5-10 mm. 

Diatomaceous earth

This is a light, porous sedimentary rock consisting of fossilised remains of diatoms. It consists mainly of silica and some aluminium. It comes in a range of grain sizes from dust all the way up to gravel size, and has a wide variety of uses. It retains water and nutrients which is slowly released as the soil dries. I don't have a lot of experience with this material but it seems a good soil additive.

Diatomaceous earth, grain size 1-3 mm.

Perlite

This is an extremely light volcanic material that occurs naturally, but is used only after being processed by firing it at very high temperatures, expanding the material greatly. It helps improve drainage and soil structure, while having a fairly low water retention capacity. It is a very useful material and fairly cheap, however its low weight can be an issue as the perlite particles will very easily float to the surface of the pot - especially if watered from above.

Perlite, grain size ca. 3-6 mm.

Vermiculite

Vermiculite is a naturally occurring siliceous mineral that expands greatly when fired - like perlite. After being processed in this way it becomes a very useful soil additive to improve drainage, for retaining water and nutrients, and for increasing soil structure. While also extremely light, it doesn't float to the surface like perlite.

Vermiculite, grain size ca. 3-8 mm.

Mineral magic

This material is something I don't quite know the usefulness of as a soil additive. It is a dust-like yellowish material that is supposed to contain 65 different minerals and 60 % of the content is supposed to be water soluble silicates. Spread around the top of the soil it is supposed to prevent algae and fungi growth, while as a foliar spray it is supposed to act as an insecticide. As a soil additive I don't know whether it is a very useful substance considering all the minerals it supposedly contains, or whether the dust size grains merely leads to reduced drainage. It is supposed to increase the cation exchange rate in the soil which should lead to more nutrients being available for the roots. I add a little bit of it in my soil mixes. If it does what it claims it should be a useful additive. 

Mineral magic. With a saguaro (Carnegiea gigantea) pictured on the label it surely can't hurt to add to the soil!

Conclusion

In the end, it isn't really critical to use one particular soil component or the other. I have tried out quite a few because I've been curious about how they work, but many of the materials talked about above do much the same thing. The important part about soil is that it should be well drained and have a good structure with plenty of air. Whether this is accomplished through an inorganic soil or an organic based one isn't that important. The amount of the various materials is only really important when it comes to growing plants without the use of fertiliser at all, since then one must strike the right balance in the soil between the various components so that all essential and beneficial nutrients are provided.

The size of the collection and which materials are most easily (and cheaply) available will usually be the deciding factors on which soil components to use.