Terrestrial orchids typically have an underground tuber or (a mesh of) thick roots to store food reserves between two growing cycles. This creates two constrains on the composition of their soil:

• dormancy: the soil shouldn't create a hostile environment for the dormant tuber or roots
• active growth: the mix should provide a comfortable reserve in nutrients and moisture.

Soil mixes not always mimic the composition which is found in their natural habitat. This is not uncommon for pot-cultured plants, for the simple fact that the micro-fauna and -flora can exhaust nutrients much faster, or one species can outcompete another on a timescale of days.
To give you an example, we'll look at bog-species. These plants occur in acidic soils created by dead Sphagnum moss on which new moss grows. The result is a compressed, submerged layer of dead organic matter that is deprived of air and light resulting in the lack of decay - called peat. In other words, the plant matter is preserved and doesn't rot - this is also the reason why in peat bogs they occasionally find remains of dead animals and even humans, preserved for hundreds of years. If you would use this soil in a pot for growing bog plants, you'll notice that after a few months or even weeks the peat has started to decay and repotting is accompanied by an unpleasant smell. This is because the wet peat is in contact with air and micro-organisms are starting to do their job - something they couldn't do in the natural habitat. So certain bacteria and algae explode in numbers and render the compost unsuitable. Ask someone who grows Sarrecenia.
This is an extreme example, but it's exactly what happens with other mixes if you were to copy the natural resource the plants thrive in. The soil is the same, but key factors such as water, air-contact, heat (exposure to sunlight) change. You should therefor not be reluctant to use mixes that do not fully reflect nature.
Our soil mixes contain 4 basic ingredients:

1. drainage material
2. organic material
3. inorganic reserves
4. pH stabilisers

With these 4 factors, we can compose every soil.

The ability of the soil to retain or evacuate water is a strict key factor in their culture, errors are punished with loss of plants. The natural habitat is a golden rule for this; most plants we grow either come from acidic bogs, grasslands, woodland or the tropics and we mimic this not only with light/temperature and watering but also with drainage material in the soil. You should look at the difference between the soil-humidity during active growth and dormancy, which is linked to the way the plants have adapted to their surroundings. Let's clarify this.

Leptocerus menziesii emerging
Courtesy of © Kent's Australian Terrestrial Orchid website

Dormant tubers can be seen as a time-capsule that don't have the ability to see what's happening above soil like plants in leaf can, they don't have exposed parts that detect air-humidity, photoperiod or photospectrum. Basically, soil temperature and humidity is their only way of sensing what's happening out there, and both factors are used intensely to synchronise their biological timers. One extreme habitat are the tropical forests where nothing much changes between different seasons (for underground tubers under the leaf debris); the soil is wet all year - mousson or not - and the temperature changes are rather subtle. The other extreme are the Ozzie and African grassland, that swing between a crisp-dry summer soil and an abundance of water during a relatively short spring. This is reflected in their culture: tropical terrorchids go dormant rather unpredictable for a short while, and the soil is composed with regards to active growth. The Australian plants have a very strict dormancy and although a medium can give excellent results during growth, you'll get punished with rotting tubers if the soil doesn't fit the shoe during dormancy. This way - by looking at natural cycles - you can learn how to create standard drainage mixes for groups of plants based on their origin.
Another thing to remember is that drainage material should be inert, i.e. it shouldn't provide nutrition, take up elements or brake down. This is not always the case but we'll point it out for the standard materials used.

Inorganic Drainage

These materials are typically quarried or are (waste-) products that are often used as construction materials. In other words, if you don't find them in a nursery have a look at your local DIY. Most of the are inert, meaning they don't leak chemical components in the soil or create chemical reactions. A lot of the materials in this list are often used in hydrophonics.


Seramis
These are basically terracotta granules and as such have the property of absorbing water which is released slowly afterwards. Disa and Slipperorchid growers report good results with a medium of pure Seramis, but this is not standard practise. Use it in media that should be kept damp but not wet as it acts as a mild buffer. Submerge it completely in rainwater (for a night) before mixing it in to get rid of excess salts and to prevent the moisture being sucked out of your new medium.
Seramis can change the pH over time.

Perlite
Perlite is a volcanic rock that occurs worldwide. The two largest deposits are in Greece and in the State of New Mexico in the United States. The raw material is shipped around the world to factories, where it is processed for local consumption. The rock is heated to 1000oC and the small amount of water contained in the rock is turned into steam which expands the rock into a light weight material almost like making popcorn. It is then graded for size, although at the present time there are no internationally recognized standards for size. The grade that is readily available is usually called Horticultural grade and this is what we use. Perlite originally saw a number of applications in construction. The product is extremely light, airy and cheap and you can pulverise it with your fingers. It retains water but is mainly used to aerate the soil so it doesn't compact after a few months. If your gardensupply shop doesn't sell it you might find it in your local DIY. References to "horticultural grade" refer to the size of the grains, not the chemical composition or quality. Use a fine mesh to get rid of the dust and small grit.
Perlite can also be applied as a top-layer of 1 cm on your pots to extract calcium from the soil (it'll turn brown) and keep out insects that might damage the roots.

Diahydro

Expanded clay, 'Hydroton' or 'leca'
Has the same properties as Seramis, but has been processed in a different way. You should submerge and wash the pebbles a few times as they often contain salts. They're normally used for drainage in the bottom-part of soil because of their size.
The pebbles have a successfull history in hydrophonics because they are completely neutral and don't change the pH.

Rockwool, stonewool and growcubes

Zeolites: Clinoptilolite and Litonite
Zeolites are inorganic silicate polymer crystals that have the property of removing nitrates, ammonia and organic phosphates from water. It captures high concentrations of mineral salts and releases them in small doses via ionic interchange. It's ideal if you're suffering from high concentrations of impurities in your tapwater and don't have an alternative. It's used widely in filter-systems for ponds/aquaria and household water but has only recently been introduced in orchid culture. There are two sources: quarried material (properties and efficiency depends on origin) and synthetic (manmade, reliable). There's no real experience in terrorchid culture with this substance but it's worth trying. Zeolites can be "charged" with nitrogen, which they release slowly afterwards.

Styropore
This foamy plastic is often used in the horticultural industry to aerate soil and counteract compaction. Adding plastic to your potting mixes is not something that should be encouraged, there are plenty of alternatives.

Sand and Grit
Probably the most basic component, but there is a large variety. As a general rule, don't use beach sand because it contains salts. Always rinse the sand or grit a couple of times in a bucket of water until the water no longer turns brown. Sand and grit both have the property of creating a medium that drains very fast. Choosing what sand you use depends on the other components of the medium; if you use a medium that contains large chunks of other material (coarse peat, twigs, perlite,…) you can't really use fine sharp sand because it'll be washed to the bottom of the pot. If the mix is very fine (leafmould, loam, fine peat,…) and you mix in grit, the soil will compact when watered.

Grit comes in many sizes and colors. The pebbles should be smooth, not sharp. Mostly used in combination with Seramis and perlite to reduce airpockets in the soil. Mixes that are high in fine organic material and regular soil are normally mixed with sand to increase drainage, but these mixes can be topped with a centimeter of grit to avoid erosion when the plants are put outside in the rain.

Lavarock

Vermiculite
Vermiculite are shiny, gold color mica-like flecks that can soak up water 3-4 times its original volume. Vermiculite has the ability to attract nutrients such as calcium, magnesium, potassium, and phosphorous. Its’ spongy quality makes it excellent for absorbing water.

Charcoal

Organic Drainage

There are a lot of waste-products from agriculture that have found a new life as a soil-component. Most of them retain their structure for a few years, but you shouldn't rely on them as stable. Use them in cultures that get repotted after 1 or two years, their properties tend to degrade. They're listed under drainage because of their structural qualities, but you should also check the pH and organic reserve header for some.

Coconut Coir
This product is putting waste from the coconut-inductry to a good use. The Coconut Coir is normally composed of coconut fibers (the strong fibrous hairs) and chopped-up shells. Other sources sell it without the without the chunks (coconut fiber) or without the fiber (Coconut Husk Chips), we normally use the mix with the shells to provide an evenly damp medium that drains excess water. The coconut decays slowly and releases a small but certain amount of natural fertiliser. After 2 to 3 years the medium should be considered as exhausted, replant every year when you're using this in large amounts.

Crushed Cork
Has the same effect as styropore, it repels water and basically just takes up space. Just as the cork in winebottles it can go bad and surrender to rot. Use this medium to evacuate excess water from tropical growing plants or for pots that should go bone-dry in a matter of days for dormancy.

Pine-needles
Walnut shell
Bark
Sphagnum
Peat

Sounds stupid, but it's better to read about it than to learn the hard way. Most people that are reading texts like this one - on how to cultivate terrorchids - allready have experience with other plant-families. They go to exhibitions / trades and lectures in the hope to find something new, and they often do. But whenever they walk into uncharted terrain, they bring in insight, experience, techniques and stupid tricks from other fields.
Most of you reading this probably tumbled out of a tree with epiphytes and landed chest down, face-to-face with a terrorchid. That's not a problem, we're open minded. What is a problem - however - is that as an epiphyte-grower you won't know how to water terrorchids. One of the expressions you'll have read about a couple of times is things like "damp but not wet" or "keep absolutely dry in winter". If left uninstructed about this, you'll make mistakes. Here are the rules:

• A pot with a plant has 4 sides: an outside, an inside, a bottom and a topside

It's not a good idea to judge the need for water on the looks and feel of the toplayer. Lift the pot before and after watering, you'll know by the weight how the center is doing. When applied to fast, water can run of from the sides - especially for soils with high sand or peat content that has become dry. Both materials make the soil difficult to water after it has dried for too long. Look at the bottomside of the pots if it hasn't been soggy wet for weeks. Materials like grit,seramis, (especially) perlite and vermiculite have a high thermal resitance. Even in hot summerdays the inside can be cool and wet while all other pots need to be revisited twice a day.

• Watering is preferrably done from the bottom, not from the top

Fine soil should collapse when watered from above, even with a "rose" spout attachment.

• Mosses are your best friends
  

If possible, stimulate the growth of mosses on the top of your soil. This is easy for media that contain fine material, organic material, peat or gardensoil. Gritty or pure inorganic soils are more predictable and don't need this trick. A low-pH wet soil such as with the use of peat should be topped with clippings of Sphagnum moss. These little plants can create a tapistry of a single species that slows down evaporation drastically. They'll create brown patches when an invaded fungus takes over your soil. They'll lose their vibrant green color and become dark if the pots are kept wet for a while, or white and dry if you water too late on a sunny day. There are good mosses, and there are those you don't want.




Liverworth: expands agressively and suffocates everything on its path. The roots probably produce unfriendly chemicals.

Sphagnum: sprinkle some of this on peat-based soils. Comes in different colors and growing speeds. Forms a very showy compact carpet in full sunlight. If you have a strain that grows too fast pull out the excess, the holes will fill back nicely. Don't throw it away: it's ideal for producing cuttings, pretty much any stem will root when placed in sphagnum moss.
A lot of growers try to grow an extra batch of Sphagnum for germinating Disa species.

Polytrichum: They can be recognised by the sporophytes and the tiny rozettes. Normally dark-green, ideal for keeping an eye on plants that prefer a place in the shadow. Some species can grow very fast, but most that germinate will be carpet-forming slow-growing.
Keep a few extra pots aside without orchids in them for germinating Bletilla and Cynorkis spp.

Cushon moss: recognisable by the sporophytes and feathery light-green leaves. Likes indirect sunlight and damp but not moist medium. Suffocates when too wet or dark, shrivels when too suny or dry.

• Not everything stays put
  
  

The image on the left shows perlite that has become brown on the top. The reason is calcium in the water. After watering perlite will start to evaporate the water on its surface. During this process the calcium is left behind (it doesn't evaporate), builds up and forms small crystals of high concentration. Because of diffusion, water gets pulled to that part of the perlite and the proces amplifies itself. After a few months you'll be able to pick up tiny chunks of calcium-stalagmites which leave brown spots on the fingers when crushed. You can also observe this on mosses.

This is an example of the many processes that happen in the chemical household round the roots of a potplant. Things build up, move or spread out.

Growers tend to develop standard mixes for a group of plants once their collection requires more than a weekend to repot. It's faster and you become able to predict watering times and fertiliser regimes. Below are a number of standard mixes from experienced people.

This soil applies for Australian species of the genera Caladenia, Diuris, Pterostylis and Thelymitra. Visit their culture & care page to get more specific info.

• 45% loam
• 50% sand
• 5% peat moss

add a small amount of blood and bone fertiliser added. It has a pH of 5.5. Growers in other states use coarser mixes containing fine gravel and with leaf-litter or buzzer chips in place of peat moss. The new tubers are replanted with the tops 20-40mm below the soil surface. A thin surface mulch of Casuarina or Pinus radiata needles, chopped to lengths of 20-50mm, prevents erosion and keeps the leaves clean and healthy, as well as improving the appearance of the pot.

A standard temperate terrestrial orchid mix, read more on myorchids.de

• 60% - 70% fine to medium grade Perlite (0 to 6 mm)
• 30% to 40% wood fibre (Toresa is recommended).

Toresa kan be hard to get hold of, but you can make it yourself: Heat a batch of old decayed (untreated !) wood-chips in boiling water for half an hour. Take the chips out and let them drip until they've lost most of the water. Use these woodchips together with some gardening compost to replace the Toresa. Bear in mind that the total amount of gardening compost should not exceed 10 % of the resulting mix, avoid composts with high percentages of peat or limestone - some genera like Ophrys aren't very fond of that.

This very simple mix has a pH between 5 and 6, and ideal for most terrorchids that grow on acidic soils. You can add up to 2 % limestone for species that inhabit calceorous habitats, but too little is better than too much. For the latter a pH between 6 and 7 should suffice.

Repot every 2-3 years during dormancy.

Free drainage, high mineral content and a low organic value are important for genera such as Ophrys and Serapias

• 40 % perlite
• 40 % seramis
• 20 % bark

or

• 80 % perlite
• 20 % Toresa

It's very important to keep the organic content below 20 %.