This guide compares the materials and systems growers use once they move beyond plain soil. It covers perlite and other materials that add air to a mix, rockwool, clay pebbles, hydroponic systems and biochar. It ends with comparison tables, the environmental footprint of each medium, the effects of Thailand’s climate and advice on choosing.
Three numbers come up throughout. Air-filled porosity (AFP) is the share of the pot volume that holds air after the water has drained. Water-holding capacity (WHC) is the share that holds water. Cation exchange capacity (CEC) is how well the medium stores nutrients between irrigations. A CEC of zero means every nutrient must arrive with each irrigation.
This is Part 2. Part 1: Cannabis Growing Media: Soil, Coco, Perlite, Rockwool — Pros & Cons
4. Aeration Amendments: Perlite, Vermiculite, Pumice and Rice Hulls
These materials are mixed into soil or coco to add air. Part 1 covers soil and coco themselves. The differences between perlite, vermiculite and pumice also have their own topic.
Perlite
Perlite is volcanic glass that has been heated until it expands. It comes in three sizes:
| Grade | Size | Use |
|---|---|---|
| Fine | <3 mm | Seed starting |
| Medium (#3) | 3–6 mm | Mixing with soil or coco |
| Coarse (#4) | 6–13 mm | Hydroponics |
Its properties are WHC 20–35%, AFP 30–55%, CEC about 0 and pH 6.5–7.5. Typical rates are 20–30% of the mix in soil and 30–50% in coco. In a hempy bucket, perlite makes up 100% of the medium. A hempy bucket is a simple system: a bucket with its drainage hole set above the bottom, so a small pool of solution stays in the base.
Vermiculite versus perlite
| Property | Perlite | Vermiculite |
|---|---|---|
| CEC | ~0 | 100–150 meq/100 g |
| Drainage | Excellent | Poor |
| Water retention | Low–medium | High (50–70%) |
| Aeration | Excellent | Medium |
| Best use | Most cannabis mixes | Seed starting, dry climates |
Pumice
Pumice has a WHC of about 30% and an AFP of 30–50%. It does not break down, so it lasts indefinitely. It is heavier than perlite. That extra weight keeps pots stable outdoors and on windy sites.
Parboiled rice hulls (PBH)
Parboiled rice hulls drain fast and hold little water, much like perlite: high AFP and low WHC. A 2008 study compared them with perlite in peat-based mixes (Reference 4). They are rich in silica, which strengthens stems and cell walls. More and more commercial farms use them instead of perlite.
Translator’s note: rice hulls are a by-product of rice milling in Thailand, so they are cheap and easy to find. Local hulls are often raw, while the research tested parboiled hulls. Parboiling is a steaming process, so parboiled hulls are sterile and free of viable weed seed when produced. Raw hulls have not been through that step. In the rainy season, choose perlite, pumice or rice hulls over vermiculite. Vermiculite holds a lot of water and suits dry climates. What to do: replace part of the perlite with rice hulls in a few pots. Compare how fast those pots dry and how the plants grow. This shows whether the cheaper local material works in your room.
5. Rockwool
Rockwool (mineral wool) is made by melting rock at about 1,500 °C and spinning it into fibres. It is the main medium in commercial hydroponic cannabis.
| Parameter | Value |
|---|---|
| Total porosity | 96% |
| AFP | 11–15% |
| WHC | 75–91% |
| CEC | 0 |
| pH (untreated) | ~8.0 |
| Reusable | 3–6 cycles |
What the numbers mean for you:
- Total porosity of 96%. Almost the whole block is pore space.
- High WHC, low AFP. Most of that pore space fills with water. Rockwool has a lower AFP than coco (15–35%), so a block that is kept soaked leaves little room for air.
- CEC of zero. Rockwool stores no nutrients. Every irrigation must carry the complete nutrient solution, as in hydroponics.
- pH of about 8.0 when untreated. New rockwool is alkaline.
How to prepare it. Rockwool must be soaked in a pH 5.5 solution before use to neutralise its alkalinity. Grodan’s wetting instructions for its slabs use a nutrient solution adjusted to pH 5.5, the same solution the plants were grown in, and a 30-minute soak. They do not give a fixed EC. Without this soak, the first irrigations push the pH around the roots too high. In hydroponics, iron and manganese become harder to take up as pH approaches 6.5.
How to water it. Rockwool works well in drain-to-waste systems and best in recirculating systems (Section 10). Keep the feed within the hydroponic pH range of 5.5–6.5 (Section 7). Because the AFP is low, let blocks dry back a little between irrigations rather than keeping them soaked. The EC and dry-back targets used in commercial rockwool are discussed in a separate topic. Like coco, rockwool does not buffer, so compare the runoff EC and pH with the feed at every check to catch salt build-up early.
Disposal. Rockwool does not biodegrade, so disposal is its main environmental problem. Some manufacturers run recycling programmes for used mineral wool. One of them is Grodan, a rockwool brand (Reference 5).
Translator’s note: rockwool is imported into Thailand. Grodan’s recycling page does not list a programme in Thailand. Before buying large volumes, ask the official distributor in Thailand whether used slabs can be returned, and plan disposal before you start. What to do: begin with one tray of cubes. Soak them, plant them and record the runoff pH every day for the first week. This tells you whether your soak fully neutralised the alkalinity.
6. Clay Pebbles (LECA)
Clay pebbles are fired, expanded clay balls.
| Parameter | Value |
|---|---|
| WHC | 10–25% |
| AFP | 35–50% |
| CEC | ~0 |
| pH | ~7.0 |
| Reusable | Indefinitely |
They work best in these systems:
- Ebb-and-flow: a tray floods with solution and then drains back to the reservoir.
- Recirculating deep water culture: see Section 7.
- Net pots: mesh baskets that hold the plant above the solution.
Like rockwool, clay pebbles hold the plant in place but supply nothing. All nutrition comes from the solution.
What to do: clay pebbles can be reused indefinitely, so clean and dry them between crops. Clean pebbles carry fewer root diseases into your next crop.
7. Hydroponic Systems
In hydroponics, the roots sit in nutrient solution or are sprayed with it. There is no medium that stores nutrients.
Deep water culture (DWC)
The roots hang in a reservoir of solution that is pumped full of air. Roots have unlimited access to dissolved nutrients and oxygen. As a result, DWC gives the fastest vegetative growth of any system, and it also leaves the least room for error.
- Solution temperature 18–22 °C. Above 22 °C, the water holds less oxygen and Pythium (a water mould that rots roots) multiplies rapidly. In hot climates a chiller (a unit that cools the solution) is a required part of the system.
- Dissolved oxygen above 6 mg/L. Check it with a dissolved oxygen (DO) probe.
- pH checked daily. The pH drifts upward as plants take up nitrate, because the roots release hydroxide ions (OH⁻).
- EC and water level checked twice a day. Plants take up water and nutrients at different rates, so both change.
If the air pump, the chiller or the pH doser (an automatic pump that adjusts pH) fails, a DWC crop can die. Heat and lack of oxygen kill it within hours. A pH crash or nutrient lockout, where nutrients are present but roots cannot absorb them, kills it within days. That is why commercial systems pay for backup pumps and alarms.
Translator’s note: power cuts happen in Thailand, especially during storms in the rainy season. A cut stops the air pump and the chiller at the same moment. Use a battery-powered backup air pump, or a UPS (uninterruptible power supply, a battery unit that keeps equipment running through a cut). A chiller draws far more power than a UPS for small devices can supply, so it needs a generator. What to do: switch off mains power briefly while you watch. Confirm that every backup starts and that the alarm reaches your phone.
Nutrient film technique (NFT)
A 2–3 mm film of solution flows continuously down a sloped channel over bare roots. The roots rest partly in the film and partly in air. NFT needs very little medium and gives roots plenty of air. However, it has almost no reserve. If the pump stops or a channel blocks, the roots dry out within 30–60 minutes. In Thailand, a backup power supply for the pump is essential for NFT.
Aeroponics
The roots hang in a sealed chamber and are sprayed with a fine mist at intervals, commonly in 5–60 second bursts every 3–5 minutes. This gives the most oxygen and the fastest growth, but it is the most demanding setup. A clogged nozzle or failed pump dries the roots out within minutes to hours. What to do: check every nozzle on a fixed schedule and keep a spare pump on site. This cuts the time between a failure and its repair.
Hydroponic pH
The target is 5.5–6.5. Many growers deliberately let the pH drift between about 5.8 and 6.2 over 24–48 hours. At any single fixed pH, some nutrient is outside its best range. Iron and manganese become less available toward 6.5. Calcium and magnesium uptake weakens toward 5.5.
8. Biochar
Biochar is charcoal made by heating plant material with little or no oxygen.
- Its CEC is 15–300+ meq/100 g, depending on the source material and how it was made.
- It has a carbon-negative footprint: it stores more carbon than its production releases.
- Mix it at 5–20% in soil.
- It is often paired with microbial inoculants such as Trichoderma.
Translator’s note: carbonised rice hulls are a common farm material in Thailand. They can be more alkaline than the biochar range above: one study measured pH 11.2 and a CEC of 7.8 meq/100 g. Biochar’s pH is 7–10 (Section 9), so it can raise the pH of a mix. What to do: add 5% to a few pots, leave the rest without it and compare the runoff pH and growth. Increase the rate only if the pH stays in range.
9. Master Comparison Tables
| Medium | WHC % | AFP % | CEC (meq/100 g) | pH | Reusable |
|---|---|---|---|---|---|
| Soil (peat) | 60–80 | 10–20 | 50–150 | 6.0–6.5 | Limited |
| Coco coir | 73–88 | 15–35 | 10–30 | 5.5–6.5 | 2–3 cycles |
| Perlite | 20–35 | 30–55 | ~0 | 6.5–7.5 | Limited |
| Vermiculite | 50–70 | 8–15 | 100–150 | 6.0–7.5 | Limited |
| Rockwool | 75–91 | 11–15 | 0 | 7.0–8.0 | 3–6 cycles |
| LECA | 10–25 | 35–50 | ~0 | ~7.0 | Indefinite |
| Rice hulls | low | high | 2–10 | 5.5–6.5 | No |
| Pumice | 25–40 | 30–50 | 5–15 | 6.5–7.5 | Indefinite |
| Biochar | 40–60 | 15–30 | 15–300+ | 7–10 | Yes |
| Management factor | Soil | Coco coir | DWC hydroponics |
|---|---|---|---|
| Feed / root-zone pH target | 6.0–7.0 | 5.8–6.2 | 5.5–6.5 |
| Buffering | High | Low | None |
| Growth rate potential | Moderate | Fast | Fastest |
| Precision of nutrient control | Low–moderate | High | Very high |
| Tolerance of errors | High | Moderate | Low |
| Main pest / disease risk | Fungus gnats, root aphids | Fungus gnats | Pythium root rot |
| Irrigation frequency | 1–3 times a week | 1–4 times a day or more | Continuous |
| Setup cost | Low | Low–moderate | Moderate–high |
| Suitability for beginners | Highest | Moderate | Low |
How to use these tables: if your pots stay wet too long, raise the AFP by adding a high-AFP material such as perlite or rice hulls. If you often miss irrigations, choose a medium with high CEC and a high tolerance of errors.
10. Matching the Medium to the System
- Drain-to-waste: the solution that drains out is thrown away.
- Recirculating: the solution returns to the reservoir and is reused.
- Water-only: you add only plain water, and the nutrients come from a living soil.
| Medium | Drain-to-waste | Recirculating | Water-only |
|---|---|---|---|
| Coco + perlite | Best | Not recommended | No |
| Rockwool | Good | Best | No |
| LECA | Medium | Best (ebb and flow) | No |
| Living soil | Yes | No | Best |
Coco-to-perlite ratios
| Ratio | AFP | Use | Irrigation frequency |
|---|---|---|---|
| 70:30 | 25–30% | Industry standard | 1–2 times a day by hand, 3–6 automated |
| 60:40 | 30–35% | High-frequency automation | 6–12 times a day |
| 50:50 | 35–40% | Dutch commercial standard | Automation required |
What to do: if you water by hand, start with 70:30. Move to 60:40 or 50:50 only once you have automated drip, because these airier mixes dry too fast for hand watering.
11. Environmental Footprint
CO₂e (carbon dioxide equivalent) expresses all greenhouse gas emissions as the equivalent amount of CO₂.
| Medium | CO₂e per m³ | Main concern |
|---|---|---|
| Compost | 20–50 kg | Best produced locally |
| Rice hulls | 10–30 kg | Agricultural by-product |
| Biochar | Negative | Depends on local production |
| Coco coir | 50–100 kg | Long-distance transport |
| Perlite | 150–300 kg | Mining plus expansion in an 850 °C kiln |
| Peat moss | 200–500 kg | Destroys peatland ecosystems |
| Rockwool | 150–250 kg | Melting at 1,500 °C; not biodegradable |
Translator’s note: coco’s main footprint comes from transport, and in Thailand coco can be sourced locally. Compost, rice hulls and biochar can also be made close to most Thai farms. What to do: where local materials pass the quality checks in this guide (EC for coco, pH for biochar), use them in place of imported peat or perlite.
12. Thailand’s Climate
Soil. In the rainy season (May–October), humidity of 70–90% RH slows surface drying. Combined with warm root zones, this favours fungus gnats and Botrytis (grey mould). Indoors, a dehumidifier is effectively required in the wet season (sizing for Thai rooms). Let the top 2–3 cm dry visibly between irrigations. Outdoors and in greenhouses, cover plants from rain in late flower to prevent bud mould.
Coco coir. Light, free-draining coco-and-perlite mixes suit humid conditions. Frequent irrigation to runoff refreshes the root zone. A surface that does not stay wet attracts fewer fungus gnats than organic soil. Wet-season humidity still pushes indoor RH up. Plan for a larger dehumidifier so you can hold a workable VPD (vapour pressure deficit, a single figure combining temperature and humidity that shows how strongly plants release water).
Hydroponics. Root-zone temperature is the main problem. Dry-season air reaches 28–35 °C. Without a chiller, a 150-litre DWC reservoir warms to room temperature within hours. At a solution temperature of 26 °C, Pythium can destroy the root system within 48–72 hours. Insulating the reservoir reduces the chiller’s workload but cannot replace the chiller (options for hot reservoirs).
What to do: before the rainy season, check that your dehumidifier and rain cover are ready. If you run hydroponics, put a thermometer in every reservoir and check it twice a day in the dry season.
13. Which Medium Should You Choose?
- Beginners: use a free-draining pre-mixed soil or a peat/coco blend with 20–30% extra perlite. Pots should hold at least 11 L indoors and 20–40 L outdoors or in a greenhouse. The next step up is coco and perlite at 70:30.
- Intermediate growers: move to coco coir, watered by hand or by drip, once you are ready for frequent irrigation and closer pH and EC control. Automated drip into coco is the standard in Thai commercial production.
- Organic growers: start with super soil or Coots mix (described in Part 1) and develop it into living soil.
- Commercial hydroponics: use rockwool, or coco and perlite at 50:50 with automation.
- Growers focused on sustainability: use rice hulls, biochar and local compost.
DWC is not a beginner system in any climate. In Thailand the chiller adds both cost and complexity. Finish at least one successful soil or coco crop first. The best medium is the one you can manage consistently with your equipment and time. A well-run soil grow beats a badly run hydroponic system on every quality measure.
What to do: pick one medium from the list above and run a small batch first. Record pH, EC, runoff and watering times for the whole crop. Your own records will show whether you can manage that medium before you scale up.
Legal note
This article is for education and research only. It does not encourage recreational use. In Thailand, cannabis flower (ช่อดอกกัญชา) is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, which came into force on 26 June 2025. Flower may be sold only against a PT 33 (ภ.ท.33) prescription covering at most 30 days. Advertising and online sale of flower are prohibited. Research, export, sale and processing of a controlled herb require a licence under the Thai Traditional Medicine Protection Act B.E. 2542. The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569, in force since 30 April 2026, tightened the licence criteria for cannabis flower. Outside Thailand, follow the law where you live.
If you have a question about choosing or running a medium in your conditions, post it below this topic. Include your system, room temperature and humidity, and your pH and EC readings.
References
- HortGrow Solutions. Coco Coir as a Medium-CEC Substrate: Balancing Precision and Buffering. hortgrow.com
- Seemakram W et al. (2022). Enhancement of growth and Cannabinoids content of hemp (Cannabis sativa) using arbuscular mycorrhizal fungi. Frontiers in Plant Science 13:845794
- Malík M, Tlustoš P (2025). Soilless Growing Media for Cannabis Cultivation. Agriculture 15(18):1955
- Gachukia MM, Evans MR (2008). Root Substrate pH, Electrical Conductivity, and Macroelement Concentration of Sphagnum Peat-based Substrates Amended with Parboiled Fresh Rice Hulls or Perlite. HortTechnology 18(4):644–649
- Grodan. Recycling Solutions. Recycling Solutions