The growing medium decides how a cannabis plant gets water, oxygen and nutrients. No substrate is best for everyone: soil forgives mistakes and buffers pH, coco coir grows faster but needs frequent feeding, and hydroponic systems grow fastest but punish every equipment failure. This guide compares the main options with their measured properties, recipes and a system-matching table, then looks at what changes in Thailand’s tropical climate.
1. What a Growing Medium Actually Does
A substrate does three jobs for the root system:
- Physical support. It anchors the plant and gives roots a stable space to spread.
- Water and oxygen at the same time. Roots need both. A waterlogged, airless root zone dies quickly whatever the feed contains. Two numbers describe this balance: air-filled porosity (AFP), the share of the pot volume holding air after drainage, and water-holding capacity (WHC), the share held against gravity.
- Cation exchange capacity (CEC). The ability of particles to hold and release positively charged nutrients (Ca²⁺, Mg²⁺, K⁺, NH₄⁺) between irrigations. High CEC buffers pH and feeding errors; zero CEC means every nutrient must arrive with each irrigation.
Soil, coco coir and hydroponics differ on all three, which is why feeding schedules, irrigation frequency, pH targets and troubleshooting cannot be copied from one to another. Treating coco like soil, or running a soil feeding schedule in hydro, gives predictably poor results.
2. Soil
Soil is a mix of mineral particles (sand, silt, clay), organic matter, water, air and living organisms. Bagged “soil” for cannabis is usually a peat or coco base with perlite for air, compost or worm castings for CEC and a starter nutrient charge, plus amendments such as lime and rock dust.
Organic living soil
Living soil deliberately cultivates a soil food web — bacteria, fungi including mycorrhizal networks, nematodes, protozoa and arthropods — that turns organic matter into plant-available nutrients over time. The grower’s job is to keep that biology working: regular carbon inputs (compost, mulch, cover crops), moisture that is neither dry enough to kill it nor wet enough to turn anaerobic, stable pH, and no broad-spectrum fungicides or pesticides. A mature living soil needs little or no bottled feed, but it takes months to build. Growers and craft consumers often credit it with better flavour; peer-reviewed sensory comparisons on this point are limited.
Typical living-soil parameters:
| Parameter | Value |
|---|---|
| CEC | 50–150+ meq/100 g |
| pH | 6.0–6.5 |
| WHC | 60–80% |
| AFP | 10–20% |
Mycorrhiza. Rhizophagus irregularis, an arbuscular mycorrhizal fungus (AMF), forms a hyphal network that extends the root absorption zone. Research published in Frontiers in Plant Science (2022) found that AMF-inoculated cannabis produced yields that matched or exceeded those of NPK-fed plants.
Trichoderma. Trichoderma longibrachiatum helps decompose organic matter and outcompetes soil-borne pathogens.
Super soil and Coots mix
“Super soil” loads a high rate of organic amendments into the lower third to half of the container; roots grow down into the enriched layer and feed as needed, while a lightly amended top layer suits seedlings. It simplifies mid-cycle feeding but gives up the precise control of liquid feeding.
Subcool’s updated formula, per 6–8 bags of organic soil:
| Ingredient | Amount |
|---|---|
| Worm castings | 25–30 lb |
| Bone meal (3-15-0) | 5 lb |
| Blood meal (12-0-0) | 5 lb |
| Bat guano (0-7-0) | 5 lb |
| Epsom salt | 3/4 cup |
| Dolomite lime | 1/2 cup |
| Azomite | 1/2 cup |
| Humic acid | 2 tbsp |
Cook the mix for 4–8 weeks, turning the pile weekly.
Coots mix is simpler: one third peat moss, one third aeration amendment, one third compost or castings. Common amendments per cubic foot:
| Amendment | Amount | Function |
|---|---|---|
| Neem meal | 1/2 cup | Pest suppression, slow-release nitrogen |
| Kelp meal | 1/2 cup | Potassium, growth hormones |
| Crustacean meal | 1/2 cup | Chitin, calcium, nitrogen |
| Fish bone meal | 1/2 cup | Phosphorus, calcium |
| Oyster shell | 1/2 cup | Calcium, pH buffer |
| Basalt rock dust | 1/2 cup | Trace minerals |
| Gypsum | 1/2 cup | Calcium, sulfur |
| Malted barley | 1/4 cup | Enzymes for nutrient cycling |
| Biochar | 1–2 cups | CEC boost, microbial habitat |
| Mycorrhiza | 1–2 tbsp | Root-zone inoculant |
Commercial pre-mixed blends
Bagged peat or coco blends with a light organic nutrient charge are the easiest starting point for new growers. How long the charge lasts before extra feeding is needed depends on the product — check the label rather than a rule of thumb.
Soil pH
Target 6.0–7.0, with 6.0–6.5 the usual optimum.
- Below 5.5: aluminium and manganese reach toxic levels, phosphorus binds to iron and aluminium, and calcium and magnesium availability falls.
- Above 7.0: phosphorus precipitates as calcium phosphate, and iron, zinc, manganese and copper become insoluble.
Soil’s high CEC buffers pH: one irrigation at pH 5.0 will not crash a well-amended organic soil. The same buffering makes deliberate corrections slow. Raise pH with agricultural or dolomitic lime; lower it with elemental sulfur (slow, bacteria convert it) or acidic liquid feeds.
Pests that live in soil
- Fungus gnats (Bradysia spp.). Larvae feed on root tips and carry Fusarium and Pythium into wounds. Control with yellow sticky traps for adults, Steinernema feltiae nematodes or Bacillus thuringiensis var. israelensis (Bti) for larvae, or a hydrogen peroxide drench.
- Root aphids. More damaging, and hard to detect because they live on the roots.
- Thrips and spider mites. Not soil-specific, but use organic media as shelter between crops.
Letting the top 2–3 cm dry between irrigations is the most effective cultural control for fungus gnats.
| Soil advantages | Soil disadvantages |
|---|---|
| Highest CEC and buffering | Slowest growth of the three systems |
| Most forgiving of feeding errors and missed waterings | Pest habitat (fungus gnats, root aphids) |
| Living soil can give exceptional flavour | Less precise nutrient control |
| Suits outdoor and large greenhouse containers | Heaviest per litre; slower diagnosis-to-correction cycle |
3. Coco Coir
Coco coir is the fibrous husk of the coconut (Cocos nucifera). After washing, sieving and buffering it has almost no nutritional value of its own: it behaves like a stable, near-inert matrix, closer to hydroponics than to soil. It is the most popular alternative to soil, especially in drain-to-waste (DTW) systems.
It has two fractions: coir pith (short, spongy particles that hold water) and fibres (longer strands that create air pockets and structure). Commercial products blend the two.
| Parameter | Value |
|---|---|
| CEC | 10–30 meq/100 g (buffered) |
| WHC | 73–88% |
| AFP | 15–35% |
| pH of the medium | 5.5–6.5 |
| Water absorption | 8–9 times its own weight |
| Reusable | Yes, 2–3 cycles |
| Bulk density | 100–200 g/L (loam soil: 500–700 g/L) |
Quality grading by EC
| Grade | EC (mS/cm) | Status |
|---|---|---|
| Raw | 2–6 | Too high — do not use |
| Washed | <1 | Acceptable |
| Premium | ≤0.5 | Excellent |
Why buffering is mandatory
Raw coco carries potassium and sodium on its exchange sites, picked up from the seawater and soil where coconut palms grow. Unbuffered coco swaps these for calcium and magnesium from every irrigation, so the plant shows calcium and magnesium deficiency however much CalMag is added.
Buffering procedure:
- Rinse with clean water (EC below 0.2 mS/cm) two or three times, until runoff EC is below about 0.3 mS/cm.
- Charge with calcium. Soak for 12–24 hours at pH 5.8–6.0. Typical buffering procedures target 600–1,000 mg/L of calcium from calcium nitrate or calcium chloride. The calcium displaces the sodium and potassium.
- Drain and use. The exchange sites now hold calcium and will stop stripping it from the feed.
Most coco sold for cannabis is pre-buffered, but the degree varies by brand, region and batch, so many growers still add CalMag at the label rate during the first weeks.
pH, irrigation and EC in coco
Target feed pH is 5.8–6.2. With little buffering, the root zone follows the feed closely: corrections work fast and mistakes spread fast. Above 6.5, iron and manganese availability drop quickly; below 5.5, calcium and magnesium uptake suffers. Calibrate pH meters weekly — a meter reading 0.3–0.5 units high can keep a plant in chronic iron deficiency without anyone noticing.
The classic beginner error is under-watering coco on a soil schedule. Coco is meant to be irrigated to runoff often, keeping moisture fairly steady with a short dry-back between events. Large plants in flower under strong light may need several irrigations a day, which is why commercial coco runs on automated drip. Aim for 10–20% runoff at each irrigation to stop salts accumulating.
| Growth stage | Feed EC (mS/cm) | Notes |
|---|---|---|
| Seedling / cutting | 0.5–1.0 | Sensitive roots |
| Early vegetative | 1.0–1.6 | Rising with root mass |
| Late veg / pre-flower | 1.6–2.2 | Full nutrition |
| Early to mid flower | 2.0–2.6 | Peak demand |
| Late flower | 0.5–1.2 | Reduced feed or plain water |
When runoff EC is consistently more than 0.5 mS/cm above feed EC, salts are building up; irrigate with pH-adjusted plain water to 30–50% runoff. Regular runoff EC readings are the most reliable early warning.
4. Aeration Amendments: Perlite, Vermiculite, Pumice and Rice Hulls
Perlite
Expanded volcanic glass used to add air to mixes.
| Grade | Size | Use |
|---|---|---|
| Fine | <3 mm | Seed starting |
| Medium (#3) | 3–6 mm | Mixing with soil or coco |
| Coarse (#4) | 6–13 mm | Hydroponics |
Properties: WHC 20–35%, AFP 30–55%, CEC about 0, pH 6.5–7.5. Typical rates: 20–30% in soil, 30–50% in coco, 100% in a hempy bucket.
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
WHC about 30%, AFP 30–50%. It lasts indefinitely and is heavier than perlite, which makes it useful outdoors and in windy sites.
Parboiled rice hulls (PBH)
AFP about 70%, WHC about 23%. Rich in silica, which strengthens stems and cell walls, and increasingly used instead of perlite on commercial farms.
5. Rockwool
Rockwool is the dominant substrate 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 |
Rockwool must be pre-soaked in a pH 5.5 solution before use to neutralise its alkalinity. It is not biodegradable, so disposal is its main environmental problem; some manufacturers, Grodan among them, run recycling programmes for used mineral wool.
6. Clay Pebbles (LECA)
| Parameter | Value |
|---|---|
| WHC | 10–25% |
| AFP | 35–50% |
| CEC | ~0 |
| pH | ~7.0 |
| Reusable | Indefinitely |
Best for ebb-and-flow systems, recirculating deep water culture and net pots. Like rockwool, clay pebbles hold the plant in place but supply nothing: all nutrition comes from the solution.
7. Hydroponic Systems
In hydroponics, roots sit in or are sprayed with nutrient solution instead of a nutrient-holding medium.
Deep water culture (DWC)
Roots hang in an aerated reservoir. With unrestricted access to dissolved nutrients and oxygen, DWC gives the fastest vegetative growth possible — and the least margin for error.
- Solution temperature 18–22°C. Above 22°C dissolved oxygen falls and Pythium multiplies rapidly. In hot climates a chiller is part of the system, not an optional extra.
- Dissolved oxygen above 6 mg/L, checked with a DO probe.
- pH checked daily. It drifts as plants take up nitrate (roots release OH⁻, raising pH).
- EC and water volume checked twice daily, since plants take up water and nutrients at different rates.
An air pump, chiller or pH doser failure can kill a DWC crop within hours (heat, lack of oxygen) or days (pH crash, lockout). Commercial systems justify redundant pumps and alarms.
Nutrient film technique (NFT)
A 2–3 mm film of solution flows continuously along a sloped channel over bare roots, which rest partly in the film and partly in air. NFT uses very little medium and aerates roots well, but gives almost no buffer: roots dry out within 30–60 minutes if the pump stops or a channel blocks.
Aeroponics
Roots hang in a sealed chamber and are misted at intervals (commonly 5–60 second bursts every 3–5 minutes). It maximises oxygen and growth rate and is the most demanding setup: a clogged nozzle or failed pump desiccates roots within minutes to hours.
Hydroponic pH
Target 5.5–6.5. Many growers let pH drift deliberately between about 5.8 and 6.2 over 24–48 hours, because at any single fixed pH some nutrient sits outside its best availability window — iron and manganese availability drop towards 6.5, calcium and magnesium uptake weakens towards 5.5.
8. Biochar
- CEC 15–300+ meq/100 g, depending on feedstock and production
- Carbon-negative footprint
- Mix at 5–20% in soil
- Often paired with microbial inoculants such as Trichoderma
9. Master Comparison Table
| 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 | 20–23 | 69–70 | 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 |
| Nutrient control precision | Low–moderate | High | Very high |
| Error tolerance | High | Moderate | Low |
| Main pest / pathogen 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 |
| Beginner-friendliness | Highest | Moderate | Low |
10. Matching Medium to System
| 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 |
11. Environmental Footprint
| 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 850°C kiln expansion |
| Peat moss | 200–500 kg | Destruction of peatland ecosystems |
| Rockwool | 150–250 kg | 1,500°C melting, not biodegradable |
12. Thailand’s Climate
Soil. Rainy-season humidity of 70–90% RH (May–October) slows surface drying and, with warm root zones, favours fungus gnats and Botrytis. Indoors, dehumidification is essentially mandatory in the wet season; let the top 2–3 cm dry visibly between irrigations. Outdoors and in greenhouses, rain cover in late flower is critical against bud mould.
Coco coir. Light, free-draining coco and perlite mixes suit humid conditions: frequent irrigation to runoff refreshes the root zone, and a surface that does not stay wet draws fewer fungus gnats than organic soil. Wet-season humidity still pushes indoor RH up, so plan for larger dehumidifier capacity to hold a workable VPD.
Hydroponics. Root-zone temperature is the dominant problem. With dry-season air at 28–35°C, a 150-litre DWC reservoir reaches room temperature within hours without a chiller, and at 26°C solution temperature Pythium can destroy the root system within 48–72 hours. Insulating the reservoir reduces the chiller’s load but cannot replace it.
13. Which Medium Should You Choose?
- Beginners: a free-draining pre-mixed soil or peat/coco blend, with 20–30% extra perlite, in containers of at least 11 L indoors or 20–40 L outdoors and in greenhouses. Coco and perlite at 70:30 is the next step.
- Intermediate growers: coco coir, hand-watered or on drip, once you are ready for frequent irrigation and closer pH and EC work. Automated drip into coco is standard in Thai commercial production.
- Organic growers: super soil or Coots mix, developing into living soil.
- Commercial hydroponics: rockwool, or coco and perlite at 50:50 on automation.
- Sustainability-focused growers: rice hulls, biochar and local compost.
DWC is not a beginner system in any climate, and in Thailand the chiller adds both cost and complexity. Get at least one successful soil or coco crop first. In the end the best substrate is the one you can manage consistently with the equipment and time you have: a well-run soil grow beats a badly run hydroponic system on every quality measure.
Legal Note
This article is for educational and research purposes only. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, and cultivation requires a valid licence; the Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569 tightened licensing conditions from April 2026. Elsewhere, follow the law where you live.
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