Mycorrhizal inoculant is one of the few living-soil inputs where timing and what you do afterwards matter more than the product. Growers often pay for it and then switch off the symbiosis with a feed. This article covers what arbuscular mycorrhizal fungi actually do, where they can and cannot work, how to apply them, the common mistakes that waste them, and what published research says about commercial inoculant quality.
1. Ecto and endo: the distinction that decides what you buy
There are two broad classes of mycorrhizal fungi, and only one matters for cannabis.
Ectomycorrhizal (ecto) fungi form a sheath around the root without entering root cells. They associate mainly with trees such as pines, oaks and birches, and they do not colonise cannabis.
Endomycorrhizal fungi enter root cells. The group that matters here is arbuscular mycorrhizal fungi (AMF), named for the arbuscules: branched structures inside root cells where nutrients are exchanged. AMF associate with most herbaceous plants, including cannabis. They cannot be grown without a living host, so products contain propagules (spores, colonised root fragments or hyphae) that sit dormant until a root is available.
Practical consequence: read the species list, not the marketing copy. Look for genera such as Rhizophagus, Funneliformis, Glomus, Claroideoglomus or Gigaspora. Rhizophagus irregularis is the most studied AMF species for herbaceous crops. Ecto species such as Pisolithus tinctorius, Suillus or Rhizopogon do no harm, but they will not colonise cannabis. They inflate the species count on the label without adding anything. A label that lists only ecto genera is a tree product.
2. What the symbiosis does, and what it doesn’t
The plant gives the fungus carbon (sugars). The fungus extends past the root’s reach and sends minerals back. The commonly cited share of the plant’s fixed carbon that goes to the fungus is roughly 4–20%, which is why the plant only keeps paying when it gets something back.
The nutrient that matters most is phosphorus. Phosphorus barely moves in soil, so the root has to reach it. Roots quickly empty the soil right around them of P, leaving a depletion zone. AMF hyphae are finer than roots and extend several centimetres beyond that zone, so they reach P the root cannot. Their ability to mobilise organic P is limited and still debated. Mostly they take up the same dissolved phosphate that roots use, just from a larger volume of soil. In a container where soil volume is the limit, that extra reach is useful.
Secondary benefits are less consistent: better water uptake under drought, and some protection against root pathogens such as Pythium and Fusarium (see topic 1263).
What the evidence does not support:
- “Increases potency/yield”: we know of no robust peer-reviewed evidence that AMF raise cannabinoid content in cannabis that is already well fed.
- “Improves uptake of all nutrients”: the evidence is strongest for phosphorus and much weaker for N, K and micronutrients.
- “Promotes root growth”: a colonised plant may actually invest less in roots, because the fungus is doing part of the foraging.
3. Where AMF can and cannot work
| Substrate | Chance of useful AMF establishment | Why |
|---|---|---|
| Coco coir on complete liquid feed | Low | P arrives in solution at every watering, so the plant has little reason to invest in the fungus |
| Rockwool, clay pebbles, DWC | Very low | No soil matrix, and fully soluble nutrition |
| Pre-amended soil plus full-strength bottled feed | Low | High soluble P suppresses colonisation |
| Living soil / organic soil with slow-release P | Moderate to high | Conditions favour the symbiosis |
| Native or compost-rich ground beds | High | A background AMF population is usually already present |
Sterile or fresh media (new coco, peat mixes, pasteurised soil) have no background AMF. An inoculant can colonise there because there is no competition, but only if the feeding regime allows it.
Tropical conditions to account for:
- Root-zone heat. Pots in Thai sun can sit well above 30°C at the root zone. Most published optimum ranges come from temperate work, and we found little data on how commercial strains perform under sustained 30–35°C. Shade the pots, use light-coloured containers and use larger soil volumes.
- Acidic and lateritic soils. Colonisation tends to fall in strongly acidic soil (below about pH 5). Raise pH with agricultural lime or dolomite. Gypsum is a neutral salt and does not change pH. In alkaline soils, elemental sulfur or acidifying fertilisers lower it.
- Monsoon waterlogging. AMF need oxygen. Beds that stay saturated for days are poor ground for colonisation, so use raised beds and free-draining mixes.
4. Why one well-timed inoculation is usually enough
Colonisation is an establishment event. Once the fungus is inside the root system, it grows with the plant. One well-timed inoculation is usually enough. Re-inoculating may make sense when you move a plant into fresh sterile media, which brings no AMF of its own.
Timing:
- Seedlings: inoculate as soon as the seed has germinated.
- Clones: inoculate when the cutting has rooted and goes into soil.
| Method | How | Effectiveness |
|---|---|---|
| Into the transplant hole / mixed into the root zone | Granular product placed where roots will grow | High: direct root contact |
| Root dip | Bare roots dipped in a slurry before transplanting | High, transplants only |
| Starter plug / propagation mix | Inoculated rooting medium | High: colonisation starts before transplant |
| Seed coating | Product applied to seed | Variable, depends on moisture at germination |
| Surface drench on established plants | Liquid poured on top | Low: propagules mostly stay far from the growing root tips |
The principle behind the table: AMF need to contact actively growing root tips. Soluble powders should be mixed with non-chlorinated water (section 5.2).
5. Common ways to waste your inoculant
5.1 Soluble phosphorus in a living-soil pot
High P suppresses AMF. This is well documented, but it is narrower than “P kills myco.”
In a sorghum pot-culture study, Li et al. (2025) compared several nitrogen–phosphorus combinations. They found the highest colonisation, spore production and arbuscule abundance at 30 mg/L P (with 150 mg/L N). The authors also report that above 60 mg/L P the decline in colonisation “could not be reversed even with sufficient nitrogen supply.”
The mechanism is economic, not toxic. When phosphate is easy to take up directly, the plant reduces the carbon it invests in the fungal partner, and the symbiosis is scaled back.
Counter-evidence: in a field trial on onion and carrot, Ilyas et al. (2024) report root colonisation of about 40–50% in both crops. Soil phosphorus level did not change the extent of colonisation, though it shifted some fungal taxa in carrot. Keep the units straight. A soil-test P value (ppm, i.e. mg/kg of soil) is not the same as mg/L of P in a feed solution, and a high soil test does not necessarily mean high P in solution at the root surface.
What to do with that:
- Living soil that relies on AMF: avoid sustained soluble P in the root zone. As a rough threshold, solution P sustained above about 50–60 mg/L strongly reduces colonisation. Check the P figure on your feed chart at the dilution you actually use. Supply P as slow-release solids (rock phosphate, bone meal, high-P guano in top-dresses), which is the approach living soil is built around (topic 1091).
- Coco or hydro on a complete mineral feed: every watering already delivers soluble P, so do not expect much benefit from AMF. Whether to use a PK or bloom booster is a separate decision (see topic 1338).
5.2 Chlorinated and chloraminated water
Free chlorine dissipates from standing water. Chloramine is designed to be more stable and does not dissipate this way, so “let it sit 24 hours” does nothing. Carbon filtration (catalytic carbon handles chloramine better) or RO removes it. Some utilities in the region (e.g. Singapore) use chloramine; check with your local water authority.
The low chlorine residual in ordinary tap water is not a soil sterilant. In a soil–plant microcosm irrigated continuously with low-concentration chlorinated water, Yu et al. (2023) found no lasting change in microbial diversity or composition after 14 days. Some rhizosphere functions dipped and then recovered. Tap water is not ideal for a living-soil pot, but it is not a catastrophe.
Where the caution does apply: mixing a soluble powder into chlorinated water leaves unprotected spores sitting directly in the solution, with no soil to buffer them. Use filtered, RO or rainwater for the slurry. It is a cheap precaution.
5.3 Old or heat-damaged product
Many products carry a shelf life of around two years if kept dry and sealed. We found no independent viability study establishing that as a general figure, so treat it as a working assumption. What is not in doubt is that viability declines with age and heat. Liquid formulations are generally more vulnerable than dry granular carriers. In this region heat is the main factor: a product shipped across the tropics and then kept in an uncooled warehouse has had a much harder life than the same product on a temperate shelf. Buy small, buy fresh, store sealed, dry and cool, out of sunlight, and write the opening date on the container.
5.4 Soil-applied fungicides, and a note on barley
Fungicides that reach the root zone can harm AMF, though effects vary by compound and the data are mixed. On cannabis in Thailand, the bigger issue is regulatory. Flower that is sold must come from a GACP-certified site, and GACP places conditions on pesticide use. Check the current rules before applying anything (topic 324).
Barley straw is used against pond algae, though Penn State Extension notes that the mechanism is poorly understood (Swistock, 2025). We found no study showing that barley products harm mycorrhizal fungi.
6. Do commercial inoculants work?
The biology is sound, but the product category has a documented quality problem. Three recent papers address it:
- an assessment of 23 inoculants (Koziol et al., 2024);
- “Poor Quality of Commercial Arbuscular Mycorrhizal Inoculants Used for Agriculture and Home Gardening” (Boussageon et al., 2025);
- a meta-analysis concluding that globally sourced commercial inoculants fall short (Koziol, McKenna & Bever, 2025).
The studies consistently report low viability and inconsistent results.
What to check before buying:
- Species list: AMF genera, not just ecto species (section 1).
- Propagule count per gram: a propagule is a spore or fragment that can start colonisation. “CFU” (colony-forming units) is a bacterial measure. If AMF are given only in CFU, ask the supplier to clarify.
- Manufacture date and, ideally, a batch-specific viability test. A supplier who will not state species, propagule count and date is giving you information by refusing.
- Mixed products: Trichoderma or Bacillus in the same pack act independently of phosphorus, so any benefit in a high-P system probably comes from them rather than from the AMF (see topic 1030).
The most reliable source of propagules is soil that already has them. Recycled living soil that has grown mycorrhizal plants carries its own inoculum.
7. Sourcing in Thailand and Southeast Asia
Importing a heat-sensitive biological product across the tropics creates exactly the storage problem in section 5.3. Options, roughly in order of preference:
- Recycled living soil from your own previous cycle. It carries established propagules at no cost (topic 1091).
- Locally produced AMF biofertiliser. Thailand’s Department of Agriculture has researched and tested arbuscular mycorrhizal biofertilisers, including a granular form, for field crops. Ask your provincial agricultural office or the Department about current availability. Agricultural-grade product is often cheaper per propagule than retail horticultural packs, but the same quality checks apply. Note that the Land Development Department’s well-known “Super LDD” microbial products are composting and soil microbes, not mycorrhizal inoculants.
- Imported branded product. Accept the storage and cost penalty, and buy only from a seller who can tell you when the batch arrived and how it was stored.
8. Checklist
- Buy an AMF product. Check the species list and ignore the ecto species.
- Check the propagule count (not CFU) and the manufacture date.
- Store sealed, dry, cool and out of sunlight. Heat is the main enemy here.
- Inoculate once, early: at germination for seedlings, at rooting for clones. Inoculate again only when moving into fresh sterile media.
- Put the product in contact with roots, not as a surface drench.
- Mix soluble product with filtered, RO or rain water.
- In living soil that relies on AMF, keep sustained solution P below roughly 50–60 mg/L and supply P as slow-release solids.
- On coco or hydro with a complete mineral feed, don’t expect much from AMF.
- Consider skipping the product if your recycled living soil already carries propagules.
Has anyone in the region actually verified colonisation, by root staining or a lab check, rather than assuming it happened? Given what the quality studies report, this is the one input where “I used it and the plants looked fine” is the weakest possible evidence. If you have looked at stained roots under a microscope, we would very much like to see it.
References
- Boussageon, R., Eigenmann, M., Frommenwiler, J., & van der Heijden, M. G. A. (2025). Poor Quality of Commercial Arbuscular Mycorrhizal Inoculants Used for Agriculture and Home Gardening. Journal of Sustainable Agriculture and Environment, 4(4), e70107. https://doi.org/10.1002/sae2.70107
- Ilyas, U., du Toit, L. J., Hajibabaei, M., & McDonald, M. R. (2024). Influence of plant species, mycorrhizal inoculant, and soil phosphorus level on arbuscular mycorrhizal communities in onion and carrot roots. Frontiers in Plant Science, 14, 1324626. Frontiers | Influence of plant species, mycorrhizal inoculant, and soil phosphorus level on arbuscular mycorrhizal communities in onion and carrot roots
- Koziol, L., et al. (2024). An assessment of twenty-three mycorrhizal inoculants reveals limited viability of AM fungi, pathogen contamination, and negative microbial effect on crop growth for commercial products. Applied Soil Ecology, 202, 105559. Redirecting
- Koziol, L., McKenna, T. P., & Bever, J. D. (2025). Meta-analysis reveals globally sourced commercial mycorrhizal inoculants fall short. New Phytologist, 246(3), 821–827. https://doi.org/10.1111/nph.20278
- Li, P., Liu, J., Zhang, S., Zhu, Y., Yin, X., Xing, L., Wei, D., & Jin, L. (2025). Effects of Nitrogen and Phosphorus Levels on Arbuscular Mycorrhizal Symbiosis and Associated Bacterial Communities in Culture. Journal of Fungi, 11(11), 757. https://doi.org/10.3390/jof11110757
- Swistock, B. (2025, updated). Barley Straw for Algae Control. Penn State Extension. Barley Straw for Algae Control
- Yu, Y., Zhang, Q., Zhang, Z., Xu, N., Li, Y., Jin, M., Feng, G., Qian, H., & Lu, T. (2023). Assessment of residual chlorine in soil microbial community using metagenomics. Soil Ecology Letters, 5(1), 66–78. Client Challenge
This article is technical horticultural information for readers operating legally in their jurisdiction. Cannabis cultivation is regulated differently in every country. In Thailand, cannabis is a controlled herb under the Ministry of Public Health Notification Re: Controlled Herbs (Cannabis) B.E. 2568; cultivation requires the applicable permission. Check your local law before growing. Nothing here is legal advice.