Growers argue endlessly about soil versus coco versus everything else. The peer-reviewed evidence is narrower than the debate: most studies test specific commercial mixes in containers, alongside a specific fertilizer program.
In a University of Guelph trial, two coir-based mixes that held different amounts of water performed the same during vegetative growth, and the organic fertilizer rate mattered more than the mix. A German study comparing peat, coco coir and a peat and green-fiber blend found the effect depended on the genotype: one cultivar produced substantially less flower outside of peat, while CBD content stayed the same in every substrate. For cuttings, rockwool far outperformed other propagation media in a large hemp trial.
The takeaway is that a growing medium works as a system with its watering and feeding. A substrate result is only as transferable as the nutrient program that came with it.
Gaps we’re still filling: controlled studies of living soil, soil microbiology and amendments for drug-type cannabis.
Key terms
mg/L
Milligrams per liter, a measure of nutrient concentration in water. In dilute nutrient solutions, 1 mg/L is about 1 ppm.
Drug-type vs. hemp
Drug-type cannabis is grown for high THC. Hemp is legally limited to low THC and is often grown for CBD, grain or fiber. Research on one doesn’t always apply to the other.
Studies
4 studies
Impact of Different Growing Substrates on Growth, Yield and Cannabinoid Content of Two Cannabis sativa L. Genotypes in a Pot Culture
Horticulturae · 2020
Hemp
Controlled experiment
Open access
Substrate changed flower yield for one genotype but didn’t change CBD content.
Authors: Lisa Burgel, Jens Hartung, Simone Graeff-Hönninger
6(4):62
Study plant: Two CBD-rich genotypes in pots, indoor cultivation
Two CBD-rich genotypes were grown indoors in pots of peat, peat with 30% green fiber, or coco coir. Responses depended on the genotype: one produced substantially less flower in coir and in the fiber blend than in peat, while the other did well in the peat and fiber blend. CBD content stayed the same regardless of substrate.
Optimal Rate of Organic Fertilizer during the Vegetative-stage for Cannabis Grown in Two Coir-based Substrates
HortScience · 2017
Drug-type cannabis
Controlled experiment
In coir, a vegetative organic feed supplying about 389 mg/L nitrogen gave the best results.
Authors: Deron Caplan, Mike Dixon, Youbin Zheng
52(9):1307–1312
Study plant: Medical drug-type cannabis in two commercial coir mixes, walk-in growth chamber
Plants were fed a liquid organic fertilizer (4.0-1.3-1.7) at five rates during vegetative growth in two coir-based mixes that differed in how much water they hold. The two coir mixes performed the same. Yield, cannabinoid content and growth were best at a rate supplying about 389 mg/L nitrogen during veg, which gave 1.8 times the yield of the lowest rate.
Optimal Rate of Organic Fertilizer during the Flowering Stage for Cannabis Grown in Two Coir-based Substrates
HortScience · 2017
Drug-type cannabis
Controlled experiment
More organic feed in flower meant more bud but lower cannabinoid concentration.
Authors: Deron Caplan, Mike Dixon, Youbin Zheng
52(12):1796–1803
Study plant: Medical cultivar ‘WP:Med (Wappa)’ in two commercial coir mixes, walk-in growth chamber
In this follow-up flowering trial, a liquid organic fertilizer (2.0-0.87-3.32) was applied at five rates to plants in two coir mixes. Feeding more increased growth and yield but diluted THC, THCA and CBGA concentrations. In the drier of the two coir mixes, rates supplying 212–261 mg/L nitrogen best balanced flower yield against cannabinoid yield.
Evaluation of substrate composition and exogenous hormone application on vegetative propagule rooting success of essential oil hemp (Cannabis sativa L.)
PLOS ONE · 2021
Hemp
Controlled experiment
Open access
The rooting medium had the biggest effect, and any hormone beat no hormone by 15–18×.
Authors: Sean M. Campbell, Steven L. Anderson, Zachary T. Brym, Brian J. Pearson
16(7):e0249160
Study plant: Eight high-CBD hemp cultivars, cuttings rooted for 12 days in a climate-controlled room
Researchers rooted cuttings from eight high-CBD hemp cultivars across four propagation media and five commercial rooting hormones, then measured root number, length and mass after 12 days. The medium mattered most: rockwool far outperformed the other media. Any rooting hormone improved rooting 15- to 18-fold over none, but hormone strength and application method made little difference. Some cultivars simply rooted better than others.