D. Caplan, M. Dixon, Y. Zheng
Sep 1, 2017
Optimal Rate Of Organic Fertilizer During The Vegetative Stage For Cannabis Grown In Two Coir-based Substrates
This study investigated the optimal rate of organic fertilizer during the vegetative stage for Cannabis sativa L. grown in two coir-based substrates, aiming to determine how nitrogen (N) application affects biomass yield and cannabinoid concentration. The work was conducted under controlled environmental conditions in a walk-in growth chamber, ensuring consistent temperature, humidity, and light.
Experimental Design
Researchers evaluated five nitrogen application rates—117, 234, 351, 468, and 585 mg N/L—delivered via a liquid organic fertilizer with a 4.0–1.3–1.7 (N–P–K) ratio. The two substrates used were:
U1-HP – a high-porosity coir-perlite mix with lower container capacity and higher aeration.
U1 – a similar coir-based substrate with higher water retention and lower porosity.
Uniform rooted cuttings of a single cannabis cultivar were transplanted into 8.4-L containers filled with one of the two substrates. Plants were fertigated daily with their assigned nitrogen concentration throughout the vegetative period. After the vegetative phase, all plants were transitioned to a common flowering regime and given the same fertilizer rate to ensure differences observed were attributable to vegetative-stage nutrition.
Key Measurements
Growth Index (GI): A composite measure of plant size, incorporating height and width.
Floral Dry Weight: Biomass of harvested inflorescences after drying.
Cannabinoid Concentrations: THC, THCA, CBD, CBDA, and CBN measured in dried floral material.
Total Cannabinoid Yield: Calculated by multiplying biomass by cannabinoid concentration.
Results
No Substrate Effect: Statistical analysis found no significant differences between U1-HP and U1 for any measured parameter. This allowed data from both substrates to be pooled for subsequent analysis.
Biomass Response: Floral dry weight increased with N rate up to an estimated maximum at approximately 389 mg N/L, after which biomass plateaued or slightly declined.
THC Response: Peak THC concentration occurred at ~418 mg N/L, a rate similar to that for maximum biomass, indicating little trade-off between yield and potency at the optimal range.
Other Cannabinoids: No significant fertilizer rate effect was found on THCA or CBN concentrations, and CBD/CBDA were negligible in this cultivar.
Overall Cannabinoid Yield: Because yield and potency peaked in the same general range, total THC yield per plant was maximized at ~400 mg N/L.
Visual Growth Indicators: Plants at the lowest N rate showed signs of nitrogen deficiency (chlorosis, reduced growth), while those at the highest rate displayed no visible toxicity but offered no further yield advantage.
Interpretation
The findings suggest that, during the vegetative stage, supplying cannabis with approximately 389–418 mg N/L from an organic liquid fertilizer is optimal for maximizing both biomass and THC production under controlled indoor conditions, regardless of which of the two coir-based substrates is used. Below this range, plants may suffer reduced growth and cannabinoid output; above it, additional nitrogen offers no significant benefit and may reduce nutrient-use efficiency.
Practical Implications
Fertilizer Efficiency: Staying within the optimal N range avoids waste of inputs and potential negative environmental impact from nutrient runoff.
Substrate Flexibility: Growers can select between these two coir-based substrates without expecting major differences in yield or potency response to nitrogen.
Strain and Environment Specificity: While results were clear for the tested cultivar under controlled indoor conditions, rates may differ for other genetics, growing systems, or environmental settings.
Integration with Flowering Nutrition: Because vegetative nitrogen supply affects final flowering output, this stage should not be overlooked when optimizing for yield and quality.
Limitations and Considerations
The study was limited to a single cultivar and two similar substrate types, meaning results may not fully generalize to other varieties or more divergent media.
The maximum tested N rate was 585 mg/L; effects beyond this threshold were not studied.
All measurements of cannabinoid content were taken after a standardized flowering phase, so the results reflect vegetative-stage nutrition’s indirect impact on final product quality.
Conclusion
Under the conditions tested, the ideal nitrogen rate for cannabis during the vegetative stage is around 400 mg N/L, achieving the best balance of growth, yield, and THC concentration. This range appears effective across the two tested coir-based substrates, offering growers flexibility in media choice while maintaining optimal plant performance.