Optimal Rate of Organic Fertilizer during the Flowering Stage for Cannabis Grown in Two Coir-based Substrates

Read full article here

Study Objectives

• Determine the optimal rate of a liquid organic fertilizer during the flowering stage of cannabis.

• Evaluate performance in two coir-based substrates:

  • U2-HP (lower container capacity, better drainage)

  • U2 (higher container capacity, retains more water)

Experimental Setup

• Cultivar: Cannabis sativa L. ‘WP:Med (Wappa)’

• Fertilizer rates: 57, 113, 170, 226, and 283 mg N/L

• Fertilizer: Nutri Plus Organic Bloom (2.00N–0.87P–3.32K)

• Plants grown in a controlled indoor chamber under high-pressure sodium lights.

• Growth metrics, yield, and cannabinoid content were tracked throughout flowering.

Key Findings

Growth and Yield

• Growth index (GI) and yield increased with fertilizer rate up to a point.

• U2-HP outperformed U2:

  • 13% higher growth index

  • 11% higher floral dry weight (yield)

• Optimal fertilizer rate for yield in U2-HP: ~261 mg N/L

• U2 showed linear yield increases with no clear plateau up to 283 mg N/L

Cannabinoid Content

• As fertilizer rates increased, THC, THCA, and CBGA concentrations decreased (dilution effect).

• Despite this, cannabinoid yield per plant increased with fertilizer rate up to an optimum.

• U2-HP produced:

  • 20% higher THC concentration

  • 57% higher THC yield

  • 22% higher THCA yield

  • 20% higher CBGA yield

• Optimal fertilizer rates in U2-HP for cannabinoids:

  • THC yield peak: 223 mg N/L

  • THCA yield peak: 212 mg N/L

  • CBGA yield peak: 228 mg N/L

    
    

Substrate Behavior

• U2-HP had:

  • Higher fertigation frequency

  • Lower pH at high fertilizer rates

  • Higher oxygenation potential due to lower CC

• U2 retained more water but had slightly lower oxygen availability, possibly contributing to root issues.

Conclusions

• U2-HP is a better substrate for flowering-stage cannabis under organic fertilization.

• Optimal fertilizer rate range for U2-HP: 212–261 mg N/L

  • Higher end for yield, lower end for cannabinoid content

• For U2, highest tested rate (283 mg N/L) maximized yield, but cannabinoid concentrations were lower.

• Over-fertilization risks dilution of cannabinoid concentration even as yield increases.

• Irrigation frequency and substrate aeration likely play a major role in maximizing both biomass and cannabinoid output.