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OPTIMAL RATE OF ORGANIC FERTILIZER DURING THE FLOWERING STAGE

Deron Caplan

Sep 1, 2017

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


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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.

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