Again, when the pH drifts into unwanted territory, adjustments must be made. Below are standard corrective procedures used to modify substrate pH for plants grown in soil-less substrates in greenhouses and adapted for cannabis.
1. Low Substrate pH Correction
When Fe toxicity and Mn toxicity become a problem, raise substrate pH to the recommended pH range. Corrective procedures to raise low pH levels are listed on p. 52 (Table 1). Switching to an alkaline fertilizer when substrate pH is nearing the lower limit will help stabilize the pH.
Flowable Lime
To make small adjustments of roughly 0.5 pH units, mix 1 to 2 quarts of flow able lime per 100 gallons of water. If using an injector, avoid using higher concentrations of flow able lime as it will damage it. If higher rates are needed, then split your applications to avoid damaging your injector. To avoid leaf burn, it is best to rinse the foliage after treatment if any flow able lime comes in contact with the leaves.
Hydrated Lime
For more stubborn low-pH problems, use a hydrated lime mixture. Mix 1 pound of hydrated lime in 3 to 5 gallons of warm water. Mix it twice and let it settle after each mixing. Once the sediment collects in the bottom of the container, pour the liquid through your injector system set at a 1:15 ratio. This product is corrosive, so rinse the foliage as soon as possible to avoid leaf burn, and avoid skin contact.
Potassium Bicarbonate (KHCO3)
Potassium bicarbonate should be handled with care. It can throw off your substrate chemistry as it provides 993 ppm of potassium (K) in mixtures of 2 pounds per 100 gallons of water. Rinse the foliage immediately after application via injectors and leach heavily the following day with a complete fertilizer to reduce substrate electrical conductivity (EC) and restore nutrient balance. The 2-pounds rate will increase the substrate pH by roughly 0.8 pH units. Rates greater than 2 pounds per 100 gallons of water can cause phytotoxicity.
As always, remember to recheck your substrate pH to determine if reapplications are needed.
2. High Substrate pH Correction
The target pH for cannabis is between 5.8 and 6.2. Higher pH values may result in Fe deficiency and create interveinal chlorosis on upper leaves. Check the substrate pH to determine if it is too high. Be careful when lowering the substrate pH, as low pH can be more problematic and difficult to manage.
Cannabis plant growth is less vigorous at low substrate pH levels.
Acid-Based Fertilizer
If the substrate pH is beginning to increase, consider switching to an acidic-based fertilizer. These ammoniacal nitrogen (N)-based fertilizers are naturally acidic, and plant nitrogen uptake will help moderate the substrate pH over a week or two.
Acid Water Drench
Some growers use this intermediate correction when pH levels are not excessively high and a quick lowering of substrate pH is desired. Sulfuric acid is recommended to acidify irrigation water to a pH 4.0 to 4.5. Apply this acid water as a substrate drench, providing 5 percent to 10 percent excessive leaching of the substrate. Rinse the foliage to avoid phytotoxicity. Results should be visible within five days. Retest the substrate pH and repeat if needed.
Iron Drench
If the levels are excessively high, then a Fe chelate application can be made to the substrate.
- Iron-EDDHA can be mixed at a ratio of 5 ounces per 100 gallons of water.
- Iron-DTPA can be mixed at a ratio of 5 ounces per 100 gallons of water.
- Iron sulfate can be mixed at a ratio of 4 ounces to 8 ounces per 100 gallons of water.
Apply the iron as a substrate drench with enough volume to leach the pot, and rinse the foliage immediately after application.
Conclusions
Based on grower observations and initial experiments at NCSU, it is possible to grow cannabis with a wider soil-less substrate pH range than most other species. Cannabis plants do not appear prone to develop leaf symptomology when substrate pH is too low or too high compared to the current general greenhouse standards—only plant stunting occurs at sub-optimal conditions.
Therefore, based on research and experience with other species, a wider range of 5.5 to 6.5 may be used. When adapting these values to a monitoring system, the recommended pH zone to target would be 5.8 to 6.2. By monitoring the substrate pH over time, one can assure that plants are within the optimal range.
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