The pH and nutrient concentration of a cannabis plant’s medium is a lot like a transistor radio; you have to dial in the right numbers to unlock its full potential.
Whether you are growing with a soil or soilless setup, pH (potential hydrogen) measures the acidity and alkalinity of the medium, which in turn controls the nutrients the plant can absorb.
Students enrolled in the Cannabis Professional Series at Kwantlen Polytechnic University, based in Surrey, British Columbia, learn about the importance of ideal pH levels early in the course.
“It's something that’s misunderstood, often overlooked, and it's highly important,” says Nico Haché, one of the program's instructors and a horticultural consultant with Root to Shoot Solutions. “People talk about the nutrient lockout, that’s generally pH related.”
Cannabis plants prefer a slightly acidic environment for their roots. Growers using soil as their medium should adjust their pH to a range of 6 to 6.8. For a soilless garden, pH should sit between 5.5 to 6.5.
Allowing a pH range, regardless of the medium, ensures the plant is able to absorb the variety of nutrients required for optimum growth. For example, the plant's ability to absorb manganese increases with a more acidic medium. Numerous pH and nutrient uptake charts are available online to illustrate the ideal pH for each element needed throughout the vegetative and flowering stages of growth.
“A range allows marijuana to absorb what it likes,” says Haché. “Your pH doesn't change your concentration of nutrition, it changes the availability of nutrients. Even though the nutrients are there, they might not be up-taken, or they may be absorbed in excess.”
While many nutrient companies add pH stabilizers to their products, Haché recommends frequent water testing to ensure pH remains in the ideal range.
“Water is a big factor that can affect your pH. City water, pond water, or river water, whatever set-up you have, every water source is different,” he says.
Whether or not different cannabis strains thrive under distinctive pH levels is an area that has undergone little scientific scrutiny. Haché doubts that exploring specific acidity levels based on cannabis species would produce noticeable improvements in yield or quality.
“When you’re talking about changing your acidity like that, it’s complex. Often, they’re hybrid plants, so they're not true Indicas or Sativas,” he adds. “It becomes difficult to start playing with that too much.”
However, matching pH to the plant’s natural environment could help promote native characteristics, like terpene profiles.
“When you think of the regions of the world where these plants come from, they’re complete opposites. All the factors would be different,” Haché says. “You can treat them exactly the same, they’re both marijuana plants, but you could probably be a little more efficient by fine-tuning Indicas or Sativas.”
Unlike pH, nutrient concentration should be adjusted for different strains of cannabis. The electroconductivity (EC) or total dissolved solids (TDS) are both measurements used to determine the nutrient concentration in your medium.
“You can definitely manipulate how often and how much you feed individual strains,” Haché said. “The fast-growing plants will be very hungry. You can afford to feed them more and keep pushing them. If you were to do the same to a shorter, slower growing plant, you might push them too much.”
Overfeeding a plant will likely result in a nutrient burn, causing the plant’s leaf tips to turn yellow or brown. Left unchecked, the nutrient burn will hinder growth and yield. Underfeeding will also affect the plant's ability to reach its full potential.
To find out what’s right for the strains in your garden, Haché recommends starting with a low nutrient concentration in the cloning or seed phase of growth. The EC should gradually increase as the plant matures.
“To find out how far you can go, push. It is a matter of trial and error and knowing your plants. Make sure you take notes and keep track,” he said. “There are also early telltale signs you can see if you’re pushing too much.”
The EC should peak about halfway through the flowering stage of growth.
“With a nine-week flowering or 10-week flowering plant, I would peak around the fourth or fifth week and then begin to tone it down, so you can start your flush at the end. You slowly creep up,” says Haché.
A gradual increase, as well as a slow decline, in EC, will allow you to determine how far you are able to push your plants while avoiding possible shock caused by drastic swings in nutrient concentration.
“Never have big shifts. A change is a stressor,” says Haché. “Consistency is key with anything you do with plants.”