Cannabis Potting Soil pH Monitoring

Root Zone pH Monitoring

Peat is commonly used as an indoor gardening medium, but unlike coir it is quite acidic and will need its pH adjusted before use and monitored throughout the season unless you want problems. Peat is naturally acidic (pH 3.5-4.5) and will drift to become even more acidic over time. Pulverized lime must be added to raise peats pH and keep it stable over the growing season. Learn more about amending Peat here.

Signs of nutrient deficiency will appear when growing in peat if the grower isn’t keeping a close eye on the growing mediums pH on a regular basis. Often the nutrients are there in the soil but are unavailable to the plants because the pH has drifted down and has become too acidic for the plants to process. If this low pH issue isn’t addressed quickly the plants will develop deficiencies due to lock out and then stop drinking. The roots will start to drown in an acidic water and nutrient solution which will then add other root zone problems like root rot and other fungal diseases that will quickly spread to the rest of the plant.

The following Case Study looks at 2 common scenarios one might use when growing with an inert medium like peat. When problems become apparent it is critical to Know Your Variables and act quickly.


Scenario 1 - Growing with Chemical Nutrients


  • Growing Medium: Peat, Perlite, Pulverized Garden Lime
  • Some Form of Water Soluble Fertilizer
  • Reverse Osmosis Filtered Water
  • Gen Hydro pH Up

In this scenario the growing medium is pH adjusted and stabilized with powdered lime. The water is adjusted pH 6.8 when used by itself. The nutrient solution is adjusted up to pH 6.8 after mixing with RO water. Since all of the variables are accounted for this grower should have a positive growing season as long as the amount of nutrient being used is in line with the cannabis strain being grown. Different cannabis strains have different nutrient needs so make sure you know what you are growing and adjust accordingly. Also know that a small plant requires less PPM TDS of nutrients than a large plant.

* Hobby growers should probably avoid expensive liquid nutrients unless you are able to use the entire bottle within 6 months. Some have a very short shelf life, the minerals fall out of the solution making them useless. You are better off finding a good, dry, water soluble fertilizer that can sit on the shelf for years, you will save a lot of money in the process.

Scenario 2 - Growing with Osmocote Time Release Fertilizer


  • Growing Medium: Peat, Perlite, Pulverized Garden Lime
  • Osmocote 15-9-12
  • Reverse Osmosis Filtered Water
  • Gen Hydro pH Up

In this scenario the growing medium is pH adjusted and stabilized with powdered lime. The water is adjusted pH 6.8. Osmocote is mixed into the top layer of growing medium per instruction on bag. Pretty simple, now all that is needed is regular watering using pH adjusted RO water…right? It seems like all of the variables are accounted for but they are not. The grower in this scenario will develop a nutrient lock out problem due to drifting root zone pH that will get progressively worse over time. So what did we miss? The problem in this scenario is that pouring the pH adjusted water over the acidic Osmocote in the soil will lower the pH in the root zone leaving the plants to sit in an increasingly acidic nutrient solution after watering. So the grower using this setup forgot to account for the Osmocote Nutrients acidic pH. The simple fix here is to adjust the pH of the RO water up to around 8-8.5. Now when that water passes over the acidic granules the resulting nutrient solution in the root zone will have a pH of around 6.5 which is ideal for your plants.

Read more about growing Cannabis with Time Release Nutrients like Osmocote.


How to test the soil pH

Test your soils pH regularly throughout the season for whatever scenario you are using to know exactly what your plants are dealing with at the root zone. If the pH is too acidic adjust your water or nutrient solution and retest. Repeat until the pH is around 6.8. It is important to retest regularly during the season as the pH of the water from the tap will change throughout the year. Testing your soils root zone pH is simple:

  1. Take 1/4 cup of soil from the plant container and put it in a glass.
  2. Pour 1/2 cup of your pH adjusted water into the glass.
  3. Thoroughly mix together and let sit for a few hours.
  4. Filter the soil and water mixture into a new glass using a coffee filter or sock. Do this a few times to remove as much particulate matter as possible.
  5. Test the pH of the resulting solution using the Gen Hydro pH testing kit.

The morale of the story is to Know Your Variables and monitor the growing mediums pH closely throughout the season. Many gardening problems start in the soil so don’t make the mistake of misdiagnosing a cultural problem as a nutrient deficiency because throwing more nutrients on Scenario 2 would lead to disaster.

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