Cannabis Soil pH: Measuring and Maintaining for Optimal Growth

Understanding the pH of your soil is important for growing any crop. Providing your plants with the right pH balance in your soil is the key to unlocking the full potential of your plants. But what exactly is soil pH, and why does it matter for cannabis? Let’s explore the basics of pH and explore how you can measure, maintain, and adjust the pH levels in your cannabis garden.

What is pH?

The pH of your soil has been considered the “master soil variable.” It influences your plants’ health and the biological, chemical, and geological processes that take place in your soil. This means that soil pH levels can have an influence on a wide variety of activities within your soil. These activities influence your plants’ health, and the pH of your soil can be a good indicator of the health of your soil.

Activities that are influenced by pH:

• How a plant uptakes nitrogen in the soil.
• How organic materials and heavy metals either break down or build up in the soil.
• The process of turning organic material into nutrients that plants can absorb.
• The activity of enzymes in the soil that helps break down materials.
• Processes happening around plant roots that affect growth and nutrient uptake.
• The availability of organic materials, like compost, that improves soil health.
•  How organic pollutants break down in the soil.
• How microorganisms interact with each other and their environment, influencing soil health and nutrient cycles.

These activities can be influenced by the “potential hydrogen,” or pH, because it changes the chemical environment in the soil, which impacts how everything interacts in the soil. Keeping your soil’s pH within the optimal range can have a tremendous impact. Most plants prefer a pH around neutral, which is 7, while the pH for cannabis tends to make the soil slightly acidic, closer to 6.

Understanding the pH Scale

The pH scale measures how acidic or alkaline a substance is, ranging from 0 to 14. A pH value of 7 is considered neutral, while anything below 7 is acidic, and anything above 7 is alkaline. This scale is logarithmic, meaning each whole number represents a tenfold difference in acidity or alkalinity.  For instance, a pH of 7 is 100 times greater than a pH of 5, and a pH of 8 is 1000 times greater than 5.  

The optimal soil pH range for cannabis is slightly acidic, typically between 5.8  and 6.5. Within this range, the soil activities required for nutrient uptake and cycling can move along.  Deviations from this range can lead to nutrient lockout, poor plant health, and stunted growth.

Why pH Matters for Cannabis

The pH of your soil directly influences nutrient availability. If the soil is too acidic or too alkaline, cannabis roots cannot effectively take up nutrients, even if they’re present in the soil. When the pH is outside the ideal pH range for cannabis, nutrient deficiencies can occur, leading to common issues such as yellowing leaves, poor growth, or even a complete crop failure.

Identifying pH problems

It’s easy to mistake pH problems for pest infestations or nutrient deficiencies. However, there are some tell-tale signs that your cannabis plants may be suffering from pH imbalances. Watch out for:

• Bronzing of leaves, especially between the veins
• Slow or stunted growth
• Curling or distorted leaves
• Burnt leaf tips, despite proper nutrient-feeding
• Poor bud development

If your plants are showing these symptoms, but you’ve ruled out pests and other causes, you may be dealing with a pH issue.

It’s also crucial to consider your soil when diagnosing pH problems. Different soils can hold and buffer pH differently, which is why choosing the best soil for growing cannabis is an equally important factor.

When dealing with pH issues, you’re dealing with nutrient deficiencies that are caused by a lock-out rather than a true lack of nutrients. This can be confusing and lead you to add more nutrients, which can then cause a cascade of other problems. If you are seeing any of the issues outlined above, please check your soil’s pH prior to making any other adjustments.

How to test cannabis soil pH

Testing soil pH is straightforward and can be done with various tools, from basic test strips to digital meters. Here are some methods to suit different budgets and precision levels:

• pH Test Strips: A low-cost, easy-to-use option. Simply take a soil sample, mix it with distilled water, and dip the strip in. Wait for the color to change and compare it to the provided chart. These kits begin around $10 and are a great introduction to testing pH.
• Digital pH Meter: A more precise option that provides a digital reading. These meters can be found at different price points, starting around $20.
• Digital pH Pen: These pens are typically used to test nutrient solutions and are often used in hydroponic grows, you can use them to test the soil by creating a slurry (mixing soil with distilled water). This option is more expensive, starting around $100. 

If you’re just starting with cannabis cultivation, a digital pH pen is a worthwhile investment. Many growers opt for a pen over a meter because the pen can be used to test the pH of nutrient solutions that you may be feeding your plants. Knowing the pH of your nutrient solutions can be as important as the pH of your soil. These pens provide quick and accurate readings, helping you monitor the pH of your soil and adjust pH as necessary.

Using a digital pH pen

Digital pH pens are the go-to tool for serious growers. Follow these steps to use one correctly:

1. Calibrate the Meter: Before testing your soil, ensure your digital pH pen is properly calibrated using calibration solutions (usually included with the device).
2. Collect a Soil Sample: Dig a small sample from the root zone of your cannabis plant (around 2-4 inches deep). Mix it with distilled water in a 1:1 ratio.
3. Insert the pH Meter: Turn on the meter and insert the electrode into the soil-water mixture. Wait for the reading to stabilize.
4. Record the pH Reading: Note the pH value and determine if adjustments are needed.
5. Clean the Electrode: After each use, clean the electrode with distilled water and store it according to the manufacturer’s instructions.

Digital meters make it easy to monitor your soil pH regularly. By staying on top of your readings, you can prevent nutrient lockout and other growth issues caused by improper pH levels.

How to lower pH in cannabis soil

Cannabis growers often face the challenge of their soil becoming too alkaline, with pH values creeping above 7.0. Luckily, several methods can help lower the pH:

• Sulfur: Adding elemental sulfur is a common solution. It reacts with soil bacteria to produce sulfuric acid, which lowers the pH gradually. Use small amounts and monitor your soil pH to avoid dropping it too low.
• Compost or Peat Moss: Organic matter like compost or peat moss is slightly acidic and can help bring down the pH over time. This option is great for organic growers looking for a natural solution.
• Acidic Nutrient Solutions: Some nutrient solutions are designed to lower pH but can have negative effects on our waterways and should therefore be used sparingly. Applications should be timed around any type of rain as these nutrients are typically highly mobile in water.  

For more on watering techniques and how to incorporate pH-balancing solutions into your routine, check out our watering weed article. 

How to raise pH in cannabis soil

Raising soil pH is less common but necessary if your soil becomes too acidic (below 5.8). Here are some strategies:

• Lime: Adding agricultural lime (calcium carbonate) is one of the most effective ways to raise soil pH. Dolomite lime also contains magnesium, which can benefit cannabis plants.
• Wood Ash: For an organic approach, consider using wood ash, which is alkaline and can help raise the pH. Use sparingly and monitor your pH to avoid overcorrecting.
• pH-Adjusting Nutrient Solutions: Some fertilizers are designed to raise soil pH, which can be helpful if you’re adjusting the pH as part of your feeding schedule.

Understanding your soil pH is different from your nutrient solutions pH, to learn more about cannabis nutrients and their pH, check out our article on cannabis nutrients.