Adjust pH Level Cannabis
3 min

How To Adjust pH Level When Growing Cannabis

3 min

In this article, we’ll teach you all you need to know about the importance of pH when growing cannabis, and how to change your pH levels if necessary.

When growing cannabis, getting the pH level of your medium right is crucial. Cannabis plants tend to perform their best in slightly acidic soil with a pH between 6 and 7. If you’re growing hydroponically, the ideal pH for cannabis is between 5.5 and 6.5.

If your medium is either too acidic or alkaline, your plant’s roots will struggle to absorb nutrients, resulting in stunted growth and even nutrient deficiencies. Highly acidic soil can also encourage the growth of fungal diseases that can kill your plants in a matter of days.



pH is a key indicator of the quality of your soil, and any savvy gardener will tell you that your plants will only ever grow as well as your soil allows them too.

If you think back to your high school biology class, you may remember that the pH scale is a simple measure of the acidity or alkalinity of a medium. The lower half of the scale (between 0 and 6.5) is considered acidic, while the higher half (between 7.5 and 14) is alkaline. Lemon juice, for example, has a pH level of 2, while soapy water has a pH of about 12.

The mineral composition of your soil and the weather are the two biggest factors affecting its pH. Moist soil with a lot of naturally decaying material (such as woodland soil containing leaves and bark or compost containing food scraps) always tend to be more acidic. Meanwhile, soils from dry, arid climates tend to be more alkaline.

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If your soil is too acidic, your plant’s roots will struggle to absorb nutrients like phosphoric acid, calcium, and magnesium as they become less soluble. In alkaline conditions, on the other hand, your plant will struggle to absorb nutrients like iron, copper, zinc, manganese, and boron in the soil.



The easiest way to measure the pH is with a digital pH pen that you simply stick into your soil for a reading. Alternatively, you can also use a manual pH kit to measure the pH of the water runoff from your soil. These kits usually come with pH drops or strips which, once added to the water, will turn a specific colour to tell you the pH of your soil. You’ll find both manual pH kits and digital pens at any decent garden supply store.

If you don’t have a pH kit or pen, you can get a less specific reading of your soil’s pH using baking soda and vinegar. Simply collect some soil from your garden in two separate containers. In the first, add roughly ½ of the soil volume in vinegar to the soil. If it fizzes, you have alkaline soil. If it doesn’t, add some water to the soil until it is muddy. Then, add in some baking soda. If it fizzes this time, you have acidic soil.

Remember, these DIY method of measuring your soil’s pH aren’t very accurate, and should only be used as a last resort. Where possible, always use a digital pen or pH kit for an exact reading.


The easiest way to adjust the pH level is to use chemical pH-up and pH-down solutions. Some pH kits come with these solutions included. However, there are alternative ways to alter your soil’s pH too.


Adjusting The pH Level Of Your Soil

You can also increase or decrease the pH of your soil by adding organic materials directly to it. However, we only recommend doing this when preparing your soil prior to planting.

If your soil is too acidic, you can use limestone (either pulverised or granulated) or wood ash to bring up the pH of your soil. For best results, till the lime or ash into the soil at least 2 or 3 months before planting, and water regularly.

To decrease the pH of your soil, try using sulfur. Again, you’ll need to till this into your soil and be patient. Sulfur can take months to bring down the pH of soil. For more immediate results, try using aluminum sulfate. A chemical reaction that occurs in the aluminum will drive down the pH of your soil almost instantly.


Adjusting The pH Of Your Water

One easy way to alter the pH of your soil is to alter the pH of your water using phosphoric acid or peat moss (to lower pH), and lime, wood ash, or baking soda to increase it. This is the best way to change soil pH while your plants are growing.

If your soil is too acidic, try watering your plants with slightly alkaline water (pH 7–8). If your soil is too alkaline, use slightly acidic water (pH 5.5–6.5) instead. Don’t do this more than twice to avoid damaging your plants. Switch back to regular water and check the pH of your soil and adjust as needed.

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Adjusting Soil pH When Growing Organically

One of the best ways to maximise the potential of your plants is to take the time to prepare your own organic soil well before you germinate your seeds. This takes a lot of time and effort, but the results are well worth it.

To do this, you’ll need to separate the soil you plan to use for planting in a large container, such as a compost bin. Some growers start this process in winter to give themselves plenty of time to get their soil ready for growing.

Over the winter and spring, you’ll want to add organic materials to your soil to create a rich medium filled with all kinds of nutrients for your plants. These organic materials can include:

  • Fruit and vegetable scraps

  • Eggshells

  • Used coffee grounds and tea leaves

  • Garden trimmings

  • Pine needles

  • Lime

  • Ash

  • Sand (for drainage)

The goal here is to create a rich, well-draining soil with the right pH level to house your cannabis plants over summer. Remember, this method will take time, but can deliver great results. After all, your plants will only ever be as good as the soil they’re growing in.

Steven Voser
Steven Voser
Steven Voser is an independent cannabis journalist with over 6 years of experience writing about all things weed; how to grow it, how best to enjoy it, and the booming industry and murky legal landscape surrounding it.
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