pH for Cannabis Plants

Gardening can be pretty easy.

You put your seed or seedlings into the ground, fertilize them, and water them. If necessary, you spray them with something to keep pests away or give them some added support so they don’t fall over.

They grow, they flourish, they produce their harvest — and you enjoy it. Easy-peasy. For many people, it’s fun, relaxing, and enjoyable as well.

Then, there’s cannabis.

The basics of growing weed are the same, but marijuana plants can be much trickier to deal with. They can be more susceptible to some types of diseases and pests, and they need different nutrients during different stages of growth.

And then there’s pH.

If the abbreviation sounds familiar, you almost certainly learned about it in high school science class. pH measures how acidic or alkaline a substance is — and proper pH levels are crucial when you’re growing cannabis.

Let’s unpack.

What is pH?

pH stands for potential hydrogen, and it’s measured on a simple number scale from 1-14.

Without going too much into detail you probably don’t care about, pH measures the concentration of hydrogen ions and hydroxide ions. A surplus of hydrogen ions makes a substance more acidic, while a surplus of hydroxide ions, makes it more alkaline (or “basic”).

The lower the pH number, the more acidic a solution or substance is. That means something with a pH of 1.0 is completely acidic; a pH of 14.0 means it’s completely alkaline. 7.0 is the midpoint on the scale and indicates that a substance is “neutral,” neither acidic nor basic.

For example, pure water has a pH of 7.0, pure sulfuric acid may have a pH of 1.0, and some liquid drain cleaners containing sodium hydroxide can register 14.0. Orange juice comes in around 3.0, and baking soda around 9.0.

That’s all interesting (or maybe not). But if you don’t have to pass a science test, who cares?

You should if you plan to grow weed.

pH and Plants

Needless to say, plants need water and nutrients like nitrogen, phosphorus, and potassium to grow and thrive. Gardeners who want healthy and productive plants can help their plants by making sure they’re best able to use the water and nutrients they’re given.

That’s where pH enters the discussion.

The vast majority of plants are most efficient at absorbing and utilizing nutrients when they’re grown in soil with a slightly acidic pH.

Why is the pH of the soil so important?

It’s partly because beneficial bacteria are responsible for releasing nutrients from soil and fertilizers, and those bacteria work best when their environment has a pH between 5.0 and 7.5. It’s also because plants are best at utilizing nutrients when the pH range is between 5.5 and 6.5, and because nutrients leach out of soil rapidly when pH is below 5.0.

The pH of the water you give your plants matters as well. If it’s too low or too high it will change the pH of the soil, potentially stopping the plants from absorbing nutrients properly. Acidic water is also more likely to contain hazardous heavy metals.

The optimal pH numbers we’ve mentioned can vary from species to species, and some plants are hardy enough to be able to deal with pH levels that are too low or too high.

Cannabis isn’t one of those plants.

pH Levels and Weed

Marijuana plants need slightly acidic soil in order to properly absorb and utilize nutrients, and they’re much fussier about it than ordinary roses or garden tomatoes.

Cannabis won’t thrive unless the pH of the soil is between 6.0 and 7.0; keeping it between 6.2 and 6.8 is even better. It doesn’t have to be right at the pH midpoint of 6.5, since the plants actually benefit a bit from fluctuation between the two endpoints of the range. For a good hydroponic grow, the ideal range is a bit lower, between 5.5 and 6.5.

An improper soil pH is likely to cause one of the big issues that weed growers may face: nutrient lockout. The plants are unable to utilize the nutrients they’re given, which build up in the soil instead. Meanwhile, the plants absorb more micronutrients like aluminum and chlorine than they should. The result: growth is stunted or stopped, the plants turn yellow, and they eventually die.

The pH of water used on the plants also matters. As we’ve mentioned, it can negatively affect the pH of the growth medium if it’s too low or too high. Low pH levels can also prevent the absorption of pesticides or fungicides administered to the plants.

Clearly, pH is crucial. What can you do about it?

Measuring and Adjusting pH in a Weed Grow

Soil pH test kits are available at every garden shop and online, and they’re simple to use. Some have sample strips that turn colors, others include a more-accurate pH meter. In some locations, you can also take your soil to a county or state facility that does free pH and nutrient testing.

If you find that the pH of your soil is too high or too low, the easiest solution is to purchase one of the products often sold under a name something like “pH Down” (which contains phosphoric acid) or “pH Up” (which contains potassium hydroxide or potassium carbonate).

You mix a few milliliters (directions will be on the label) of the proper product into a gallon of water and add it to your soil, check the soil’s pH after a few minutes, and then add more if necessary. If you prefer a DIY approach, you can add peat moss to water to lower pH, or lime or baking soda to increase it.

It’s also a good idea to check the pH of the water you regularly give your plants. If it’s too high or too low you may have to compensate rather than just watering out of the tap.

What about adjusting the pH of the growth medium itself during the grow? It’s theoretically possible by adding limestone or wood ash to the soil to boost pH or adding sulfur to lower it. However, those substances can take months to have any real effect.

Changing the growth medium mid-grow could also disturb your plants or seriously hamper their growth. The right move is to adjust pH with additives that can combine with water — and leave the serious changes for the following year when you can start your grow with soil that has an ideal pH level.

pH for Weed: FAQ

Q: Does cannabis require different pH levels for different growth stages?
A: Not really. The optimal pH for weed plants remains the same throughout their growth. However, pH is just as important for the proper development of seedlings as it is during the vegetative and flowering stages, so it’s crucial to start the plants in soil with proper pH and then monitor it right through until harvest.

Q: What causes low pH in soil?
A: Soil’s pH changes naturally over time as it’s used as a growth medium. The erosion of topsoil, the natural loss of organic matter, depletion of minerals as plants grow and are harvested, and the effects of fertilizers added during the season all alter pH. That’s why it’s important to monitor and adjust the pH as plants grow, and to rejuvenate or change the growth medium in between crop plantings.

Reference

Feng, H., Fan, X., Miller, A. J., & Xu, G. (2020). Plant nitrogen uptake and assimilation: regulation of cellular pH homeostasis. Journal of Experimental Botany, 71(15), 4380-4392. [1] [2]