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Controlling pH in Container Grown Cannabis

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Part I of a 3-part series on managing pH in container grown cannabis

By Ben Holmes, founder of Centennial Seeds in Lafayette, Colorado.

I’ve been growing cannabis indoors continuously for seven years. Over the course of those years I have learned a few things about growing this crop. Without a doubt the most important lesson I’ve learned is the role that pH plays in healthy, productive plant growth.


Most cannabis gardeners think of pH as something that must be maintained in their water and the fertilizer solutions they apply to their crops. For the purpose of this discussion let’s think of pH as a condition that exists in the planting media, in and around the roots and in the aqueous film that coats the contents of the growing container.

In this first part we will identify three components of your grow that significantly impact your media pH: the growing media, fertilizers and water.

In my lab I grow in Fafard Peat-Lite media. I use a variety of Fafard mixes from their Super Fine Germinating mix to their Fafard Organic Formula FOF-30. I use their professional products in my seed production facility because they are the very best.

Peat-Lite lends some amount of acidity throughout the growth cycle, as do coco coir mixes. That acidity if allowed to progress unchecked will create unfavorable conditions at the root zone and will negatively affect plant growth.

The fertilizers we choose also affect pH in the container. More specifically, the form of nitrogen in our fertilizers greatly affects the direction of pH drift in planting media.

Nitrogen from organic sources are protein based. These proteins degrade to amino acids which degrade to ammonia and urea. Ammonia and urea can also be manufactured by industrial process. These nitrogen sources have an acidifying effect on media pH.

The second type of nitrogen, that which comes from nitrate sources has a net increasing effect on pH.

Used alone or when combined in specific ratios the two forms of nitrogen can be used to steer media pH one way or the other, up or down.

Your water and its mineral content also has an effect on media pH. The amount of calcium and magnesium carbonates present in your water determine how much buffering capacity your water has.

Water with high carbonates will work in concert with an acid producing fertilizer to neutralize the acid and to provide calcium and magnesium. Acidic water can be paired with a base producing fertilizer to counter act the increase in media pH.

This is useful knowledge, but how do you apply it to your own garden? The answer begins with a water test. If you are serious about growing cannabis you have to start with some key information about your water source. For our purposes you want to know the amount of calcium carbonates and total alkalinity in your water.

I will follow up in Part II with some examples of fertilizers that match particular water types.

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