5 Classic Questions on pH Marijuana

When it comes to growing marijuana, there are a few main concepts that each and every grower should abide by, and the one I’m going to talk about today is checking and correcting the pH of your water. Maintaining the correct pH of water for your plants is one of the easiest and most beneficial things you can do when it comes to caring for your plants.

I’ve met many beginning growers that were not checking and correcting their pH, and it showed in most of their plants. Having the wrong pH for your water or nutrient solution can cause massive problems with your plants – anywhere from minor nutrient deficiencies that show up spotted leaves all the way to plants having stunted growth or sometimes worse.

In actuality, pHing your water or nutrient solution is an extremely easy and cheap thing to do, and after reading this article I’m sure you will agree – especially when you realize how simple it really is!

What is pH and how do we measure it?

PH is the measure of how “acidic” or “basic” (alkaline) a substance is. This is measured on a scale of 1-14 where 1 is most acidic and 14 is most basic.

Pure water is considered to have a pH of 7, right in the middle of the acidic and basic scale.

There are a few different ways that you can measure your pH. 

There are electronic pH pens available on the market that automatically measure the pH of the fluid they are put into. These are usually extremely accurate but can be expensive (50-300$) and need regular calibrations to maintain their accuracy. For the record, this is what I use because it’s the quickest and most accurate measurement.

The other method is using pH test strips or drops. These are inexpensive and easy to use but are much less accurate, which I’ve found can cause minor issues especially in certain types of continuous-feed hydro systems. That being said, I did use them for the first two years of my growing career with great results. You don’t need to spend lots of money on a pH tester to be successful.

Why do we pH our water?

A marijuana plant needs many different macro and micronutrients throughout its lifecycle, meaning lots of some and very little of others. Some soils have many of these nutrients but, because of the high rate that marijuana grows, often these nutrients are gone from the soil in 1-2 months. In a hydro or soilless system, you must provide all of these nutrients to the plants yourself through your nutrient routine.

The pH of the nutrient solution you give your plants essentially makes certain nutrients “available” (basically, with an incorrect pH, the plant refuses to absorb these nutrients even if the nutrients are present in the grow medium or nutrient solution). With the correct range of pH, depending on your grow medium, this will allow the plant to take in all of the nutrients that it needs at the different stages of its life.

If your pH gets outside the normal range in a hydro or soilless system, you get what is known as “nutrient lockout,” where essentially the nutrients are provided to the plant, but the plant won’t drink them. This causes deficiencies within the plant.

As the pH of a material changes, the nutrients will actually take on slightly different forms on a chemical level, and these different forms aren’t what the plant wants/needs. By keeping our pH within range, all of our nutrients will stay in whatever form it is the plant needs.

What is the correct pH for growing weed in soil?

As a general rule for soil, you want to run between a 6.0 and a 7.0 pH. This will vary a little bit depending on the soil you are using. Soils that are good for growing marijuana (such as fox farm, happy frog, roots etc.) will let you use between a 6 and a 7 except in extreme cases.

If you are using cheaper soils designed for other plants like household potting plants, you will have to monitor your plants closely to make sure you catch potential pH imbalances early. You don’t want it to cause any nutrient deficiencies!

As I mentioned, these soils often have some kind of chemical time-release fertilizer that will alter the pH of your soil as the plant uses up the nutrients. This means that your soil could start at around a balanced 7 but change up or down through the life of your plant. Note that if you keep the pH of your water or nutrient solution constant through the whole grow, you will most likely see some problems before you are done with your grow.

There are soil pH testing kits on the market, but I’ve personally never had them be very helpful. I just closely monitor the plants, looking for any tiny signs of issues, so that I can correct the pH issues before they become a large problem. If determining the pH of your soil becomes important (such as you continuously have nutrient deficiencies and you think its the soil pH) you can measure the pH of the fluid before you give it to the plant and measure the pH of the run off to get an idea of where your soil is pH wise. We will be discussing this more in a later article.

What is the correct pH for growing marijuana in hydro or soilless systems?

The rule of thumb in these kinds of systems is you want to use a pH of between 5.5 and 6.5. Due to the fact that you don’t get any natural nutrition from the medium itself, the ideal pH shifts down in the scale to ensure that the plant has access to all nutrients.

When I have grown hydro, I use what is known as a continuous monitor pH sensor. This sensor stays immersed in your nutrient reservoir that is feeding your plants so you can instantly and continuously know what pH they are receiving. This is especially important in hydro systems because the pH will change as the plants absorb nutrients out of the solution.

The pH can change somewhat quickly depending on your ratio of plants to volume of nutrient solution. With less solution volume in your nutrient reservoir combined with many plants to feed, pH can change daily. This doesn’t mean that you have to correct this each day as I’ll discuss in the next section.

Does pH have to be exact all the time?

The answer to this question is no, as long as you keep it within the recommended ranges I gave you earlier in the article.

The pH can vary inside that range, especially if it varies around the middle of the range (soil 6.5 and hydro 6.0). 

This does not mean that, by letting your pH vary a little bit between feedings, you are going to have plant problems.

However pH issues can arise quickly and show up in the roots before you see it in the leaves of the plant. Obviously in soil and certain types of soilless systems, you can’t get a good look at the roots, but in most hydro systems you can see the roots easily.