Weed Grow Guide by Royal Queen Seeds

By Steven Voser Reviewed by: Gloria Payá

While it can be a daunting topic to try and wrap your head around, understanding soil pH is key to growing healthy cannabis plants. In this article, we’ll outline all you need to know about soil pH and how to get it right when growing weed.

Cannabis And Water Quality Part 3: Choosing A Water Source
Cannabis And Water Quality Part 3: Choosing A Water Source

What Is Soil pH?

pH is a measure of how acidic or alkaline something is. The pH scale ranges from 1–14, with a pH of 7 being neutral (the pH of pure water). If pH is lower than 7, a substance is considered acidic (think vinegar or lemon juice). If the pH is higher than 7, the substance is alkaline, as is the case with soaps, bleach, and ammonia.

In more scientific terms, pH level has to do with the concentration of hydrogen ions, say in the water you give to your plants. The pH scale is logarithmic to the base 10, which means that water with a pH of 6 is already 10x more acidic than water with a pH of 7.

Below is a basic chart of the different pH levels of common items:

1.0 - Battery acid
2.0 - Lemon juice and vinegar
3.0 - Orange juice and soda
4.0 - Tomato juice
5.0 - Black coffee and bananas 
6.0 - Urine and milk 
7.0 - Pure water, not tap or bottled water
(the pH running or bottled water can vary considerably)
8.0 - Seawater and eggs 
9.0 - Baking soda
10.0 - Milk of Magnesia and the water of the Great Salt Lake, Utah

Why Is pH Important When Growing Cannabis?

So, you now know what pH is. But how exactly does the pH of your growing medium affect the growth and health of your plants?

As you already know, all plants require nutrients for healthy growth. They require the macronutrients nitrogen, phosphorus, and potassium, as well as micronutrients and minerals including calcium, magnesium, iron, copper, and a whole lot more. If plants cannot access these nutrients, it will lead to deficiencies and other serious health problems.

The issue with cannabis plants is that they are only able to take up nutrients within a small pH window, which ranges from about 6–7 when growing in soil.

If the pH is lower or higher than that, the plant cannot take in nutrients, even if they are present—thus spurring nutrient deficiencies via "nutrient lockout".

In those places where cannabis thrives in the wild, the soil is normally slightly acidic; therefore, homegrown cannabis plants also prefer a slightly acidic environment. However, the way that you grow cannabis also plays a role in the optimal pH level for your plants. Cannabis grown hydroponically or without soil needs an even lower pH than a soil grow.

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The Benefits of Maintaining the Perfect pH

The benefits of caring for and maintaining your plants’ pH is pretty straightforward; you’ll have healthier plants that demonstrate more vigorous growth and, as a result, produce better harvests. Plus, you’ll also ensure that the time and money you’ve spent fertilising your plants is paying off.

By regularly checking the pH of your growing medium, you’ll be able to ensure that your plants are able to take up all the nutrients you’re giving them. You’ll also be able to catch any pH imbalances early, minimising your risk of running into nutrient deficiencies later on in your grow (more on that below).

The Problem With pH Imbalances

pH imbalances are one of the most common causes of nutrient deficiencies in cannabis plants. As we mentioned earlier, cannabis plants can only take up certain nutrients within a small pH window. If the pH of your medium shifts below or above that ideal window, your plants won’t be able to take up the nutrients in their fertilisers and will start to show signs of a nutrient deficiency.

Understanding and Preventing Nutrient Lockout

Nutrient lockout (sometimes referred to as “nutrient lock”) occurs when your cannabis plants can’t absorb nutrients from the soil or the fertilisers you’re using to feed them. One of the primary causes of nutrient lockout is pH imbalance, but it can also be caused by salt buildup near the root zone as a result of feeding with mineral fertilisers (which tend to have a high salt content).

The Best pH for Growing Cannabis

We’ve alluded to it already, but the best pH for growing cannabis resides within a narrow window. But, within that window, is there an optimal reading you should try to achieve? And, does that reading change depending on how you choose to grow? Let’s explore further.

  • Soil pH range: 6.0–7.0

If you grow in soil, the optimal pH level for the root zone is between 6.0 and 7.0. However, there is no set number within this range that is “best”. Instead, it can be good to allow for some natural fluctuation within this window to support optimal nutrient uptake.

So, as you adjust, try a slightly different reading each time. You can, for example, adjust your pH to 6.2 for one watering, then 6.6 the next. As long as it stays within 6.0–7.0, you should be fine. Soil is also more forgiving when it comes to pH imbalances, but it can only give so much.

If you grow purely organically—where you do not administer mineral nutrients—pH is less of an issue. If you’re using amended or composted soil with organic matter, the microorganisms within will make the nutrients more available to the roots. However, most growers using standard potting mixes and mineral nutrients will indeed have to reckon with pH.

The Best pH for Growing Cannabis
  • Go Organic and Forget About Measuring pH

At RQS, we’re big proponents of organic cannabis growing; not only because it delivers a far superior product, but because it takes some of the hassle out of growing, especially when it comes to pH.

While using chemical nutrients might seem simple, it can actually take some time and practice to get the hang of properly fertilising your cannabis plants with liquid mineral fertilisers. Organic nutrients, on the other hand, naturally promote the health of your plants by supporting the development of healthy microbial life within your medium.

Using natural fertilisers like compost, worm castings, and bone meal creates a breeding ground for healthy bacteria and fungi that keep the conditions of your soil optimal, so there’s usually no need to monitor the pH of your soil as closely as you would otherwise.

  • Hydroponics and Soilless pH range: 5.5–6.5

Hydro and soilless grows are a different beast when it comes to pH. If you grow soilless, say in coco, the optimal pH level at the root zone should be somewhat lower than in soil, between 5.5–6.5. The same goes for all methods of hydro.

With these methods, it is just as important that you allow the pH level to fluctuate across the acceptable range to support nutrient uptake. For example, in hydro, calcium and magnesium are mostly absorbed at pH levels above 6, while other nutrients like manganese prefer a slightly lower pH.

Then again, this shouldn't be an issue since pH levels will naturally fluctuate slightly with each feeding in a hydroponic setup. You will only need to correct if the pH level exits the optimal 5.5–6.5 range.

When growing in coco, perlite, or hydroponically, you are in charge of administering nutrients directly to the root zone via the water, which means huge pH fluctuations are more of a risk than in soil. The inert media used in hydro and soilless grows merely retains water and provides support for the roots. So, when administering nutrients, be careful that you don’t overload your plants.

The Best pH for Growing Cannabis

How to Test Cannabis Soil pH

Managing pH level means testing the water or nutrient solution and adjusting it accordingly. This may sound complicated, but it really isn’t.

To test pH, you can use a digital pH meter or a pH measurement kit with drops. Opinions here differ as to which method is “best”. Some prefer digital pH meters because they are accurate and easy to read, while others like the drops as they are super simple and don’t require calibration. Try them both and see which you prefer.

Cannabis pH — FAQs

Let’s break down some of the most common questions related to cannabis pH. Feel free to refer to these if you encounter any pH-related issues during your grow.

Questions & Answers: Ph

? Do I Test the pH of My Fertiliser Before or After Adding My Nutrients?
Always measure the pH after you add any nutrients or amendments as they will change the pH value of your water. After you mix your nutrient solution, use a pH meter or drops to test its pH level.

If you are growing hydroponically, test a sample from your water reservoir a few minutes after you add your nutrients.
? Do I Need to Measure the pH of My Runoff After Feeding My Plants?
Yes. Always remember to test the pH of your nutrient runoff as this will give you an idea of the pH of your medium.
? How Exact Do I Need to Get My pH Levels When Growing Cannabis?
Don’t get flustered if your nutrients are slightly below or above the optimal conditions we mentioned above. Only react to big changes in pH that may inhibit your plant’s ability to uptake nutrients.

Measuring pH With Drops

pH measuring kits usually contain a test tube, a bottle of testing solution, and a colour-coded pH chart. Testing the pH of your soil with these kits is super simple:

1. Prepare your fertiliser as per usual and stir it gently. Be careful not to over oxygenate your fertiliser as this may throw off your pH reading.

2. Half-fill your test tube with your fertiliser and add 3 drops of testing liquid into it.

Understanding pH and How It Affects Cannabis Plants

3. Gently shake the test tube to mix the pH testing solution with your fertiliser.

4. Use the colour chart to read the pH of your fertiliser and, if needed, use pH up/down products to adjust it.

5. Repeat this process with the runoff from your fertiliser. If the pH reading from your runoff is far below or above that of your fertiliser and in the danger zone (below 5 or above 7), you may need to regulate the pH of your soil.

Understanding pH and How It Affects Cannabis Plants

Measuring pH With a Digital pH Meter

Measuring pH with a digital pH meter (like our pH tester) couldn’t be any more simple. After you’ve calibrated your device, simply stick it into your fertiliser, runoff, and soil to get an accurate reading of the pH in your garden.

How to Adjust pH When Growing Cannabis

If the pH of your fertiliser, soil, or water is far out of the optimal range, you’ll need to adjust it quickly. This can be done using “pH up” and “pH down” products, which you can obtain at any grow shop. If your pH is too high, you will of course need to add some pH down, and vice versa.

  • Using pH Down

Given the alkalinity of most tap water, you’ll usually need to lower your pH rather than raise it on most occasions. To do this, add small amounts of pH down solution (there are many different brands on the market) either to your water or nutrient solution, stir, and test the pH again. Repeat the process until your pH levels are right.

  • Using pH Up

The process of raising the pH of your water or nutrient solution is the same as lowering it. Simply add small amounts of pH up to your nutes or water until your pH readings are on par.

  • Alternative Ways to Lower or Raise pH When Growing Cannabis

Besides using pH adjusters, you can also lower or raise the pH of your soil using a variety of natural products. Manure, compost, worm castings, compost teas, pine needles, and wood shavings all help gently lower the pH of your soil over time.

And, while these products tend to take longer to take effect, they also help to establish healthy microbial life around your plant’s roots, which helps promote the healthy growth of your plants and keeps them protected from pathogens and pests.

Lemon juice and vinegar are also very effective at driving down pH. Just remember to dilute either in water to avoid driving down the pH too drastically and/or damaging your plant's roots.

The most common product used to raise the pH of soil is lime or limestone, which can come in powder, pellet, or hydrated form. Alternatively, we recommend using wood ash, which helps to raise pH more gradually and also contributes potassium, calcium, and other micronutrients to your soil.

How to Adjust pH When Growing Cannabis

How to Correct pH for Cannabis (In Soil and Hydroponics)

  1. Always test the pH of your soil or reservoir before making any adjustments to it.
  2. If your pH is too high (too alkaline), we recommend adding small amounts of lemon juice or vinegar into your water when watering your plants (when growing in soil). For the best results, dilute the juice or vinegar in at least 5–10 parts water, apply it gradually, and continue testing your pH until you get a level you're happy with. If you’re using hydroponics, we recommend adding small amounts of pH down (1–2ml) to your reservoir at a time until you get the right pH reading.
  3. If your pH is too low (too acidic), we recommend using a liquid dolomite lime product to raise the pH of your soil. If you need a quick fix, you can also try raising your pH by adding baking soda to your water, but most growers find the results to be short-lived and end up turning to dolomite lime for more stable results. In hydroponics, we recommend using a natural pH up solution to regulate your pH levels.
Understanding pH and How It Affects Cannabis Plants

Cannabis and pH — The Takeaway

Monitoring and regulating the pH of your soil or reservoir is one of the keys to growing healthy cannabis plants and producing a great harvest. If you’ve got your own tips for regulating the pH in your cannabis garden, make sure to share them in the comments below!

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