The ideal pH value for growing weed in soil is 6.3. But why is the pH actually important? What does this vital information mean when growing weed? Is the best acidity level also different when growing in something other than soil, such as coco or rock wool? This article contains all the answers to these questions.


What is pH?

The pH value is another word for the degree of acidity. How acidic a liquid such as your growing medium, plant food or water is, is determined by the amount of dissolved hydrogen ions. Fortunately, you don't need to remember this to become good at growing weed. What is important to know is the pH scale. The pH scale runs from 0 to 14; where 0 is acid and 14 is alkaline. Base or alkaline means something like chalky.

Lemons taste sour because they contain citric acid (you can't make it up). Citric acid has a pH of 2.2, which is very acidic. At the other end of the scale you have, for example, caustic soda which is used as a strong cleaning agent. Because this substance has a pH of 12, special equipment and safety standards are required to handle caustic soda. After all, such an alkaline liquid can be just as dangerous as caustic acid!

Also, pH neutral is a term that is good to remember. Rainwater, tap water and spring water contain minerals which influence the pH. Water that contains nothing at all (demineralized water) has a pH of 7.

Why is the pH value important when growing weed?

The acidity level influences and limits how easily (weed) plants can absorb nutrients. After all, plants do not only need water and light to grow, but also minerals. Potting soil or garden soil also has a natural pH value. This is close to the value that the weed plant feels comfortable with, so that these minerals can also be absorbed. Weed plants need nutrients that can only be absorbed at a pH between 5.5 and 6.5.

The right pH value is important for your plants, because they need to absorb nutrients.

If the pH varies too much when growing weed, your plants will show all sorts of strange colours and the leaves will slowly but surely die. There are all sorts of symptoms of nutrient deficiencies, but also excesses. But the foundation of plant nutrition is the correct pH value.

What is the best pH value when growing Weed?

The best pH value depends on what the weed plant is growing in; the growing medium:

  • Growing in soil: pH-value of 6.3
  • Growing in stone wool: pH value of 6.0
  • Growing hydro or coconut: pH value of 5.8

The better the medium can retain moisture, the higher the pH can be. Good garden soil holds water well, and stone wool slightly less. Coconut does not hold moisture well. Hydro or clay granules do, but because they are so large, the roots cannot spread them out properly.

There is another rule of thumb. The better the medium retains moisture, the broader the pH will be. That is why it is much easier to grow in soil than in a hydroponic system where you do not use soil. So in most cases you can grow weed in soil without worrying about pH. But if you see leaf problems such as brown spots or yellow dots, then that is a very good indication that the acidity is not right. With hydroponics, the correct pH level of the water is extremely important. Take a look at the chart below.


Measuring the pH value with a pH meter

Now that you know that the pH value is important for growing weed, you naturally want to know how you can change it. To begin with, you measure the medium or nutrient solution of your weed plant. Nutrient water is the word we use to indicate that you dissolve the liquid nutrient in the water and give the mixture. In this way you can take care of the pH!

Tap water has a pH of around 7. Do you water from the rain barrel? Rainwater is slightly more acidic and has a pH of between 6 and 7. Rainwater therefore needs less frequent and extreme downward adjustment. But since you now know that weed plants do better at a lower pH than the water naturally has, you might make the mental leap of thinking that you do not need to adjust the pH upwards.

Measurement is knowledge. Therefore, make sure you measure the pH of the water before you give it to your plant.

Plant nutrition can influence the pH to some extent, but there are also plant nutrition products that are precisely tuned to the desired pH, so that you don't have to worry about acidity. That kind of nutrition also costs a few euros and because there are more factors that influence the pH, we heartily recommend that you still measure it once in a while. Especially if your plant gives a sick impression.

One of the factors that influences the pH measurement is temperature. The measurement is a lot more accurate if the temperature is constant. A pH meter often has no built-in thermometer to take this into account, but advanced microprocessor pH meters do. Thanks to the automatic compensation, measuring and tracking the pH is a piece of cake. Measure your nutrient with the spout into the water and check if you need to adjust the pH. Do you purchase such a pH meter? Consider a storage fluid and cleaning fluid. They keep your equipment in top condition and ensure that the measurements are always reliable.


The Adwa AD12 is a micro processor pH and thermometer build in one.

Adjusting the pH value

We have already discussed this briefly: you practically always have to adjust the pH downwards. You do this with pH-min from Plagron. The purpose of this acidic additive is to lower the pH value of the nutrient water so that your weed plant can enjoy all that nutrition that you give it. You only need a few drops of pH-min to lower the pH value. Use a pipette to dose properly and then follow the instructions on the bottle and on the pH-min product page. Tip: First add plant food to the water before adjusting the pH value. Plant food can cause the pH value to drop even further. 

Would you rather have no hassle at all with pH value and pH meters? Then take a look at the growth and bloom nutrients from Advanced Nutrients with pH Perfect technology. Both Sensi Grow and Sensi Bloom bring the pH value of the nutrient water to the correct value within a few seconds. Without having to do anything yourself.

This is as hard as it gets to tackle the subject of the pH value of weed plants, but as you have read, it is of vital importance to your weed plant! Want to know more about specific applications when growing in soil, coconut or other hydro systems? Then check out these articles: