How to water your Marijuana plants

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Nutrients, pH, Elecro-conductivity and more

Scared of over watering or under-watering you plants? Can’t figure out what ingredients to give them? In this guide we’re going to go over the best ways to water your plants at the different stages. Here’s what we’re going to cover:

How much water to give your plants

The amount of water to give to your plan depends on how much soil you have. In general you give about a quarter of the volume of soil you have. Here are some of the most common sizes used:

  • For a 1L pot give 0,25L of pH’d water
  • For a 4L pot give 1L of pH’d water
  • For a 12L pot give 3L of pH’d water
  • For a 30L pot give 7,5L of pH’d water
  • And so on

Your should see about 20% run off in your tray, that water will be sucked back up in the next hours.

What is the optimal pH for the marijuana roots

The short answer is between 6 and 7. The sweat zone being around 6.5. You wouldn’t think so but the pH of your water is actually one of the most important things. Marijuana can only take in certains elements at certains pH’s, this is why you really have to be careful, and a trustworthy pH tester is your closest friend.

Here’s a graphique showing the important nutrients and what pH level the roots will be able to take them in at

The 12 main nutrients necessary for marijuana and their pH level availability
Optimal pH range per nutrient

It’s important to note that if you decide to go into the lower levels of humidity you’ll need to keep them well hydrated, or else this will be lived as a huge stress.

In any case you go for, as with the temperature you should aim for steady relative humidity levels.
If you decide to reduce or increase the humidity levels you should do so gradually

Elements and the root pH needed for cannabis intake – Recap

 Minimum pHMaximum pH
Nitrogen (N)5.57.5+
Phosphorus (P)6.27.2
Potassium (K)5.47.5+
Ca (Calcium)6.57.5+
Magnesium (Mg)5.87.2
Sulfur (S)5.47.5+
Copper (Cu)– 5.07.1
Iron (Fe)– 5.07.1
Manganese (Mn)– 5.07.5+
Molybdenum (Mo)5.87.5+
Zinc (Zn)– 5.07.1

You really just need to focus on the 3 first nutrients (N,P,K), but by your pH between 6 and 7 you’re sure that she can take in all the elements needed.

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Importance and Measuring Electro-Conductivity (EC)

Electro-Conductivity is a measure that allows you to get the levels of salts (nutrients) within your watering solution and your soil. 

As a grower this can be super useful to make sure you have enough nutrients within your soil, that your plant can intake them, that you haven’t saturated your soil (and need to flush it)

You can measure the EC levels with an EC pen, which is very similar to a pH pen. 

EC levels dropping thanks to flushing
Measuring EC levels of runoff

Best Electro-Conductivity levels

Optimal EC levels will depend on each strain, as they all have different needs, so you will always need to adjust and learn depending on each strain. That said, Royal Queen Seeds has a pretty nice table laying out an average EC level depending on your stage of development. Below you can find the table but remember, you may have to adapt this depending on your strain.
PPM EC (mS/cm2) PPM EC (mS/cm2)
Early Growth 350 – 400 ppm 0,7 – 0,8 400 – 500 ppm 0,8 – 1
Seedling 400 – 500 ppm 1 – 1,2 500 – 600 ppm 1 – 1,3
Transition 550 – 650 ppm 1,3 – 1,5 600 – 750 ppm 1,2 – 1,5
Vegetative Stage 650 – 750 ppm 1,6 – 1,7 800 – 850 ppm 1,6 – 1,7
Vegetative Stage 750 – 800 ppm 1,7 – 1,8 850 – 900 ppm 1,7 – 1,8
Vegetative Stage 850 – 900 ppm 1,8 – 1,9 900 – 950 ppm 1,8 – 1,9
Flowering Stage 900 – 950 ppm 1,9 – 2 950 – 1000 ppm 1,9 – 2
Flowering Stage 950 – 1050 ppm 2 – 2,2 1000 – 1050 ppm 2 – 2,1
Flowering Stage 1050 – 1100 ppm 2,2 – 2,3 1050 – 1100 ppm 2,1 – 2,2
Flowering Stage 1100 – 1150 ppm 2,3 – 2,4 1100 – 1150 ppm 2,2 – 2,3
Flushing 0 – 400 ppm 0 – 0,8 0 – 400 ppm 0 – 0,8

What nutrients during the Vegetative stage

So, what nutrient does your plant need during her vegetative stage. The amount per Liter really depends on her size, but here’s what I’ve been doing

  • Root booster: I take the minimum given on the bottle. At the size of the current one’s (available here) I’ve been giving them 5ml for every 2L of water.
  • Nitrogen: During the vegetative stage it’s important to get as much nitrogen to your plant as possible. As soon as you start the stretch she’ll reduce her intake of Nitrogen little by little, until stopping complete. That said, I like to wait a at least until the third or fourth watering before giving them any nitrogen, by fear of nutrient burn (really got marked by that during the Big Buddha Cheese growth ^^)There are some good grow mixes available out there that you can find. The important is to stick to the same brand, since their elements are generally developed to work together.

That’s really all you need to give her during the vegetative stage.

What nutrients during the Flowering stage

During the flowering stage your marijuana plant hardly takes in anymore Nitrogen, but her intake of Phosphorus (P) and Potassium (K), thus your mix should adapt with it. You can also stop giving it root booster at the end of the stretch, they won’t be growing much more from now on. So to recap, you’re going to need to:

  • Reduce gradually the Nitrogen
  • Increase Phosphorus and Potassium levels to your mix
  • Stop root booster

At the start of the flowering stage I was giving about 5ml of Flowering mix for every 2L, but I was going pretty low, new turn I’m going to increase that level a little bit

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 Alright folks that’s all for this one!

Until next time, be safe and grow easy 😉

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