How to Grow Marijuana – Lesson 6 – Watering Soil Grown Plants

Water Splash

We will get more in-depth and specific with nutrients and certain problems later on, but for now, we are going to talk about one of the most important factors for a successful harvest: water.

The Building Block of Life

Periodic Table

Obviously, you can’t really build something out of the water unless you freeze it, but living things require water. Humans use it to keep their cells hydrated, which allows normal bodily functions to occur, like production of blood cells, metabolizing of food through the stomach and intestines, and filtering of toxins through the liver and kidneys.
Plants take water from the growing medium (in this case, soil) through their roots. Dissolved in the water that the roots take in is the elements that it will need to properly grow and sustain itself, that cannot be taken in through the leaves:

  • Macronutrients (major elements that make up most of the plant’s structure):
    • (N) nitrogen
    • (P) phosphorus
    • (K) potassium
    • (Ca) calcium
    • (S) sulphur
    • (Mg) magnesium
  • Micronutrients (trace elements measured in parts per million, or ppm):
    • (Fe) iron
    • (B) boron
    • (Cl) chlorine
    • (Mn) manganese
    • (Zn) zinc
    • (Cu) copper
    • (Mo) molybdenum
    • (Ni) nickel

There are others, but they are seldom required by any strain of cannabis.

Plants also use water during photosynthesis, where the water is drawn through the stem up to the leaves and exits through stomata (openings in the leaves that close at nighttime), where it is replaced by air. From the air, the plants obtain the macronutrients, (O) oxygen, (C) carbon (from carbon dioxide) and (H) hydrogen.
So, as you can see, plants need water to get every element they need to grow.

The Importance of Pure

Pure Water

Water can dissolve a huge number of things, including heavy metals, pollution and excess chlorine. When this happens, small particles of what was dissolved are suspended in the water itself and get carried along with it. That includes when it is taken in by plant roots.

Most impurities are either directly harmful or add too much of a good thing for your plants (too much chlorine or another element).

The impurities that water can contain will cause problems, such as stunted growth, nutrient deficiency or toxicity. Not only do they add harmful elements to the water, but they leave less room for beneficial elements to dissolve and reach the plant.

Even with soil, you want your plant to get only what you intend to give it, in order to get the absolute best harvest with the highest yield.

To achieve the best possible water quality, you can use tap water filtered with the reverse osmosis filter in our recommended list, found here: What You Need to Grow Marijuana. We will keep the list updated with the highest quality, best value products available, as we find them.

To Feed or Not To Feed

If you have followed our tutorial up to this point, are using the “Super Soil” method that we recommend, and this is your first crop, then you can use the pure water that you have filtered for your plants.

If you are using just soil, or you are onto your second (or more) crop, then you will need to supplement with nutrients. We have recommended nutrients for soil growing in the list mentioned above. If you are feeding, follow the directions on the package, but start at a quarter of the recommended amount. Less is better until you get to know your plants. Every strain likes a different mix. Slowly increase the amount every time you feed until you see the tiniest amount of “nute burn” on the leaves. Look for a tiny amount of browning near the tips of the leaves, or possibly the leaves to bend downward at the end.

Immediately record the amount of nutrient used in the last feeding and put it with your water and nutrients. That is the amount that will kill your plants.

On the next watering (there is nothing you can do until then), use pure water, then on the next feeding, give slightly less than the last SAFE amount.


It might also be helpful to keep a feeding journal. Record how much of each nutrient you added to each watering, and keep track of any changes you make. Then record the results. If the plant shows signs of stress, you know it wasn’t a good change and you know how to change back.

How Often?

Marijuana has adapted to grow perfectly in subtropic to tropic climates, making it somewhat resistant to drought. You do NOT want to over water. Overwatering will reduce the amount of oxygen making it to the roots (bad), and make an ideal environment for mould or non-beneficial fungus (also bad).

Let the soil dry out to about an inch down before you think of watering.

Do NOT saturate the soil.

If using nutrients, only feed every second or third watering.

pH in Soil

Just about everything has a pH level. pH is the measurement of the acidity or alkalinity of something (usually liquid). The scale ranges from 0 to 14.

The measurement is the number of hydrogen ions, when (usually) water (H2O) breaks down into OH (hydroxide ions) and H3O (hydronium ions, simplified as hydrogen ions).

Pure water, which you are hopefully starting with using the filter recommended in What You Need To Grow Marijuana, is pH 7.0, exactly in the middle of the scale. It has neither more hydroxide nor more hydrogen ions. It is balanced.

Battery acid is a pH 0 and consists of only hydrogen ions, where liquid drain cleaner (a base) is pH 14, consisting of only hydroxide ions.

The scale is logarithmic, ranging from 10,000,000 hydrogen ions compared to pure water, to 0.00000001, each number on the scale representing a decimal place.

The reason all this is important is that every living thing has an ideal pH for its well-being and ability to thrive on a cellular level. Humans are ultimately perfect at 7 (as always, we base our findings on ourselves), the body tissues being around 7.2 and saliva or blood being around 6.8.

Different plants like different pH levels, depending on where they have adapted to live. Cannabis tends to grow best at 6.5, or slightly acidic when grown in soil. This means you want to keep your soil between 6 and 7 to get the most out of your plants.

If your pH is not in the proper range (6.0-7.0 in soil), your plants will not be able to use the nutrients you give it, which can lead to overfeeding instead of fixing the pH. Always test!

When your pH is perfect for your plants, and your feeding schedule is at the right amounts and times, your plants will give you the best harvest.

Take your time and get to know your plants. It pays off!

Watering is much simpler than most belief. Now, let’s move on to Lesson 7 – Training Your Plants Part 1