Growing marijuana already has its challenges and when you grow hydroponically, it doesn’t get any easier. The important thing to remember is that all plant life needs a grow medium with the correct acidity, proper nutrients, and water. These three fundamentals work hand-in-hand to help your plants grow.

The acidity (pH) of the growing medium determines the quality of the bacterial life as well as the metabolic rate of the marijuana plant. The bacteria are necessary because they help convert fertilizers into absorbable substances. In addition to maintaining a healthy environment for bacteria, your plant also needs to absorb nutrients from its water. This ability is measured by its level of dissolved salts (EC).

When you aren’t growing hydroponically, you can purchase premade growing mediums that guarantee the proper pH and EC. In a hydroponic setup, however, that environment must be created and carefully maintained. This guide will teach you the best PH and EC values for marijuana plants and provide guidance on how to maintain them.

Let’s Start with the Nutrients

Let’s Start with the Nutrients
Let’s Start with the Nutrients – Image powered by Royalqueenseeds.com

PH and EC levels are all related to your plant’s health; specifically, it’s ability to absorb nutrients. When growing hydroponically, you need to pay careful attention to both values; however, you must still think about the nutrients you are providing as well.

The best nutrients for a hydroponic setup will have no organic matter and are explicitly made for hydroponics. They should include chelated mineral nutrients, as they are easier to absorb by most plants. They should also contain significant amounts of micro-nutrients since your plants won’t be absorbing these things from the soil.

You’ll want to avoid ingredients such as:

  • Guano
  • Worm Castings
  • Fish Emulsion
  • Blood Meal

Not only are these organic materials messy, but they can also introduce bacteria that your setup isn’t prepared to fight against. Plus, your ecosystem likely doesn’t have the necessary microorganisms to break down organic matter into useful nutrients. If your choice of nutrients causes your water to look cloudy, you do not want to use it.

Your nutrients should also have the correct NPK ratios. NPK stands for NitrogenPhosphorusPotassium, and they are the three most essential nutrients for plants. The ideal NPK ratio for cannabis plants is 7-5-5 during the vegetative stage and 3-10-10 during the flowering stage; however, the requirements are slightly different when growing hydroponically. Because soil naturally contains more Nitrogen than water, and Phosphorus is easier to absorb in water as compared to soil, hydroponic nutrients tend to have more nitrogen and less phosphorus than ordinary marijuana nutrients.

Nutrients for hydroponic growing should also include:

  • Sulfur
  • Magnesium
  • Iron
  • Calcium

You can also have Boron, Copper, Manganese, Cobalt, Molybdenum, and Zinc, especially if you have soft water. These micronutrients are often found in trace amounts in soil and may need to be supplemented for plants grown hydroponically.

The pH of Hydro Growing

pH is the rate of positively and negatively charged water ions. It measures absorption within a solution. Acidity represents the number of positively charged ions. If the negatively and positively charged molecules are at a balanced ratio, then the environment is considered neutral. Depending on what materials are added to a solution, the acidity level may become more alkaline or acidic. How well positively charged ions absorb into a solution determines the pH level (Wikipedia).

No matter which nutrient brand you’re using, even with chelated nutrients, cannabis roots are better able to absorb some chemical forms of nutrients better than others. This is where pH comes into play. When the pH at the roots is too high or too low, it actually changes the chemical form of the individual nutrient compounds, making it more difficult for your plant to absorb the nutrients it needs.

The pH scale goes from 0 (acid) to 14 (alkaline). For a solution to be neutral, which means it is non-alkaline and non-acidic, it needs to be at pH level 7. This is a mathematical scale with a consideration of 10. For every point that the pH level shifts, the solution either increases in acidity or becomes more alkaline. When you have a pH level of 7, a liquid that has a pH level of 6 is literally 10 times higher in acidity. As for a pH 5, it would be one hundred times higher in acidity.

When growing hydroponically, the ideal pH is around 5.5 to 5.8. However, 5.3 to 6.5 is good, and 5.0 to 7.0 is acceptable. If you start to see numbers under 4.5 or higher than 8.0 you will begin to have nutrient absorption problems. If the pH reaches 3.5 or 9.0, there’s likely permanent damage to your roots.

The pH of Hydro Growing

Creating the Ideal pH

Your marijuana plants need a specific pH to thrive. When creating this ideal pH, be very cautious when blending acids and salts. What you assume is a small change can easily become a lot quite fast. Here are some ways to safely bring the pH back to normal levels.

pH too low. If the pH level is 5 or beneath that, then it is clear that certain elements, especially aluminum are shifting to the point of where they are becoming toxic to your plant. You can stop this from happening by making sure the nutrient liquids stay at a pH of 5.5 or higher. Always take a measurement of the pH of your nutrient water and make the necessary changes before feeding it to your plant. One way to raise the pH is by mixing caustic potash into the water, which you can find at a local garden center.

pH too high. When you have a pH of 6.5 or higher, the calcium, iron and phosphorus ions are constrained to each other. Even though these elements are existing in the solution, they cannot be absorbed by your plant, leading to a severe loss of growth. To fix this, try adding diluted nitric acid to the nutrient water during the growth stage. Once it reaches the flowering stage add diluted phosphoric acid. You can also find these in your local garden center labeled pH- growth and pH- flowering.

After adjusting your pH, always measure your nutrient water to ensure it is right. You may find that the pH levels may continue to fluctuate, especially if you use Rockwool slabs, coco, or mapito as your substrate. These (and other hydro growing mediums) tend to hold onto excess salts and acids. Rinsing your hydro growing medium can help maintain consistent pH levels and improve the health of your plants.

Sometimes cannabis plants need trace elements when water has a high pH level; however, it is important to remember that something that is good for one part of your plant may not necessarily be good for another. For example, the needs of trace elements in the roots differ, so you must figure out a pH that works for every part. You are better off choosing water and nutrient liquids that are somewhat acidic with a pH around 5.5 – 6.5 and adjust the levels by half a point as needed.

Maintaining pH

Rinsing your hydro growing medium from time to time will ensure that you maintain healthy roots. This is critical because if the pH becomes too low, the roots can be damaged. Even worse, in Rockwool specifically, if the pH drops down as low as 4.5, the Rockwool itself can be chemically affected. At this level, the Rockwool will release alkaline substances with a pH value somewhere between 7 and 14. This can be far too strong and will almost certainly burn the roots of your plant. If the roots are burned, your plant won’t be able to absorb the nutrients it needs, and it won’t be able to grow.

It’s also possible for the pH to be too high in your Rockwool slab. Tap water can cause this, as it usually has a pH somewhere between 7-8, depending on your location. Purchasing Rockwool that is specifically designed for marijuana such as these from Cultilene, can help prevent this problem, but regularly testing and rinsing can also help.

Hydro EC and TDS values

EC stands for Electric Conductivity. It is a measurement of the amounts of dissolved salts in a solution. In terms of plant nutrition, the EC represents just how healthy a nutrient will be. The amount of dissolved nutrients can also be measured by TDS, which stands for total dissolved solids.

EC values are difficult to determine because they can be impacted by many variables. The size of your plant, the amount of water, the lighting, and even the amount of time that you drip can impact EC. You’ll also need to consider the EC values of the water (especially if it is tap water) as well as the humidity in the environment.

Hydro EC and TDS values

Measuring EC

Before you start thinking about creating the correct EC value, you’ll need to understand how to measure it. An EC meter can measure the electrical conductivity of water, providing a useful measurement of the amount of salts present; however, many marijuana growers use a tool called a TDS meter, which measures a thing called PPM.

PPM stands for Parts Per Million; specifically, the concentration of minerals in the water you are giving your plants. The purest water (reverse osmosis or distilled) will have 0ppm, whereas ordinary water will have between 200ppm and 400ppm. PPM is not EC, but it is still a useful measurement, and it is often measured alongside EC on many TDS meters. When using a TDS meter, choose one that measures both TDS and EC.

Creating the Ideal EC

Finding the best EC for your plants will take a little trial-and-error. Some growers prefer unusually high nutrient solutions while others do better with a weaker, more diluted solution. The best fertilizers for cannabis plants tend to have an EC value between 1.2 and 1.5 mS (600 to 750 TDS), but you can see as much as 3.0mS (1500 TDS) for more mature plants. In terms of altering the EC, it only gets complicated when you have hard or soft water.

The scenario may look like this:

  • Grower 1 has tap water at an EC of 0.5 (250 TDS). They prefer an EC of 1.8 (900 TDS). They add a fertilizer that is 1.3 (650 TDS).
  • Grower 2 has hard water with an EC of 0.8 (400TDS). They also prefer an EC of 1.8 (900 TDS). They add a fertilizer with 1.0 EC (500 TDS).

While it looks like both growers would end up with the same results, Grower 2 actually ends up with 0.3 EC (150 TDS) fewer than Grower 1 due to the acidity of the water. For Grower 2 to have the same result as Grower 1, they would need to first change their EC to 2.1 because their water will dissolve 150 TDS fewer than the water used by Grower 1.

Because acidity levels can impact EC, it is critical to maintaining the pH levels in your setup. If salts cannot be absorbed, their concentration will be impacted. For this reason, rinsing your hydro growing medium can also help maintain proper EC for your plants. Monitor your plants by evaluating the EC and pH of the water that comes out of the slabs every two days. Use a syringe to get a sample of water from the slabs on each 5th and 6th day.

Your EC value will also be influenced by the number of drip feedings. So, for instance, having a high solution that is at 2.7 EC, spread over a period of 500 ml per plant is not as much of an issue as drip feedings of 150 ml with the exact solution, six times each day. Using the last situation, the salts end up releasing from the solution causing the EC to go up sky high.

Summary: the more you drip, the stronger your nutrients will be.

Should You Use A Reservoir or a Drip System?

Now that you understand how pH and EC impact your hydroponic garden, you may be wondering which mechanism works best for your plants. Truth is both systems work just fine.

Use A Reservoir or a Drip System


In a reservoir system, you use hydro grow medium, (such as horticultural clay pellets or pea-sized volcanic lava) in plastic pots. These pots are put in a tray containing a water-nutrient solution that is about one-quarter of the container’s height. A 10-inch high container will have about one inch of soaking grow medium. A large portion of the medium stays above the waterline. The medium remains moist as long as water stays in the reservoir, since capillary action keeps drawing the water up.

With this setup, as soon as the water-nutrient solution is added to the tray, plain, unenriched water needs to be added to keep the water level and strength of the nutrient solution at equilibrium. As water evaporates, the concentration of nutrients rises. Adding water will lower it.

Many experts say roots should not be sitting in water. There are two reasons why this is occasionally true. First, the sitting water might not have dissolved oxygen. The roots utilize oxygen and will incur diseases without it. Second, some mediums are incredibly dense and take up so much water that there isn’t any air left. The roots then suffer from a lack of oxygen. That is why, in a reservoir system, large air spaces are left in between the growing medium so that there is enough air and the roots can access the oxygen more readily.

Reservoir systems can also infuse gas into the water using an air stone. The small device attaches to an air pump to create a column of rising bubbles. These pumps are safe to use as they do not produce much heat. (To stay safe, measure the temperature of your water to ensure it is under 72 degrees Fahrenheit.) The bubbles that an air stone produces moves the water around, allowing more of the surface area to absorb oxygen while releasing CO2. If the roots in a reservoir setup are extremely dense, they may not be getting enough oxygen and may benefit from an air stone.

Drip System

The never-ending drip system also uses hydro growing mediums in containers that sit in a tray of water that drains into a reservoir below. However, a pump also supplies a constant gentle stream of water through the tubing to each container so that fresh water is always trickling over the stones. This system is rather easy, while still promoting vigorous growth. Keeping the drain hole blocked with the tubing that raises the drain to the desired height makes for an easy combo constant drip/reservoir system. By doing this, marijuana plants receive a constant stream of water, and the roots have the space that they need in the reservoir.

Rinsing Your Hydro Growing Medium

Rinsing Your Hydro Growing Medium

Rinsing your hydro growing medium is essential for maintaining proper pH and EC levels. To do it, you’ll need a pH meter, a TDS meter, a syringe and a measuring cup. Use the same syringe and measuring cup that you’d use to water your plants. All of these supplies can be purchased at any gardening shop.

Rinsing Hydro Growing Medium from Robert Bergman (ILGM) on Vimeo.

First, you need to take some water from various points in your growing medium. Measure its pH and TDS. This isn’t a foolproof method, but it’s a good first step that will often highlight any issues you are having. You should notice right away if any of the measurements are way off.

Measure each syringe of nutrient solution separately, then mix it all together to see what the totals are. This will give you a good generalidea of how nutrients are flowing through the root systems. If you do notice any issues, just adjust the ratio of nutrients you’re adding into the nutrient solution. Add plenty of water to the medium. This keeps the plant hydrated and flushes out some of the dissolved solids that may have collected.

A major downside to this technique is how time-consuming it is, especially if you have a large growing operation. Another method is to simply choose a few slabs as your ‘reference’ and monitor these specific slabs over time. It will help you keep track of the general pH and TDS of all your plants, assuming you are feeding them and watering them the same.

Rinse your slabs when their pH has changed by at least 0.5 from the pH of the nutrient solution you’re using. The same deviation holds true for TDS values. Rinse the slabs until the value of the slab is the same as the value of the water you’re rinsing with. This should be somewhere around pH 5.5, and 250 TDS. Use the methods mentioned above to balance your pH and EC.

Hydroponic growing has its challenges, but it also has its rewards. Once you understand how pH and EC can affect the growth of your plants you are well on your way to growing high-quality marijuana plants.


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