Monitoring the Water Quality in Your Cannabis Grow

Knowing the amount of nutrients in the soil and the pH is key to larger yields. Luckily for you, there is an easy-to-understand system that calculates this information, and that is what we’ll be exploring today.

What do  EC, PPM, and TDS Mean?

Cannabis plants have grown in nature for thousands of years without the assistance of humans. We know from our previous article that pH (water/soil) is important because essential nutrients are only available to the plant in certain pH ranges. 

In nature, there are many factors, like rainfall, decomposing foliage, and microbes, that help balance the soil pH. We can replicate some of these functions indoors, but we still need to monitor the soil and water to ensure the correct pH levels for cannabis plants.

Along with water/soil pH, it is also important to monitor the amount of nutrients they contain. To express this, we have EC, TDS, and PPM for you to choose from. Let’s break down what these are and how they work before I give you my recommendation for which of the three is the best method to monitor cannabis nutrient levels.

EC is short for electrical conductivity.

Important plant nutrients like nitrogen, phosphorus, and potassium (to name a few) all have electrical charges. The more nutrients in the solution, the more that solution can conduct electricity and, therefore the higher the EC measurement. In other words, measuring EC won't tell us about specific nutrients but it will enable us to measure if our plants are “eating up” nutrients or not. When solutions or soil are low in nutrients, the EC will also be lower. Cannabis grows best within a specific EC range, and we will give you the target EC for hydroponics, coco, and soil in just a few moments.

PPM stands for parts per million. PPM is used to represent the nutrient load of a solution. There are a few different scales for ppm, but the most common are the 700ppm scale and the 500ppm scale, which is commonly referred to as TDS.

TDS is short for total dissolved solids. TDS represents the level of dissolved solids in the solution. TDS (500ppm scale) and PPM (700ppm scale) seem similar, but they are technically formulated in different ways. TDS measures the sodium chloride (NaCl) in a solution. PPM measures the potassium chloride (KCI) in a solution. Both measurements are nonspecific, so other minerals are also part of the final values.

In reality, the measuring devices we are using aren’t actually determining the true ppm of the solution; you would need chemical analysis for that. Instead, all of the ppm meters we are using to grow cannabis calculate the EC of a solution and then, with simple math, convert that to ppm. 

ppm500 scale = EC x 500 

(1.2EC = 600ppm)

ppm700 scale = EC x 700 

(1.2EC = 840ppm)

We Recommend EC and PPM, Why?

EC and PPM for cannabis are the most beneficial to use, but commonly, EC is the preferred method for measuring cannabis nutrient levels. EC is a universal language, which is why it is the choice of most professionals. When a nutrient company suggests adding 200ppm of their special sauce, are they using the 500ppm scale, the 700ppm scale, or one of the other ppm scales? This unclarity can lead to costly mistakes, but if you say 1.2 EC, there is no confusion about how much 1.2 EC actually is. 

How pH and EC Are Linked

Growing amazing cannabis is about balance. This mantra rings true in the relationship between pH and EC. If the pH falls outside of the desired range, cannabis plants aren’t able to uptake certain nutrients. Although there may be plenty of nutrients in the media, the plant can’t access them, causing the plant to show signs of a deficiency, which is misleading. The EC was fine, but the pH was in the wrong range.

A plant with what appears to be Manganese deficiency on leaves

Conversely, if the EC levels are too high, it can negatively affect pH over time. High EC levels can lead to salt buildup, which affects the soil pH by causing it to drift out of the correct pH range for cannabis growth, and can lead to nutrient lockout. I’m happy to say that keeping EC and pH in balance is easy with the right tools.

What Tools Do You Need?

The most effective way to measure EC and PPM for cannabis is by using a specialized probe, often referred to as an EC/PPM pen. These devices can be purchased for well under $50.

pH and EC/PPM meters

Every grower will also want to have a dedicated pH pen in their toolkit. PH pens often look the same as EC/PPM pens but they are specially equipped to measure pH instead. 

Some pens will have the ability to measure all three, but I’ve found having a dedicated pen for pH beneficial. Accuracy is important, and cheaper all-in-one pens tend to be less accurate and have a wider margin of error, which can be detrimental to your plants' health.

Correctly Monitoring the Solids in Water

Hydroponic growing uses water as the medium. Measuring the pH and EC levels is straightforward. Dip your dedicated pen into the reservoir, and you will get the current levels. When growing in soil or coco, there are two times when monitoring the pH and EC are most beneficial.

Monitoring the Water Coming Into Your Grow

As a home growers we often get our water from the tap to fill our watering cans. It is beneficial to know the pH and EC out of the tap before any nutrients are added. Most water sources will fluctuate levels during the year, so don’t assume it will always be the same. Get into checking these as part of your routine!

Some growers will keep their water out for 24 hours to get rid of potential toxic chlorine before applying it to your coco or soil. If you do this be sure to check the pH one more time before watering to confirm that it is still the correct pH for cannabis growth. 

When adding nutrients to your water, always be sure to check the pH and EC before and after adding all of the nutrients into the solution and go slowly and steadily. Always double-check these numbers before applying it to your media. “Measure twice, pour once,” and you will avoid mistakes and achieve the proper EC and pH!

Growing in Soil Requires a Slurry Test

Soil chemistry is highly complex with millions of microorganisms working within. Soil runoff, isn't the only test needed and we recommend you also get into the habit of performing a slurry test. This gives you more data points to follow your grow effectively.

Measuring soil (slurry test)

1. Collect a representative soil sample, breaking up any large clumps and removing large pumice

2. Add distilled water (pH of 7.0) at a 2:1 ratio to the soil - eg; 500ml of soil to 1,000ml of distilled water

3. Stir and shake thoroughly to mix the material into a slurry. Let the slurry sit for 15 minutes or until the solids have separated and created a liquid layer

4. Measure the pH and EC of the water layer

Measuring Your Runoff Water (Coco, Soil, Hydro)

Above you see an example of different options to measure for various setups

Runoff water is simply the extra water that escapes the bottom of the pots after being watered and when the medium is saturated. We usually recommend watering plants until you can see some runoff. This runoff water can give you valuable insight and data into the nutrient and pH levels surrounding the root zone. That’s right, homegrowers also use data-driven methods!

1. Record the pH and the EC of the solution being used to feed the plants

2. Provide enough water to the top of the pot to obtain 10% runoff in the drip tray

3. Use a dropper or turkey baster to remove that water and place enough of the runoff water in a cup to cover the tip of your measuring device(s)

4. Compare the pH and EC of the runoff to the pH and EC of the input solution

Decrypting the Results

Once you have your initial measurements and the soil slurry, or runoff measurements it is time to analyze!

Understanding Runoff Water Data

Checking the cannabis nutrient levels and correct pH for a cannabis grow is pretty simple. Ideally, the runoff readings should be lower, indicating successful nutrient uptake. If runoff readings are very low, you may need to increase nutrients.

If there’s no change between the solution and the runoff, your plants likely aren’t absorbing nutrients due to pH spikes or drops.

A higher runoff reading usually points to salt buildup around the roots. As you feed your plants, these salts dissolve back into the runoff, increasing PPM or EC. To fix this, use an enzymatic line cleaner or repeatedly flush the roots with pH-neutral water to remove excess salts.

If you are growing in coco coir expect the coco to be high in salts and so these measures can be less clear. In coco aim for about the same EC and PPM coming in and out of your grow. 

EC and PPM slightly Lower in Runoff = Normal

EC and PPM unchanged in Runoff = Plants aren't eating enough

EC and PPM higher = PH imbalance and potential salt buildup

If the runoff EC or PPM is much higher than the input, this indicates a salt (synthetic nutrients) buildup in the root zone. You can remedy this by using one or all of these three methods;

1. increase watering frequency (EC rises as the soil dries out)

2. run plain water through the root zone to wash away excess salt buildup

3. reduce the EC or PPM of the feedings until the runoff returns to a normal range

Understanding Soil Slurry Test Data

Soil chemistry is more complex than coco which is why we need to do the slurry test instead of just runoff water. The results, however, are easier to manage. The pH and EC of the slurry is a representation of the root zone. Corrective action is similar to that of coco.

If the soil pH is low, you can moderately adjust the pH of the input higher until a slurry test indicates the ideal root zone pH. If it is too high, adjust the input pH lower slightly. 

If the slurry test shows a high EC, run plain water through the soil and feed with a lower EC until the slurry test returns to the proper range. If the EC is lower than the normal input EC, increase the EC value for the next few feedings.

Ideal EC, and PPM Ranges for Each Grow Stage:

The target EC and PPM range for cannabis will shift in-tune with the various life stages of the cannabis plant. Below, we have a chart for the four main grow styles covering each stage. The pH and EC/PPM values shown are for the solution being added to the reservoir or soil surface. Each week of growth, the grower will increase the EC or PPM values within the ranges below until just before harvest when they will come right down again, otherwise known as flushing.

*pH will remain the same through each life stage, but each grow style has a different target range

**Pre-harvest is defined as the final 10 days before harvest

Extra Tips

Hitting the exact range is important for pH because the pH scale is logarithmic, not linear. This means that there is a 10X difference in acidity between each number. A pH of 5 is ten times more acidic than a pH of 6, and 100 times more acidic than a pH of 7. Be sure to hit the target pH levels for cannabis plants to achieve proper cannabis nutrient levels.