PPM and You: A Quick Guide

While we try to make cannabis growing as simple as possible, some technical terminology is just unavoidable. Lately, we’ve had many novice growers ask us to explain PPM, TDS and EC readers, and how they all work together to help you grow better cannabis. Understanding these concepts isn’t the easiest thing in the world, so we’ve decided to write a definitive guide in three parts: the 10 second Cliffs Notes, 30 second explanation, the two minute talk, and the five minute deep-dive into the world of PPM. Enjoy!

PPM in 10 Seconds

1. PPM is a measurement of nutrients in the soil or growing medium.
2. Organic soils are too comple—x to be analyzed with ordinary PPM readers. Our growing system is scientifically calibrated to provide the ideal nutrients at the right time.

PPM, TDS, and EC in 30 Seconds

PPM stands for Parts Per Million, a measure of concentration. By concentration, we’re referring to how much of any nutrient, or ‘soluble’, is dissolved in the water you feed your plants with. It’s a way to measure tiny amounts of dissolved nutrients in water. PPM does not measure the type of soluble in water, only the total amount.
Measuring PPM matters because cannabis plants are extremely sensitive to proper nutrition. High PPM can cause nutrient burn. PPM are measured using a TDS or EC meter. TDS stands for Total Dissolved Solids, while EC stands for Electrical Conductivity. Both effectively measure PPM, though EC readers require some basic math to convert their units to PPM.

The 2 Minute Talk: What is PPM?

Next, let’s get to the science of soluble nutrients and cannabis growing. PPM, or parts per million, aren’t really a class of nutrient or an ‘ingredient’ in any true sense. As its name suggests, a PPM is simply a unit of measurement, like a percentage. In fact, a percent is a ratio of one part per hundred, or ‘cent’. If you’ve ever cooked rice, you’re probably familiar with ratios. For every cup of rice, you use two cups of water. That’s a 2:1 ratio of water to rice, or two parts water per single part rice. If you weren’t clear on the term before, hopefully you can see where it comes from now: a ‘PPM’ is a single unit (say, a millilitre) of any given substance per million units (one million millilitres, or 1,000 litres) of another substance.

As you can see, that’s a tiny ratio. Take a look at the image below:

That’s a 100 pixel by 100 pixel black image, with a single white pixel near the centre. Can you see it? This square is a total of 10,000 pixels, one of which is white. Only a single part-per-10,000 is white. That tiny white pixel actually represents 100 parts per million in that image. So take that pixel and make it one hundred times smaller. That’s a part per million, or PPM.

That’s Pretty Small.

Yes, it is. It seems impossibly small. One mL in 1,000 litres isn’t just a drop in a bucket; it’s less than three quarts in an olympic sized swimming pool. How can a PPM — or even hundreds of PPM — matter whatsoever in growing your cannabis? Even 10,000 PPM equals just one percent. It matters because plants don’t eat their food by the mouthful; they eat their food by the molecule.

Cannabis plants, like all leafy plants, absorb nutrients through their roots. Roots are porous membranes capable of drawing water and nutrients out of the soil, which the plant then uses to produce food through photosynthesis, and as ‘building blocks’ for all its structural and chemical components, from leaves to flowers and, yes, cannabinoids. Plants can only ‘eat’ so much from the soil at once; if a plant is flooded with certain solubles beyond what it can handle, the roots can become gridlocked and unable to take up any nutrients. Other nutrients, when taken in abundance, can actually damage the plant, causing issues with chlorophyll production or even effectively poisoning the plant.

That’s where PPM come in. Cannabis plants have different nutritional needs throughout their lifespan. For each stage of growth, a different concentration of nutrients is considered ideal. Using a TDS or EC meter to measure PPM is an accurate way to ensure your plants receive exactly the right amount of nutrients, and prevent them from soaking up too much of the wrong stuff.

The Five Minute Deep Dive: PPM and Organics

The 30 second guide covers only the most basic understanding of PPM and their relation to cannabis cultivation, while the two minute guide further explains the scientific concept of PPM. Think of them as glossary entries — it helps you understand roughly what’s going on, but you’ll need to know more to really put that knowledge into practice.

This five minute guide aims to go through the process of purchasing and calibrating a TDS (Total Dissolved Solids) or EC (Electrical Conductivity) meter, measuring PPM in your water, and ensuring your plants are receiving the nutrients they need — no more, no less. It also talks about how PPM and organic growing interact, and why your readings may not be accurate when you grow organically.

Purchasing and Calibrating TDS or EC Meter

TDS and EC meters are finely calibrated tools for measuring solubles in water. Both TDS and EC meters measure electrical conductivity in the water, and determine the soluble levels based on how conductive the solution is. TDS meters give you a measurement in PPM, while EC meters require a simple mathematical conversion to PPM (multiply the EC number by 1000 then divide by two).

TDS and EC meters can be purchased from any hydroponics or growing store, or from many online retailers such as Amazon. They can cost anywhere from $15 to $50 depending on their features and intended use. While almost any TDS or EC meter can be used, you may want to shop around to find one with the features, shape, and size you think will work best for you.

TDS and EC meters may not work perfectly right out of the box. You should always calibrate your meter to ensure it’s accurately measuring PPM. Calibration is done by inserting your meter into a solution verified at a certain level of PPM. These can be purchased online or at many growers supply stores. To calibrate your meter, be sure the calibration fluid is at testing temperature, usually around room temperature. Test the fluid with your meter according to the package instructions. If it displays the correct PPM, it’s properly calibrated. Otherwise, follow the meter’s instructions to calibrate it.

Measuring PPM in Your Soil and Fertilizer Solution

Measuring your nutrient solution PPM should be just as simple as calibrating the meter, right? Close, but not quite. You need to remember that unless you use distilled water, your water will have a PPM count of its own. First, measure the untreated water you plan to dissolve nutrients into. Hopefully, its PPM are fairly low. If they’re high (say, above 400 PPM), you may need to filter your water prior to dissolving nutrients into it. While the solubles in tap water aren’t necessarily nutrients, they still count toward the total PPM; you must account for the existing solubles in your math to ensure you don’t harm your plants.

Latent solubles also exist in your soil, and can similarly burn your plants if they are too concentrated. In order to test soil PPM for cannabis growing, take a sample of several ounces of soil and mix it with distilled water in equal measure. Much like pH testing your soil, you want to mix the soil and water thoroughly to ensure the water absorbs the solubles, but don’t get too soupy. Let this mixture sit for 24 hours, stirring occasionally. Once you’ve allowed everything to meld together, use a simple filter like a coffee filter to strain out all the soil and fine particles. Strain the mixture until the water runs clear; this may take several rounds of filtering. Once the water runs perfectly clear, test it with your meter. The reading is your soil nutrient content in PPM!

Another quick and dirty method that’s pretty accurate is to make the same distilled water and soil mix described above. Place the mixture in a plastic baggy for 24 hours to allow the interaction between distilled water and media. Then, use a pin or paper clip size tool to poke small holes into the bag— five or six holes should do. Try not to make the holes too big. You don’t want to get any dirt in the solution. Gently squeeze the baggy over a small container, collecting the filtered solution for your measurement.

I’m growing organically and my readings are way off!

Yeah, this gets a little complicated. PPM are a straightforward measure for hydroponic or standard, synthetics-based growing. There’s a set number that’s ideal for each stage of growth, and that’s it. Organic growing is a little different. Part of the philosophy of organic growing is that the nutrients don’t just come from some refined, synthetic fertilizer. The soil itself should be fortified and made nutritious for your plants using natural, organic ingredients. Why does this affect PPM readings?

Rich, healthy organic soil is more complex than artificially fertilized soil or hydroponic solutions. The nutrients are stored in more complex molecules than with artificial fertilizers, that are released slowly over time. EC meters work by measuring the electrical conductivity of salts in water; organic nutrients haven’t yet broken down into simple salts. It’s like the difference between complex carbohydrates found in whole grains and refined white sugar. The organic nutrients get broken down by microorganisms over time, much like complex carbs get broken down by our digestive system. Therefore, the PPM readings you get from organic soil won’t reflect the actual nutrients that soil contains.

However, they may not be lower, as you might expect them to be. Why? There are plenty of non-nutrient solubles found in organic fertilizers that can confuse your readings. Remember how we talked about solubles in tap water earlier? This is a similar concept; many organic soil additives can end up adding acids and carbohydrates that are electrically conductive, thus throwing off your TDS meter’s EC readings.

Is it worth testing organic soil for PPM?

Good question. Due to the complex nature of organic soil, EC readings will be too inaccurate to determine whether the appropriate amount of nutrients are present in the soil. To determine if organic soil is well-balanced for your growing system, you’ll need to carefully monitor pH, watch the plant for signs of nutrient deficiency or imbalance, and learn from experience. If you have the resources, you can send a soil or plant matter sample to a lab for analysis. Alternatively, you could always rely on our simple four-step process for growing flavorful and potent organic cannabis.