Grab Your Lab Coats: What Is PPM?

Growing plants doesn’t seem like a particularly hard-science kind of thing. Seeds, light, water, and nutrients. It’s that easy, right? Spoiler alert: it’s not. In fact, a little bit of number crunching is going to make a world of difference for your grow. That’s because when you take the hydroponic route, you take complete control over everything your plant gets – the light, the airflow, the support. You also have to manage the nutrient solution. And managing that solution takes — you guessed it — science!

For the nutrient solution, it’s all about ppm. We know you’re all dying of curiosity: what is ppm?


What Is PPM?

Good question! It stands for “parts per million” and it’s a unit of measure for the concentration of one substance dissolved in another substance – like the concentration of salt in the ocean. One ppm is one part by weight of a mineral in one million parts of solution. Here’s how it works: the concentration of salt in the ocean is 35,000 ppm by weight. So if you have 1 million tons of seawater, salt accounts for 35,000 tons of that weight.

What Does This Have To Do With My Grow?

Now that we’ve recapped the technical definition of ppm, it’s time to put it to use.

The whole point of hydroponics is growing plants with a nutrient solution rather than in mineral soil. Unfortunately, water does not come out of the tap pre-loaded with the right nutrients. So, we add  nutrient concentrates to the water to make sure we’re giving our plants what they crave (which is not Brawndo, although plants do need electrolytes). These nutrients are necessary for the creation of plant cells and management of plant metabolism – water and light alone just don’t do the trick.

There are several types of essential elements that plants need to grow, which are split into two categories: macronutrients and micronutrients. Macronutrients are nutrients that plants need in larger quantities and include nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Micronutrients are needed in smaller amounts but are still important for plant growth, and include elements like boron, manganese, iron, zinc, and copper. Plants that are grown in mineral soil are able to pull some elements from the earth directly, but in hydroponics all the essential nutrients need to be added to water in order for plants to thrive.

Without those nutrients, plants can’t grow. But if that’s what plants need, can we just dunk them into a nutrient concentrate  and sit back while they turn into Jack’s beanstalk? Nope! While that kind of fairy tale would probably be hilarious, it’s not gonna happen. Too low of a nutrient solution concentration, and plants may run into nutrient deficiencies and poor growth. Too high a concentration of nutrients is just as bad as too low. Some types of nutrients can actually cause toxic effects when too much is available to plants in a nutrient solution.  And for the record, if someone hands you magic beans, those probably also need the right concentration of nutrients to grow.

How do you get the right concentration? Start by figuring out what your plants need. That’s right – get on the Google (we know that’s your favorite thing) and get a sense of what types and concentrations of nutrients are best for your grow. Young, tender plants need lower amounts of nutrients while more mature plants need greater amounts. Generally speaking, leafy vegetative types of plants like lettuce and herbs need a lower concentration of nutrients compared to fruiting or flowering plants like tomatoes and peppers.

Once you get a feel for what your plants need, find a nutrient program that fits . Now, let’s talk about how to measure the concentration of your nutrient solution. That’s where a TDS meter and ppm comes into play.


Put A TDS Meter On Your Shopping List

A TDS (total dissolved solids) meter will give you a measurement of the concentration of total nutrients, measured in ppm. Most TDS meters work by measuring the electrical conductivity (EC) of the nutrient solution. The conductivity of water increases as salts and minerals (aka nutrients) are added. So, the higher the nutrient concentration, the higher the EC. Seawater, for example, conducts electricity better than freshwater (if for some reason you need to choose between those two during a thunderstorm, in which case we wish you the best of luck and we thank you for your contribution to science). EC is measured in  milliSiemens per centimeter, for the record.

A TDS meter converts mS/cm to ppm for you, which saves you calculator time. There are also EC meters that measure just in mS/cm and combo TDS/EC meters that do both ppm and mS/cm. So many choices! Depending on your nutrient program, it may reference a target concentration in either ppm, EC, or both. So (obviously), choose a meter that meets your needs.

Before you even think of using your new TDS or EC meter, it needs to be calibrated with standard reference solutions – like zero-ing out a scale in order to make sure you’re getting an accurate weight. Different models may require different standards, so make sure you’re getting the right ones. A meter is only as good as its calibration – you can’t trust the numbers from a non-calibrated meter. That’s a waste of time and a threat to your plants, so don’t skip this step! There are no shortcuts in science, folks.

Once you have your fancy new meter calibrated, you’re ready to start using it. Go ahead and test your nutrient solution. Now, compare that measurement with the target concentration you found when you were doing your homework about what your plants need. Too high? Add some more water. Too low? Add some more nutrients. Rinse and repeat until you hit the Goldilocks zone. By the way, you should always stir your nutrient solution when you measure it – those minerals may sink to the bottom or rise to the top and give you a goofy measurement.

For the record, this is also a good time to check your pH and make sure that it’s in the right range. The pH of the nutrient solution will impact what nutrients are available to the plant. So even if the concentration is spot-on, the wrong pH may mean your plant can’t absorb some of those nutrients. For most plants, a nutrient solution pH of 5.5 to 6.5 is recommended and can be measured with a pH meter.


Black Magic Takes Science (So Quit Complaining)

If you’re going to go to the trouble of managing a grow, you might as well do it right! Give your plants the healthy diet they need so that your grow is as happy and healthy as possible. And if that means getting a little technical, it’s totally worth it! Besides, science is totally in right now.

It’s not likely that they will need to do these conversions

I included it as part of the general primer on the idea, but we can cut it or cut it down. Maybe just “Heads up – you may occasionally run into something measured in ppb (parts per billion) or ppt (parts per thousand). It’s the same idea and you can pull up a calculator online to do the conversions.”