After my recent 'obsession' with writing about the different types of water you can use for hydroponics, whether you can grow with tap water, or what to know about distilled water, I figured it'd probably be smart to talk about how to measure what's IN your water.
It's one thing to understand all of the different compounds and substances that could be in your water, but it's a totally different thing to be able to measure it.
The Best Measure: Total Dissolved Solids
As its name suggests, total dissolved solids (TDS) is a measure of the minerals, metals, cations, anions, or salts that are dissolved into the water you're measuring.
Essentially, it's a measure of anything that isn't the H2O molecule in your water.
Before we go any further, it's important to know that any suspended solids - like wood pulp or crushed rock - are not included in a TDS reading.
A Good Rule of Thumb
The total dissolved solids concentration usually equates to the sum of the cations and anions in your water or nutrient solution.
- Cations - positively charged ions
- Anions - negatively charged ions
Where Do Dissolved Solids Come From?
It's a good idea to have an understanding of where all of the dissolved solids in your water come from, especially if you're trying to grow plants!
A lot of the common dissolved solids you'll find in your water come from organic sources:
- Industrial waste
- Road salts
However, dissolved solids also come from inorganic materials:
Usually these materials form salts that end up in your water. Chemically speaking, salts are compounds that are made up of a nonmetal and metal. Just as common table salt (NaCl, or sodium chloride) dissolves in water, other salts dissolve in water and contribute to the amount of total dissolved solids.
How TDS Is Measured
The hardcore way to measure TDS is to take a sample of water and evaporate all of the water.
Then, you would weigh what's left and compare that with the total weight of the solids and water.
That's nearly impossible for the average gardener to do, which is why TDS hand meters can make measuring your TDS an absolute breeze.
To understand how they can do this, we need to learn another important concept: electrical conductivity.
Introducing Electrical Conductivity (EC)
Simply put, electrical conductivity is a measure of the degree that a material conducts electricity. Simple, right?!
Pure water is very unique in that it has a conductivity of almost zero.
A TDS meter uses the fact that pure water has zero conductivity to measure the amount of total dissolved solids in the water. Because the dissolved solids in the water do have conductivity, by measuring this conductivity and doing some simple math, the meter can estimate the TDS in your water.
Important: While they are called TDS meters, technically they are actually EC meters that convert the EC reading into a TDS reading. Fun fact!
What About Parts Per Million (PPM)?
Some of the more hydroponically experienced might be yawning at this point in the article, but beginners are probably wondering how TDS plugs into another acronym that they're more familiar with...PPM, or parts per million.
PPM is simply the way that TDS is expressed. It's also the most common way that gardeners will describe how they're feeding their plants.
Let's say you're using the Bluelab Truncheon to measure your TDS (it's the one I use), and it gives you a PPM reading of 1100.
What does this 1100 reading mean?
It's simple! 1100ppm means that for every liter of water, there are 1100 milligrams of dissolved solids.
Why Does Measuring TDS Matter?
When it comes to growing plants hydroponically, aquaponically, or even in soil, knowing the amount of dissolved solids in the water that you're giving them is critical.
Without this knowledge, you wont be able to troubleshoot any common plant problems that are related to nutrients.
For example, you wouldn't be able to know if your plant is nitrogen deficient unless you knew the amount of nitrogren that you'd been feeding your plant.
In that example, nitrogen concentration could be measured by knowing what percentage of the nutrients you added to your water were nitrogen, and cross referencing that with common nitrogen requirements for the plant that you are growing.
See how it all comes together?
How to Reduce the Amount of TDS In Your Water
If you've decided to grow with tap water or just want to lower the latent TDS in your water, there are a bunch of options available to you.
Carbon Filtration - This is a very common technique and one you're likely very familiar with if you have a Brita filter in your home. Activated charcoal sticks to or absorbs many compounds, making it a good choice to remove dissolved solids from water.
Reverse Osmosis - Often an expensive option, it is also one of the most effective ways to remove dissolved solids from water. Water that goes through an RO system typically ends up with a PPM below 10.
Distillation - Read my article on distilled water and gardening.
Deionization - Passing water between negative and positive electrodes. This causes positive ions to move towards the negative electrode, and visa versa.
I hope this has been a good primer on what total dissolved solids are and how they plug into everything else you need to know to grow plants successfully.
If you have anything to add, leave it in the comments!