TDS, PPM, EC, And What They Tell Us

What is PPM?

PPM stands for Parts Per Million, and is a measure of how many particles are in your reservoir water. This helps you know how much of the water is actually water, and how much of it is particles (such as nutrients) floating around in it. There are different names for it; EC (Electrical Conductivity), TDS (Total Dissolved Solids) etc, but without going into the fine details of it all that matters is that these meters help you figure out how many particles are suspended in any given volume of water.

Why is this important when growing hydroponically?

When growing plants in water, it is essential to remember that plants CAN NOT LIVE IN WATER alone. They need a balanced supply of water to drink, oxygen to breath, and nutrients to eat. Water comes from, well, the water you put in there, the oxygen comes from an airstone which constantly pumps bubbles of air into the water, and nutrients are added to the solution until the appropriate ratio is met.

So we already have oxygen covered with an airstone, but now we have to choose our water source. Why does the kind of water you use make a difference you ask? Well the answer is that, ideally, you want to start off with completely clean and filtered water (also known as Reverse Osmosis water), that is ~0pm by default. This means it is just water, with no calcium, fluoride, iron, chlorine etc floating around in it. The benefit of this is that when you start with 0ppm, you can keep track of what is in it at all times. For example, if I add 400ml of nutrients to the water, and the nutrient solution bottle says that this particular nutrient contains 50% calcium and 50% hydrogen, then when we insert our TDS pen into our reservoir and get a PPM of X, we know that X is comprised of half calcium and half hydrogen.

In other words, if you start with a clean slate, you can better account for what is in your water, and can therefore more easily make sure your plant is receiving the optimal balance of nutrition at all times.

What about tap water?

If you start with tap water for example, it causes some potential issues. The tap water at my house for example reads as 400ppm. This means that when I fill up my reservoir with water from the hose, there is 400ppm of something in there already. But what? Is it all calcium, magnesium, iron, hydrogen? Who knows? And this is the problem, because you will be adding nutrients to it. Lets say that the 400ppm in your water was mainly calcium, than you run into problems when adding your nutrient solution into it since your nutrient solution already contains the ideal amount of calcium for your plant. Now you have “ideal amount+400ppm” of additional calcium. You can see how this all gets very difficult to track and keep stable, because you can never truly identify what was in that tap water to begin with (unless you pay for costly tests in a lab, which is cost-prohibitive and time consuming).

And don’t think a Britta filter will save you either. My Britta filter takes my 400ppm tap water and filters it down to about 200. A big improvement, yes, but I still don’t know what is in that 200ppm.

This is why I always say one should invest in an R/O filter, so that they can start with clean water. That way you know what is in your water because you started with a clean slate. If you need to know how much calcium is in there, just look at the nutrient bottle and it will tell you how much calcium is in there. No need for additional guesswork to find out what was contained in your tap water prior to you supplementing it.

But does PPM tell us the whole story, even assuming we start with R/O water and measure our nutrients carefully?

No, not really, because after a week or so your plants are going to have consumed certain nutrients at a faster rate than others, or they may have drank more water than usual because it was hot one day. This means that over the course of the week, you see your PPM slowly dropping, but you don’t know what specific nutrient is being depleted. If it was 900 PPM last week, and is 800ppm this week, all that tells you is that you lost 100PPM of some nutrient, but you still don’t know which one. Unfortunately, this is the case with hydroponics as the TDS/PPM/EC meter is only going to count the total number of particles in your reservoir and tell you the ratio of water:particles. It can not discern between different types of particles, nor can it identify them individually.

So what should I do?

It is highly recommended that you completely drain your reservoir once per week (or two weeks if you are using a larger reservoir). At this time you would put in fresh water and re-measure your nutrients, that way each weak you know that your plants have all that they need thrive. If you let it go for too long, you may check your reservoir and say “Hey it’s still got 800ppm, so there’s plenty too eat in there!”, but all the while the plants may have eaten the calcium at a faster rate than the other nutrients and your solution has now run low on calcium, causing a deficiency and maybe some blossom end rot on your tomatoes.


  • A PPM meter will give us a rough estimate of the water:particle ratio, but does not tell us exactly what ingredients make up that PPM.
  • Change your reservoir weekly or every two weeks and start over fresh, that way your plants will never be deficient and always have plenty of the things they need to bring that explosive growth you are seeking.

Here is the meter I use & recommend: HM TDS-4 Pocket Size TDS Meter

For additional, more scientific explanations of what is going on with EC meters etc, check out this great detailed breakdown from Electrical Conductivity and Monitoring Plant Nutrition