PPM is an acronym for Parts Per Million, and when this term is used in cultivation, it refers to the concentration of solubles (minerals) in the water you give your plants. This is a way for growers to refer to exactly how much minerals and other substances are in their water which is useful since plants can only intake so much at a time. This is also useful for DWC growers who need to know the current condition of their reservoir water.
One part per million is the equivalent of 1mg of solubles per liter of water which is a minuscule amount to measure! Drinking water testing in the US can measure as low as 0 (for reverse osmosis, or distilled water) or as high as 700 (poor quality tap water; old, out-of-shape pipes), but most tap water will fall in the 200ppm-400ppm range.
Simply put, a TDS meter is the tool a grower would use to measure the ppm in the water they’re checking.
There are many TDS meters of varying cost and abilities; some also measure temperature or have hold buttons, some measure a higher range of ppms than others.
For those looking into purchasing a TDS meter, know that the range of ppm a particular meter measures shouldn’t be much of a factor for you. Some TDS meters measure up to 5000ppm and some go up to 9999ppm, but for cannabis cultivation, you will rarely see anything above 3500ppm. In fact, in all of the General Hydroponics feeding schedules, the highest ppm is calls for is 1500ppm max. This doesn’t mean you need a meter that only reads up to 1500ppm, but it does mean the ability of a meter to read over 5000ppm isn’t needed.
EC meters measure the electrical conductivity of the water they’re testing. This reading is useful for measuring the amount of solubles in your water!
So what’s the difference between a TDS meter and an EC meter?
When it comes to growing, there isn’t much of a difference at all. In fact, many TDS meters actually measure the electric conductivity of liquids and then convert those results into ppm. In other words, many TDS meters are actually EC meters in disguise!
Electrical conductivity is measured in S/m (Siemens per meter; no giggling!), but since we’re dealing with stuff on such a minuscule scale, readings will show up as μS (micro - or one millionth - siemens per meter) or mS/m(milli - or one thousandth - siemens per meter).
Although EC meters function just as well as TDS meters, I would strongly recommend against getting one. Most feeding charts or sources of information that talk about water quality will give measurements in ppm. EC meter readings (mS/m) can easily be converted to ppm, but you’ll save yourself some trouble by starting in the right unit of measurement.
For a large part of my growing career, I ignored using a TDS meter or even learning what they do. Although I don’t recommend that strategy as a way to deal with any problem, I can say that I still managed to grow sticky, potent bud despite my resistance to learn.
In short, I’m confident that anyone can grow a beautiful set of plants from seed to harvest without ever needing to invest in a TDS meter.
However, having a TDS meter can help to prevent or solve problems with nutrient burn and/or overfeeding your girls. If you frequently find that your plants suffer from nutrient burn despite giving them a very small amount of nutrients, you might want to invest the TDS/EC meter.