Trying to understand PPM in and out

Why is testing ppm in and ppm out, when to start testing? Some say EC is better and some PPM, which do you all prefer? I bought the blue lab combo ph/ppm pens and they will be arriving this week. Can someone break this down for me?


With out getting too technical, 0 ppm is what you can start with. Reverse osmosis water is usually almost a 0 ppm. Some water from a well has some Total Dissolved solids (TDS). This is made of limestone or calcium. (And other stuff but we are keeping it simple)
When you add nutrients you’re adding more TDS, which makes the number go up. Let’s say you start with 0 PPM water, add nutrients now you’re at 1500 PPM.
Dump in 1500 TDS/PPM water and nutrients into your soil your plant is growing in. You check run off, and you get only 500. This means your plant is probably hungry, and needs more nutrients.
If you dumped in 1500 and got 2,000 back, you’re feeding too much, or the PH is off, and your plant is not absorbing nutrients correctly.
So you begin to see how these numbers can be utilized, it’s less of a guessing game when mixing solutions together. I hope this helped you understand a bit better.
Welcome to the forum.


EC and PPM are just different ways of reading the same thing. If your meter is on the 500 scale, every 500 ppm is 1ec (1000ppm is 2ec). If the meter is on the 700 scale, every 700ppm is 1ec (1400 ppm is 2ec).


@Covertgrower @BobbyDigital thank you, yes very helpful. What is ideal ppm for seedlings?

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PPM is slightly more accurate then EC from my understanding and if your on this forum you’ll want to use TDS/PPM because that’s what everyone here uses.

There is no 100% agreed approach to values, and it sort of matters what your feeding thus follow your feed chart that came with your nutrients.

Seedlings: 100-250 (nutrients aren’t needed here, hence there’s not a lot of particles needed)
First Half of Vegging Cycle: 300-400 (this is usually after you transplant, which still doesn’t require many nutrients)
Second Half of Vegging: 450-700 (you’ll start giving your plants more nutrients at this stage)
First Half of Flowering: 750-950 (your plants will be eating more as they grow, so they’ll be taking in more nutrients)
Second Half of Flowering: 1000-1600 (this is when your plant’s eating the most, especially if you give it additives)
End of Flower, Entering Harvest: As close to 0 as possible (this is when you’ll be flushing your plants, so you don’t want there to be a lot of particles leftover)

That all being said my seedlings go in pots of fox farms ocean forest soil that runs about 4000 TDS

Here is my feed chard

But then here is a very common one


To explain this more accurately, ppm and EC are like celcius and Fahrenheit. They mean the same just sound sounds different. The way the pen works is it passes a small charge of electricity through the liquid and measures its conductivity. Pure water is not conductive, surprise! Many folks dont know that. What make water a conductor is molecules of other things in it. Those things must be conductive, sodium, magnesium, phosphorus, etc are all conducive, whereas chlorine is not. This is important because, the pen will not detect chlorine from tap water. The pen does not display what its detecting so if your tap water is 200 ppm you won’t know what that is without a test report.
To start with, take a reading of your water. Decide what strength you require and deduct the starting value from the target. If your water is 200 ppm and you want 500 ppm you can only add 300 ppm of food.
I hope this helps and if anyone sees an error in my explanation please correct it.


@Cannabian @Nicky very informative thank you. One question, my tap well water is 200 so I should not need to add anything the first few weeks? Another question, when I do start feeding say the ppm going in is 400 but the ppm out is 800, in this case what would you do or vise versa? My understanding is you want the same out as is going in correct? Thanks again. Also my ph out of tap is high 7’s ok per one gallon how much ph down should I use?

Are you using Fox farms ocean forests? If so, you can get away with the tap water as it is. You may want to PH it? But it has buffering qualities so… ? I dont use ffof so I’m not an authority on it. But dont use fertilizer for at least 5 weeks from what I understand.

Like I said before, just because your tap water is 200 ppm does not mean that can be used as plant food. It could be calcium or arsenic? Without an analysis it’s just a guess. Therefore, if you want the largest overhead for plant food it may be necessary to use distilled water at first until the plants require higher nutrient load. Also, you could use reverse osmosis.

What that says is there is already nutrients in whatever you are using? Do not leach it out! Water as needed and do not do drain to waste. That is a technique used with straight coco or peat based mixes when running nutrients. You can do that at the end stages when you feed flower food and a final flush… if you do that?

@Cannabian I will be going straight to coco perlite mix with the rapid rooters. Thanks for your guidance.

In that case you will be doing hydro. You will require a cal mag supplement too. Target ph of 5.8 and EC will need to be monitored as per plant age and the food you plan to use. Not all fertilizer uses the same loading.

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I plan on using advanced nutrients sensi coco. When would you start the feed? At the time of transplant?

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In straight coco yes, but start in a small container so you dont over/underwater and use very light solution. The best thing to do is put feelers out for anyone here that runs that brand of fertilizer and soiless mix. There will be a few for sure.


First thing is to test your well water, I know I know that costs money… Well Geuss what you want to make sure you don’t have high heavy Metals because if you do you should NOT be using it on plants your going to consume or drinking it. The tests are like 20$ or something so go get your water tested and then put in a in line water filter from your local water store totally worth it will save you tons on water you drink and give your plants much better water.

As far as PHing your water it depends if your in hydro or soil if your coco then your hydro so this chart will help

If you grow in coco I’m not your guy to help you with I’m a soil guy, @Cannabian Ps he would want to water at 6.3-6.8 swings until flower not worrying about runoff unless suspected lockout. Come flower run 6.5-6.8 PH


I guess coco is a bit different than pure hydro isn’t it.


From what I hear it is, but also it isn’t…
Again it’s not my fortay maybe
@BobbyDigital @aaa @Myfriendis410
@Covertgrower or someone else can tell you

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Don’t get Latewood started: he’ll school you on this haha. No; coco is definitely NOT hydro. It is technically ‘soilless’ and you would use a ‘Drain to Waste’ hydro grow schedule but there the similarity ends.


As far as PH is concerned? Are they similar


Yes. That said; I haven’t seen much issue with holding everything at 6.0. I have used coco, Promix, soil, RDWC and DWC and all of them are at 6.0 ± .2. (I will bias down in coco or hydro and bias up in soil.