Question about TDS/PPM testing

Sorry if this has been covered, and I’m sure it certainly has, but I’m still in a mystery state as to how to navigate the flushing piece of this thing.

I’ve been using the Fox Farm Trio feeding schedule w/CalMag as supplement, and a I used molasses on the auto because “I read somewhere” that it had a hand in the frostiness of the flower. I don’t know if it works, but these look like their on their way to frostyville. Of course the ultimate test will be the smoke.

I only put a couple of tsps per gallon because it’s hard to see the color of your RO because it’s so dark.

I flushed yesterday and my TDS was 416 on the Auto, and the RO water looked like motor oil. I’m on the second flush on everything except the auto.

BTW, I Phd about 20 gallons of rainwater at 6.0, and used Clearex.

So my crop was anywhere from 950 to 1,800 before the flush, and after 416 to 750ish.

The Fox Farm schedule I use is from 2019 is use only the Trio, Cal Mag, and I’ve tried Molasses on the Auto.

The ranges they show are much higher than a schedule someone posted here. Assuming you guys use soil, do you test the ppm after mixing up your nutes before you feed them, and then test the RO after you’ve fed them, or wait a few days and test the soil or RO again?

Also, how often do you test the soil?

There’s a lot of info out there, but I believe getting this correct is key.

Thanks in advance.


Firstly… gorgeous plants!!! Man i wanna grow outside! Beautifully done. Good work.

Flushes… once u get under 500… especially from 1200+ ur pretty good. Could go lower but not really necessary. Its alot of work and you got good results.

Those feed schedules are on steroids. Dont really need your ppms that insane usually. But if the girls like it i love it. Also they have diff schedules for different scales of TDS… i believe there’s a 500 and a 700… @elheffe702 @DoobieNoobie i think could explain it better. @Hellraiser knows his sciency stuff too.

Dont go to far down the rabbit hole with ppms… its a good guestimate. U dont even know which solids are which. More important to listen to your leaves.

But! I get ur question. You measure ppms after mixing before pouring to know what you are feeding. You measure runoff after pouring it (hopefully the last bit of it, not the first few drops) to see how’s she eating WITH her new food ontop of it. You flush going into flower to release alot of unnecessary nutrients from veg (namely Nitrogen) that they dont need in as much excess… and when you start back feeding, its a more bloom based diet.

That cover it? Any more ?’s?


Good stuff and thanks for your kind word. Hellraiser has been my go to, and has answered some of these questions, so I wanted to get as much feedback as possible.

I gotta be honest, Im ready for them to get their flower on because they’re getting huge! Those photos are from mid June.


O they are into flower now. Get ready for the weight


I’m out of likes, but great advice, and this part is the best advice. The scales are how different meters do the math to come up with the PPM number they display. They ALL measure the EC (electrolytic conductivity) of what you’re testing, then simply multiply to display the PPM value. You’d need to check with the manufacturer or the manual that came with your meter to know which scale it uses. I’d use the EC chart to eliminate any confusion as to which scale is used, but the main thing is to watch the plants. I’d use about half (at most) of what the manufacturer recommends, and see how they react, then adjust as needed.


That’s what I’ve done, I’ve been really conservative with the fertilizer.

My biggest worry now is the humidity and all the fun things that go with it.

Airflow airflow airflow. A nice boxfan would tune that worry right up. N some selective defoliation… dont strip-her tho


Yup, just a trim to let everything catch some of that breeze. They look great! :+1:


Purp got you covered. I agree about not going to far down the rabbit hole on measurements though. The plants will tell you what you need to know more than any meter will.

But if you want to do some learning about ppm here’s a link.

Edit to add notice it says pure water will not conduct electricity. RO water is close to pure as you can get at home. It’s absolutely a must to add any nutes or supplements before adjusting PH.


BlueLab makes the professional testing gear right?
I’ve already had to replace the $15 entry level Chinese ph pen, with the $40 version. I’m sure I’ll figure out what everyone eventually figures out, “gee, I could’ve paid for half of the Blue Lab model that will last a few years.”

I’ve also got three oscillating fans on them pretty much 24/7. The auto has its own dedicated fan.

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Thanks, you gents are best!

This is basically home growing in a nutshell. Everyone wants to start out on a beginners budget. Not realizing they get over $500 quickly. Then u learn n upgrade. Then learn more n upgrade. Something breaks then u save n upgrade. I just hate saying ‘go ahead and buy this over that’… yes it’ll obviously be better for the price, but u gotta learn what you need and what you dont.

N they are the cats ass. Thanks Jefe n Doob. :+1:t5::ok_hand:t5:


Nice @DoobieNoobie! Next time PPM come up, I’ll ask "is that the potassium or the sodium scale? :thinking: :face_with_monocle: " haha. :+1:

And I’m definitely some sort of ass, @PurpNGold74 ! :rofl:

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