pH goes low in some plants

I had this posted on Grow Journals, but I’m having issues and need advice!

Plants are about 3 weeks old from breaking ground. Just installed the new HLG light and watered with 5mg of 20-9-20 fertilizer in 1 gallon with 5ml of CalMag added.

pH of the nute mix was about 6.5, ppm about 1400.

Up to this point the plants have gotten the same treatment but I have some tuning to do as my runoff is pretty wild.
Here are the runoff numbers:
Northern Lights: 5.8 and 1800
Amnesia Haze: 6.17, 2033
Blue Dream: 6.13, 1842
Amnesia: 5.9, 2176
Lee Roy: 5.12, 1300
Jack Herer: 5.0, 1326

What do you guys think I need to change? That low pH with low ppm is kinda interesting…

Soil is FFOF.

Light was just an ES200 for the last week until the HLG Scorpion RSpec Far Red came in. Before that, I had them in my other tent with the ES300’s so light was a lot less over the last 7 days.

20-4 light schedule

Should I water with some higher pH water on those 2 plants that are near 5.0? Flush the hell out of them with tap? Wait it out?

I checked the calibration on the pH meter, it was good.

Thank you in advance for the advice!


FFOF runs 4-6 weeks with no nutrients needed. Runoff should be around ~1000 before feeding.

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Dont feed anything grown in FF soil for at least 4 to 6 weeks. Its loaded already. Wait till water runoff is bellow 1000 tds then feed.

Nutrient in soil drives ph down, it will rise as nute levels drop. Also increase feed ph to 6.8

Dont worry about flushing until between weeks 5 and 6, 8 and 9 then water only last two weeks.

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Should I flush? Or just water with pH of 6.8 with no nutes until ppm is below 1000?

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Your numbers aren’t outlandish, water only next once or twice

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I typically recommend 2K in soil as your cutoff point and 1K in soilless. If your runoff is higher than 2K in FFOF I’d simply do water only or water/cal mag until TDS drops below 2K.

Low PH has been a common problem reported on this forum multiple times with OF. I don’t see anything that the plant/plants can’t handle though. Good to watch and media is going to acidify anyway as roots die during flower.


What about the lowest pH being in the plants that have the lowest ppm?

Seems like those plants with good nute usage should have higher pH as the nutes are consumed. But, these show the opposite, they are eating AND the pH is dropping.

So it’s maintaining the 2k once it drops to around that point in soil? After a couple of weeks of feeding using Jacks, my runoff stays around 11-1200. I feed ar around the same. Would feeding sooner, around the 2k point, at same tds, keep it around the 2k point? Sorry this is kinda loaded. I seem to hit slight deficiencies in flower even bumping up the pt A.

I’ve only just started watering these plants to the point of runoff. I’ve been using the 20-9-20 fertilizer since they were transplanted at about 2 weeks above ground and they were a bit light green.

So, I’m seeing the pH the LOWEST on plants with the lowest ppm. Since nutes lower pH, it makes sense that my low ppm plants would have a higher pH.

I don’t have that. I have low ppm and low pH.

Water with just water on a couple days and see what happens?

Yeah; soil buffers differently than soilless media. You certainly could have plants showing a def at 1K but you could also see an excess with some plants at 2K. But remember if you checked runoff in fresh OF it would be 4 or 5k. It’s a good rule of thumb but your mileage may vary lol.

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Thinking of avoiding the FFOF next grow. I’ve read a few posts on this forum complaining about pH issues. Very strange that some of my plants are great while others have low pH and they were all treated the same as far as watering/nutes.

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I’ve slowly been steering away from it. I’ve been using Roots Organic 707 more lately. Did a couple slurry tests from different bags and it measured 4-500. Mixed couple 3 gallons with the RO and FF, and have done one or two in just RO. When using straight FFOF, I’ve had to do a flush even with Jack’s. When mixing or just using the RO, no issues.

I’m flushing out the 2 low pH plants. Chasing pH is really frustrating. Maybe these two plants got the same bag of FFOF, I fill two planters with a single bag. The other 4 plants were ok but these 2 were way off. Makes sense that they would be the same bag.

I’ll probably mix my own soil next round.