hello mate
how are you collecting the run off to take readings?
what line of nutrients and EC you feeding them?

Trash_2002 said:

hello mate
how are you collecting the run off to take readings?
what line of nutrients and EC you feeding them?

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The plants are on a wooden 4 x 8 table lined with panda film, the table is at a slight angle so the water drains off the one side in a tote with a submersible pump running outside. They are on 15mm pvc pipes to keep them off the "tray". The nutrients are just plain generic salts... a part a and part b (calcium nitrate). I add I pk booster in flower. Ec of nutrients is 1.8, ph 5.6. The runoff ec isn't much higher then the input so I'm not concerned about that at all, it's the PH that is breaking my brain. I've used the same nutrients for years. Only thing that changed is switching to wool

phill93 said:

The plants are on a wooden 4 x 8 table lined with panda film, the table is at a slight angle so the water drains off the one side in a tote with a submersible pump running outside. They are on 15mm pvc pipes to keep them off the "tray". The nutrients are just plain generic salts... a part a and part b (calcium nitrate). I add I pk booster in flower. Ec of nutrients is 1.8, ph 5.6. The runoff ec isn't much higher then the input so I'm not concerned about that at all, it's the PH that is breaking my brain. I've used the same nutrients for years. Only thing that changed is switching to wool

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so you using something like plant prod with cal nit?
that runoff reading is skewed cause it passes through the tray before you measure it tho.

I would up temp in res to 21c if possible and PH to 5.8 in rockwool.

RO water buffer capabilities is zero so you rely on buffer capabilities of your nutrients witch is fine, the drift between 5.8-6.1 inside the cubes is ok and beneficial. Just make sure to ph your reservoir right before feedin. When you used tap water you had this up overnight in PH in the reservoir too?

how many feeds per day? raising feed numbers and lowering runoff and/or giving it more dryback to acidify the cubes could help, you have to test there with your setup. Nitrates are acid so the dryer the cube the more acidic and higher media EC it gets to some extent.

EDIT - Also get a hanna soil PH tester to really test inside the cube and not the run off,you gonna get much more accurate readings.

The sad part is I bought 4 boxes of 12 8x8x8 inch cubes that I won't be using if I can't get to the bottom of this

Trash_2002 said:

so you using something like plant prod with cal nit?
that runoff reading is skewed cause it passes through the tray before you measure it tho.

I would up temp in res to 21c if possible and PH to 5.8 in rockwool.

RO water buffer capabilities is zero so you rely on buffer capabilities of your nutrients witch is fine, the drift between 5.8-6.1 is ok. just make sure to ph your reservoir right before feedin. When you used tap water you had this up in PH in the reservoir too?

how many feeds per day? raising feed numbers and lowering runoff and/or giving it more dryback to acidify the cubes could help, you have to test there with your setup.

EDIT - Also get a hanna soil PH tester to really test inside the cube and not the run off,you gonna get much more accurate readings.

Click to expand...

Thank you for the reply. These are mother plants so I don't have any drippers etc installed in this tent, I have a open room that I use to flower plants in and a separate tent for veg. They are being top fed with a water wand atm, I was feeding this way every second day untill the PH issue, I've been feeding daily with significant runoff to try and get the cubes to stabilize but that hasn't happened yet. When I take the runoff I usually let the tray drain for a minute or two before taking runoff so some contamination is possible I suppose so it would be fairly limited because most of it would already have drained by the time I take the readings.

I stay in South Africa so I didn't mention the nutrient brand because it probably won't be available in other countries, it's a basic two part hydroponic nutrient with monopotassium phosphate as a bloom booster. I will attach a photo of the ingredients in this post.

Since I've noticed the issue a couple of days ago I test the res ph every time before I feed. When I was using tap water I was using ph down to get the ph in range (it would be at around 7 after I mixed-up nutrients) but noticed that it drifted up in the res. I've since switched to RO water and have to use PH up to get it to where I want it but at least now there isn't a drift upwards anymore like what happened with the tap water.
I can run TA flashclean through the cubes with oh water to try and lower the PH although I'm not certain it would work. At this point I've ran gallons through the blocks and the ph still comes out 0.4 to 0.5 points higher.

Edit: a bluelab substrate meter is on my wish list, I'll pick one up in the next month or two. I already spent a lot of money in the last month on grow equipment, I won't be able to justify spending more. Gotta keep the old lady happy

phill93 said:

The sad part is I bought 4 boxes of 12 8x8x8 inch cubes that I won't be using if I can't get to the bottom of this

Thank you for the reply. These are mother plants so I don't have any drippers etc installed in this tent, I have a open room that I use to flower plants in and a separate tent for veg. They are being top fed with a water wand atm, I was feeding this way every second day untill the PH issue, I've been feeding daily with significant runoff to try and get the cubes to stabilize but that hasn't happened yet. When I take the runoff I usually let the tray drain for a minute or two before taking runoff so some contamination is possible I suppose so it would be fairly limited because most of it would already have drained by the time I take the readings.

I stay in South Africa so I didn't mention the nutrient brand because it probably won't be available in other countries, it's a basic two part hydroponic nutrient with monopotassium phosphate as a bloom booster. I will attach a photo of the ingredients in this post.

Since I've noticed the issue a couple of days ago I test the res ph every time before I feed. When I was using tap water I was using ph down to get the ph in range (it would be at around 7 after I mixed-up nutrients) but noticed that it drifted up in the res. I've since switched to RO water and have to use PH up to get it to where I want it but at least now there isn't a drift upwards anymore like what happened with the tap water.
I can run TA flashclean through the cubes with oh water to try and lower the PH although I'm not certain it would work. At this point I've ran gallons through the blocks and the ph still comes out 0.4 to 0.5 points higher

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Runoff readings are overrated imho, you could be in range and reading wrong results this way.

get this and all calibration/storage solutions too, calibrate it monthly.
Hanna PH Soil Tester

Just wanted to do an update for anyone who might have wondered what happened.
I ended up transplanting the plants into 25L pots with coco/perlite and the issues with runoff ph being high went away. The 6 and 8" rockwool blocks was a cheap brand (the 4" blocks were grodan) and I think there may have been an issue with the blocks because of the lime binder or sometimes. Anyway, I'm done with rockwool, I'll stick to coir. I ended up keeping 4 of the plants after I took the cuttings I wanted to and flipped them to flower about a week back while I was waiting for the clones to root for my main flower room. Attached are pictures i took today of the 4 i kept. I have these under 2 x 1000w mh's. Happy and healthy

Rockwool has a high PH anyway, doesn't it?

That's what I've always read anyway.

GanjaJack said:

Rockwool has a high PH anyway, doesn't it?

That's what I've always read anyway.

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Correct, slightly higher then neutral at 7.5 but after the cubes have been soaked in ph adjusted water they should be stable at 5.5 or 6... don't know, I won't mess with rockwool again untill I've personally spent some time at a facility that uses it successfully. My clones root perfectly in rockwool but trying to grow in the stuff was a failure