pH really low in Uni Slabs

Iam having a problem with some of my plants in 4" short blocks on top of Grodan Uni Slabs. The problem plants are 3 and 1/2 weeks into 12 - 12 flowering photoperiod. Visual symptoms are yellowing and dying leaves starting at the bottom and working up. All fan leaves that are not yellow or dead are light green. Inner bud leaves are healthy. Iam feeding a simple 2 part grow and bloom full profile base and recently switched to the bloom after 2 weeks 12 - 12 of grow base. Feeds have been steady between 700 and 800 ppm using the Hanah 500 EC to PPM conversion scale. pH input solution is 5.6 to 5.9. This is a non recovery drain to waste system. Slight trickle of runoff and this is probably where my problems started weeks ago. Apparently not enough runnoff. I just went downstairs and took a big syringe sample from one of the most yellow and dying leaved plants from several spots from within the rockwool slab. I tested both PPM and pH on the test sample (Bluelab Combo - cleaned and calibrated) and return was a whopping 1450 ppm and a very low pH of 4.5. I confirmed the pH sample test using bromothymol blue drops/vial and color chart. It was reddish pink. Any experienced rockwool growers have any suggestions? Should I make up a tank of base nutrient solution (tap water is 120 ppm at 7.7 pH) at 600 to 650 ppm total and no pH adjustment? It comes out at 6.3 to 6.5 after adding the nutrient fertilizer before adding pH down. Fertigate the slabs until slab samples hit close to target ? (5.6 - 5.9 pH and 700 to 800 ppm???)

Definitely need more runoff.

In between irrigations the nutrient solution partially evaporates. This pH should drop to around 5.3-5.4 at it's lowest. Naturally, your starting pH will vary depending on your setup and environment.

That being said, too strong of a nutrient solution burns roots and causes a pH drop. (I suspect this is why cannabis grows so dark and vigorous in highly amended, high pH organic soil, after the initial stall in growth. The roots are burning and rotting, causing the pH to drop. When the pH reaches the right point, super healthy growth begins. lol)

At any rate, flooding them with plenty of runoff should reset the situation. Once you've done this, check the health appearance and root zone pH again. Your feed may even be too high at 700-800. With the wrong reflector, even a 1000w HPS grow can have 500ppm be too much.

Douglas.Curtis said:

Definitely need more runoff.

In between irrigations the nutrient solution partially evaporates. This pH should drop to around 5.3-5.4 at it's lowest. Naturally, your starting pH will vary depending on your setup and environment.

That being said, too strong of a nutrient solution burns roots and causes a pH drop. (I suspect this is why cannabis grows so dark and vigorous in highly amended, high pH organic soil, after the initial stall in growth. The roots are burning and rotting, causing the pH to drop. When the pH reaches the right point, super healthy growth begins. lol)

At any rate, flooding them with plenty of runoff should reset the situation. Once you've done this, check the health appearance and root zone pH again. Your feed may even be too high at 700-800. With the wrong reflector, even a 1000w HPS grow can have 500ppm be too much.

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Yep. Iam going to leach these rockwool slabs tonight and get it balanced out. Thx. And yes. I recently got new reflectors and bulbs for my 8x600 watt room. From a multiple t5 veg room (they were super green and healthy ...not a purple stem or petiole anywhere) to the new flower room with basically new 600 watt hps fixtures (the old reflectors and bulbs were old and dirty) they lost that deep healthy green color. And I will bump down my feed strength to may be 650 to 700 ppm and get the pH right. I have major salt build up and super low pH (4.5 eeekks!!) In the rockwool slabs. Right before flower I did let them slabs dry down too much and when I did fertigate...it was with little runoff. Major lower yellow leaf drop so early in flower. Leaching the substrate tonight. Thx again.

It's possible your sick plants will not get much better no matter what and not need much more than 600 ppm, and long hard flushing or weird pH or ppm maneuvers would be more stress. You may need to water more often as well? At the point you're at, I need to water twice as often, combined with 15-20% the volume of the medium in runoff. For me the starting water is worse and is neutralized with phosphoric acid.

The ratio of nitrate (-) to ammonium (+) has importance. Like GH Flora has 7x more nitrate N than ammonium. Plants essentially exchange H for NH4. Also a little runoff might preferentially leach nitrate and leave ammonium and anything else that's less mobile, and with no runoff the unabsorbed ammonium can get converted to nitrate.

So then you need to raise the Ph of your feed solution to bring your run off ph up to range that your stuff needs to be at.

Watering long, and with perhaps lower nutrients levels. Some of that high ppm levels could be more build up from the ppm of your starting water.

Then are you doing a run to waste or a recirculating system?

Drip to waste. Start water is tap at 0.2 EC or 110 ppm at 7.5 pH. My tap water is damn near perfect for just about any medium substrate and base fertilizer. Iam for sure positive now it was not enough run off now. It was drying down too much between fertigation cycles...concentrating a shit ton of salts and dropping pH. I been giving them 1.3 EC total (0.2 of this is tap) at about 6.1 pH. Tons of runoff. And the plants kicked right back in the groove 2 days later. Leaves that are damaged/yellow/crispy I reckon wont recover. I just need these fat colas anyways. Thx again people.