Week 4 DWC update, chasing a calcium lockout

Running a Blue Dream pheno I've been holding onto for two runs now, currently day 28 of veg in a 4-bucket DWC setup under a 630w CMH with an HLG 100 for side fill. Reservoir temps have been holding at 67-68F which is where I want them. PPM is sitting at 820 (Bluelab, 500 scale) and pH has been stable at 5.9 for the last week. Problem started around day 22. New growth on two of the four plants came in with slight interveinal chlorosis and some upward cupping on the newer leaves. Classic signs of calcium deficiency or lockout, even though i was running 150ppm cal-mag on top of my Megacrop base. Checked the math and my total EC was landing at 1.64, which should be plenty for mid-veg. After ruling out root issues (roots are white, no slime), I bumped pH up to 6.1 and increased cal-mag to bring calcium contribution up to roughly 190ppm. Symptoms stopped progressing within 4 days. My working theory is that 5.9 is on the lower edge for calcium uptake in DWC and I was just barely missing the window, especially with the higher transpiration rates from the VPD sitting at 1.1 kPa under the HPS supplemental heat. Curious if anyone else has seen this pattern running Megacrop in DWC. The product is potassium-heavy and i wonder if there's an antagonism happening at the root zone that compounds the issue at lower pH. Would also take input on whether 6.1 is where you'd stabilize or if you let it drift up to 6.3 before correcting back down.

3 comments

3 Comments

Experienced6d ago

Potassium antagonism is real and probably underrated in the DWC world. But i'd push back slightly on blaming the pH alone. 5.9 is not aggressively low, i've run Blue Dream cuts at 5.8 to 6.0 for full cycles without that pattern showing up. What i think is actually doing more work here is the K-heavy formula stacking against your calcium at the root site, and the higher transpiration you mentioned is pulling water faster than the plant can move the calcium along with it. That combo is nasty. On your 6.1 versus 6.3 question, i personally let mine drift up toward 6.2 and pull it back. Sitting static at 6.1 is fine on paper but in practice DWC tends to drift anyway and you want a little range built in. I wouldn't let it sit at 6.3 long though, that's where you start losing your micros in the other direction. Your roots being white with those res temps is the one part of this whole picture i'd keep watching as the win. Most people fighting this issue in summer have root slime eating into it before they even check pH.

Seedling5d ago

I’ve been running some The Seed Pharm autos in coco lately and I’ve noticed similar calcium sensitivity when pH drops too low, even though it’s not DWC. The plant response you’re describing is very consistent with what a lot of people see in hydro when Ca uptake gets restricted. How are the new leaves looking now after the adjustment, any fresh growth coming in clean?

Seedling4d ago

Nice