DWC Adjusting GH Nutes In Flower To Counteract pH Drop

[copperrein]

Blueberry Cookies (Mostly Indica)
Have two plants, only one plant is affected
Day 19 of flower in a Vivosun 2x4 tent.
5 gallon DWC started in rockwool and currently residing in hydroton
Using straight well water with a starting PPM of 400-ish
Reservoir bopping around 62f-74f depending on day or night
Growing under a Spider Farmer SF2000
Temp of tent: 70.3f () RH: 40% () Light distance from top of plants: 21" )
pH 5.3 () 2.546 EC () TDS 1.188 () Water level: 9.5" () res temp 68.7f ()
No pests. I do a res change once a week, following the GH Flora Series chart. I'm using the base nutes at 'early bloom' strength per the chart PLUS Liquid Kool Bloom and Floralicious

Two days ago this plant's pH was 5.8. This morning it's 5.3. EC has barely changed....I feel like something is getting locked out? There's zero evidence for that happening (plants look good) but I'm watching that one number drop and I'm getting worried.

This is my first ever indoor grow so know that I'm going full Chicken Little atm.

BBC 2021 Indoor DWC left

• copperrein
• Feb 5, 2021

 

[TysonOG]

pH will fluctuate like that and I adjust daily as necessary. When it's dropping like that, I'll start it at 6.0 and let it drop to 5.5 or so and adjust it back up. With a 5 gallon res (presumably about 3.5 gallon of solution) you will see more of a fluctuation than in a larger res. I grow in 10 gallon reservoirs and still see decent fluctuations. I would say your ec is very high by my experience, although the plant looks healthy so not an issue I guess, though I never exceed 1.0 ec during the grow.
I always experience this pH drop around weeks 3-5 of flower as the plant really absorbs the potassium and phosphorus. These elements are naturally alkaline so when the plant is sucking them up at a high rate, the pH tends to drop. I use GH nutes as well.
Your res temps seem pretty good and you change weekly like I do so all should be good there. Make sure you dont notice and smells in the res. I use a chiller and H2O2 so I've kind of forgotten about root rot or anything like that, but I've seen instances where pH swings are due to root health issues.

 

[copperrein]

These elements are naturally alkaline so when the plant is sucking them up at a high rate, the pH tends to drop.

Oh duuuuh. Hah! That's really...uh....simple >.> Thank you...I've been reading about cation exchange getting more and more confused.

So during veg the plant is taking a more balanced serving of nutes more slowly...thus the ratio of NPK+water stays relatively the same between res changes?

 

[copperrein]

Also if you think that EC was scary...

Blueberry Cookies day 43 veg
Left: pH 5.9 () EC 2680() TDS 1224 ()
Right: pH 6.0 () EC 3274 () TDS 1566 ()

 

[TysonOG]

That ec is scary! But hey, whatever works for ya. In veg, my pH tends to stay pretty constant.