Rockwool slabs and very low ph

[Sipulisakari]

Hi everyone!

I'm a first time grower and decided to go with rockwool slabs with drippers setup. Now i have a weird problem. Runoff ph is 4.3 while i feed the plant with solution ph'd to 5.8-6.0.

Four days ago I flushed the grow with Ph'd water but situation stayded exactly the same. Runoff ph goes down to 4.3.

I have checked the roots as much as those are visible from the bottom of the slabs and they look perfectly white andfine. There isn't any dubious scents either. That seems to me that I can exclude pythium from possible causes of rapid ph decreasing. (which on the other hand is the only reason what would explain so huge ph fluctuation)

Tap water is used for watering. My tap water is very soft at 90 ppm. It is disinfected with sodium hypoclorite by water treatment plant. The amount of chlorine is 1 mg/l according to information given by water plant. I have no possibility to leave water to sunlight to get rid of the chlorine so i have to use it as it is. I still don't think that chlorine in so little amounts would make a difference.

My tap water has the ph of 8.4. I have to use quite a lot of ph down to get the ph to desired levels. I use ph down of Hesi nutrient line which is 59% phosphoric acid.

Another thing worrying me in this grow is that plant seem to stretching too much. I have been given them a half of the recommended dosage. Anything over that will lead to dark green leaves. This might be connected to that low ph problem as I have read that low ph makes nitrogen available to plants which would explain why i need to keep so low EC.

Overall plants are growing well. I'm scared that I will have bigger problems with this low ph in near future.

Any ideas what I should do/check with this Ph situation? I'll provide more information if needed. Thank you all in advance!


General information about the grow:

1.2m x 1.2m (4ft x 4ft) tent
600W HPS
GiB dripper irrigating system giving ~2dl (1 cup) / plant 4xday
60 liter (16 gallon) reservoir
23 plants in 5 rockwool slabs
temperature 24C/76F on canopy
plants vegged for 10 days and flowering for 14 days
Hesi Hydro nutrient line as nutrients
EC of solution 0.5-0.6 which sounds a bit low but higher will give dark leaves and claws.

Here's couple pics also

 

[aridbud]

Get pH UP or DOWN and adjust accordingly.

 

[Phaeton]

I use rockwool croutons (1 cm cubes) but should be similar.
I use GH pH down which is phosphoric acid which is also similar.

My tapwater starts at 7.2 pH with 219 ppm of calcium iron and magnesium.

The rockwool is soaked in 5.3 pH for a day before draining and using. Reservoir is started at 5.5 pH and allowed to climb to 6.3 over a week's time.
EC is .8
Flood and drain is done every 4.5 hours during bud and every 8 hours during veg.
The runoff varies by less than .1 with this system. The nutrient is in and drained within fifteen minutes, leaving little time for change.

I have not used the dripper method. It appears to use a steady state flow that matches the plant's usage.
If that is so then evaporation becomes a major player due to the time spent moving slowly in the rockwool.
Different chemicals will precipitate out at different concentrations, altering the makeup of the nutrient solution.
A chemical substance is causing a pH of 8.4 and phosphoric acid is used to balance the solution, a fair amount as that is pretty alkaline at the tap.
There may be a sludge buildup in the rockwool from some low solvability substance coagulating as the solution thickens. This would leave the runoff with the phosphoric acid still there but the alkaline substance precipitated out.
A gigantic drop in pH would result.

Can a container of water with a starting pH of 7 be found and used for a week to see if an additive that dissolves poorly may be causing the present problem?