Rockwool DTW Run Off Question

Hi all,
I recently installed a 4 x 8 top feed table running 6” rockwool cubes on top of slabs. Normally I would run this recirculating but didn't have the time to properly setup my reservoir and plumbing so decided to run them drain to waste for this cycle. Slabs were properly conditioned with 5.5 PH water for 24 hours prior to use.

My initial run off PH was at 6.5 and I have slowly dropped the PH of my nutrients to 5.0 which has brought my run off down to 6.2.

My question for experienced drain to waste growers is how low can you go with nutrient PH to bring down run off? I would like to get my run off under 6 and I think I would have to feed them 4.5 PH to accomplish this. Anyone ever done that?

I am at day 10 of flower and they are looking great but I'm not used to run off above 6 hydro/rockwool.

Thanks!

I have a similar setup and would like to know the answer to this one as well.

I've continued to feed 5.0 ph nutrients and run off is still 6.2 - 6.3 ph.

Using an Apera PH pen that is freshly calibrated so I know the numbers are correct.

If anyone with DTW experience has any input it would be greatly appreciated.

well, what I do is soak the cubes in low 5.0 ph water. then drain the water out and flush the cubes out with ~5.5 ph water half strength nuit mix while monitoring the runoff (blue lab guardian) until the run off is where I want it. then I'll water them with desired nuit mix a little.

I never adjust my ph due to the run off ph. I keep it between 5.7 - 6.2 depending on growth stage. I think maybe the rockwool hssnot been completely ph neutralized and still slowly breaking down causing high ph.

I'm not sure how long it's been since you've transplanted but maybe try mixing a low base feed and flush the cubes with 5.5 ph water. then the following day plan to do it again if it's still high.

If you're using grodan, I've noticed that the cubes coming from Canada are much easier to neutralize vs the ones coming from holland.

Nutrient ph can go as low as it needs to be too get to desired run of level(I use granulate).

Dreaded thanks for the advice.

I'm gonna try 4.0 ph nutrients next feeding to see if I can get run off below 6

Update:
Yesterday I fed with the nutrient PH adjusted to 4.0.
EC was 2.2 and I fed until about 30% run off.
Run off PH tested at 5.8

So far no negative reactions and plants are drinking heavily.

thanks for the help!

In my experience runoff PH in rockwool will tell you more about your feeding strength than anything else. Too heavy and the runoff is lower, too light and the runoff PH will be higher. the accumulated and concentrated salts are acidic and lower the PH.

Eli
Thanks for the response. I believe you are right. As I've increased the EC the PH has dropped.

I am now feeding EC 2.7 and pH is much more stable.

Thanks

Dronefreak said:

Dreaded thanks for the advice.

I'm gonna try 4.0 ph nutrients next feeding to see if I can get run off below 6

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Grodan strongly advises against this.

Grodan has extensive documentation about how to effectively use rockwool on their web site. I strongly encourage people to read their information. They know exactly what to do to help you dial your system in.

Start with the one called 'Water'.

GRODAN Literature and Brochures

Here's my recent experience with conditioning the Hugo blocks, fwiw. I had read somewhere that they changed the manufacturing process so that pre-conditioning was 'not as necessary' to bring the ph down. The blocks themselves have instructions that say to soak in ph 5.5 water for a half hour.

My problem with that method is that it doesn't flush the block, it just saturates the block. When the water is added from the top it flows through and out the bottom, which seems like what you really want, IMO.

In my case the other night, I was admittedly super high when I got excited and decided to get going with my Hugos. I poured ph 5.5 water through the block, only about a half gallon, and let it drain off. That's pretty laughable and lame of me in retrospect. The next day the runoff was sitting at about ph 7.

I fixed it by first running another 3/4 gallon of water through each at ph 5.4. After letting them sit level to settle for a couple minutes, I found that the blocks drain much more quickly at this point when turned so one corner points downward. I perched them this way in empty 5" pots for that and it worked great. I rotated the blocks 90 degrees after a few minutes to get the other corners draining too. It still takes quite a while to drain, but had I done that from the start I would have been good to go much faster.

After this I mixed up my nutrient solution. I adjusted this to ph 5.4. I then poured about 3/4 of a gallon through each block. After letting them sit level for a couple minutes I let them sit corner-down for quite a while until only occasional drips came out, because those early stage roots need plenty of air. My final runoff was at ph 5.7.

Today the runoff was at ph 6. So still trending upward a bit, but now within range, and I suspect that the upward swing will now stabilize. I will top-feed 8 ounce amounts of nutes at 5.5 once or twice a day right around the clone's base for a few days to keep them hydrated as they establish, then shift up to ph 5.8.

Summary: in the future I will run at least a full gallon of ph 5.4 water through each block first, and point the blocks corner-down to speed the flushing process. After that I'll run 3/4 of a gallon of nutrients through the block at ph 5.4 to fully flush it with that, then drain it really well and consider the blocks conditioned.

I've got kindergarten pencils in 8 days stacked hugo ftw

Update: my Hugos are completely stable at ph 6 now. I hit them with 5.6 nutrients. That will probably be adjusted upwards to 5.8 for the rest of the grow. The plants are going ape shit crazy; thick white roots out the sides and bottom. I gave them some KLN, so that sped things up a lot.