Drip irrigation schedule for young seedlings

New grower here. Quick background on my setup. I'm shooting for NFT but I'm still in the seedling care stage, trying to develop a nice root ball. My seedlings are now in 3" rockwool cubes. I rigged up a small ebb-and-flow system that ran every few days.

Eventually when my seedlings reached their second set of true leaves they started showing signs of deficiency. I began feeding them 1/4 conc. nutrients and they continued to degrade. I checked the pH inside the rockwool cube and it was at 6.4, which seems a little high. I lowered the res pH to 5.5 and fed with ebb and flow once more. No obvious improvements, plants were stunted, continued degrading, roots stopped growing for several days.

Yesterday I manually top-fed and I'm already seeing big improvements. Number of root tips nearly doubled out the bottom of the cubes and plants are looking much happier. Today I rigged up an automatic drip irrigation system in conjunction with my NFT and dropped in my cubes, so I've now got a nice combo setup.

My guess is my problems stemmed from the rockwool pH rising during the dry days and the upper portion of the rockwool keeping that high pH. To keep it down, what I'd like to do is top feed more often with the lower pH water. This seems pretty reasonable to do with drip irrigation since I'm not flooding my cube at each watering.

My seedlings just now have their third set of true leaves coming in strong with the 4th leaves peaking through. How often and how much should I be watering with drip irrigation at this point, given that I'd like to increase feeding frequency to keep that pH down. Is there a good heuristic for dialing in a drip feed schedule?

Thanks!

Tons of people like rockwool and get good results, honestly i'm not even going to fuck with that shit. I use rapid rooters and have never had any problems. Might want to look into those instead of rockwool next time.

Yeah not sure how people get good controllable results with rockwool, other than finding or following a procedure that "just works" and/or growing alkaline tolerant strains. I'm doing this based on taking lots of measurements and some experimenting.

I'm using RO and have soaked the rockwool in pH'd water for 24 hours. Based on experiments with pure rockwool I can confirm the pH drifts. And it retains soo much water. So it presents a catch 22. It's raising your pH yet you can't control for that without overwatering.

I would love to know more about what's going on in my cubes. They're surely more stable at full strength nutes and more frequent watering. As it is, they're wreaking havoc on my seedlings.

At the last watering I actually used a pipette to suck water right out of one cube containing a particularly unhealthy plant, and replenishing it with some dilute 1/4 conc pH adjusted solution. Kind of like plant CPR! We'll see how it goes.

Just to follow up on this for anyone else who comes along with rockwool and seedling problems. Don't let young seedlings sit in unwatered rockwool for multiple days! If you're using a saturating watering technique such as hand-dip or ebb and flow, you'll likely run in to the above problems. With top-drip you can more easily keep the root zone within reasonable parameters. I ran a 1gph drip feed system for 3 hours and totally flushed my cubes. After 3 hours my cubes were at 90% saturation. By the next day they had dropped to 75% saturation and I put them on a daily watering routine of 15 minutes in the morning, which represents ~3x the water weight of the cube. To really counter pH swings you need to flush, not just soak, so technique seems to matter.

The pH can rise from 5.8 to 6.4 in a day. If you only water every few days, it will stay at 6.4 and you'll get nutrient lockout. if you water every day (I set my res pH to 5.5-5.6), you get this nice 24 hour sinusoid in your cubes between pH of 5.8 and 6.3, and a total saturation of 75% - 90%. Pretty great.

Keep in mind this is with a 3" rockwool cube and at local conditions (75% rel humidity, 22 deg C). In terms of control theory you will need to "tune" your system accordingly. If you search Google you'll find a lot of over simplified advice saying to just water when your cube has dropped to 75% weight. This may be true, but not at the expense of letting your pH raise. Also there's both "volume" and "frequency" of watering. You can increase frequency and decrease volume. According to J. Benton Jones' Hydroponics: A Practical Guide for the Soilless Grower, increasing frequency means you should also decrease nutrient concentration.

I think if the community began thinking in terms of tuning your system parameters instead of broad sweeping subjective advice and rules-of-thumb, we'd have a lot more quality bud from even new growers--irrespective of technique or local conditions.