Help identifying seedling problem

I've had some wildly different responses on this when asking around and thought I'd ask here. These are 16 days from breaking the surface. Slow growth, roots no longer growing either and there are less than a dozen roots showing. I'm in RO water and started feeding them 1/4 concentrated solution (final 120ppm, 5.8 pH) once the second set of true leaves began to form. Water extracted from the cube itself has a pH of 6.3 and 150ppm. Lots more detailed information can be found in my newbie post, but thought I'd find more folks interested in plant diagnosis here.

Original post: https://www.rollitup.org/t/rockwool-seedlings-not-looking-great.840620/

Try adding cal/mag to your ro water. Pretty much mandatory. They need it for nutrient transport.

Those brown spots just beginning are Ca deficiency.

nute burn...too early to feed

Only one plant has the burning on the tips and that came several days after the light green leaves started, which I thought was deficiency.

In a previous experiment with bag seed I noticed signs of deficiency around second leaves, which seemed to be about the time the plants began seeking nutrients in the medium. That was with tap at 150ppm, and I began feeding at 120ppm in the RO this time. Having switched to RO it seemed reasonable to begin 1/4 conc. at second leaves.

One thing to note is that I recently discovered that my pH was climbing from 5.5 to 6.4 in the rockwool. One theory I have on my experimental attempt and this one (which showed very similar problems with no nutrients at all) is that my pH is drifting out of range and causing lockout even of its own nutrients. Could this be why I might see signs of deficiency and it not being able to absorb nutrition from its cotyldons, which remain green? It's using that higher pH water for transport I assume. Is this substantiated?

Is this nute burn because it generally looks like nute burn or is that an assumption based on the fact that I began feeding at second leaves? I've seen responses suggesting both toxicity and deficiency with regard to these photos.

Since plants normally have some nutrients in soil right away and don't need feeding for nearly a month, I'm confused as to how in soilless medium light nutrients would have this kind of impact?

It is hard to tell if its a deficiency or toxicity...we can come to an educated guess by what you are telling us youre doing to the plants

NeonTetra said:

Since plants normally have some nutrients in soil right away and don't need feeding for nearly a month, I'm confused as to how in soilless medium light nutrients would have this kind of impact?

Click to expand...

The nutrients in soil typically can't be utilized by the plant as quickly as bottle nutes. Additionally the plant already has some goods packed in the cotyledons to help grow through the seedling stage...it's like drinking a couple beers and feeling fine - then you take a couple shots of liquor and shit changes quick.

Looks like some PMS / too much water too imo...

Makes sense. I'm always trying to distinguish between those responses based on personal experience and those based on sound and thorough understanding of cannabis and plant botany. What can kill one plant can make another thrive from what I've seen online, so important to take in to account all the factors. In my case I'm curious as to seedling care in RO vs. regular tap, I've not seen much consistent information.

ayr0n, I'm watering by weight every 2-3 days when I see around 20-40% total weight loss.