If you can, I would mix in an excessive amount of pure, light, pearly white perlite (like 1.25:1), with some extra worm castings (like 1:70) and some epsom salt (like 1:300). Azomite is also very good to add (like 1:100) if you can get easily. My soil base is usually ffof, but adding a 1:1 of ffhf is recommended. Also adding scotts seedling mix at (about 1:3) is something I do frequently for seedlings.
Anyways I think you should also cut holes in pot to get air pruning as I've had seedlings in my original trials with seeds grow exactly like this. I had bagseed in 2l soda with duct tape and mg soil with moisture control. Was under 40w of cfl 2700k. At 30 days the roots were to the bottom but plant was 4.5 inch tall. Roots were 95% vertical tap roots (like 10in) with very minimal horizontal rooting (like .5 in either way. Roots were also only on pot walls. Inadequate aeration and high nutrient content is what made mine look like that.
@J Bleezy some of the solo cupper do. The photoperiod plants only flower when mature, so some plants get 2.5-3ft tall. As long as the roots are healthy and consist of a majority of feeder roots you can achieve similar results in soiless as in hydro. And all a hydro plant needs to grow huge is a couple inches of healthy roots and an appropriate nutrient solution and in correct dilution.
