He's talking about the root that is curling in a circle in the upper righthand corner. There were a bunch of those growing out from where the 4" cubes connected to the slabs. They just hung out absorbing moisture vapers because the flood tray was covered in panda film.
i personally like dry amendments as wizzle suggested so would go down that route also add wormcastings with the amendments you use it works well every 2 weeks to a month is a good window depending on the stage your plants are in.
for example topdress with something like 50%neem meal mixed with 50% wormcastings in veg wait 2-3 weeks then a week before flipping add 40-50% bat guano/20% neem/30% wormcastings etc and just use straight water on them they’ll be happy with that regimen. you can replace neem with another high N amendment and you can replace bat guano with another high P amendment these are just what i have.