Anything you can add to the nursery soil to lighten it would be great. I've used rice hulls, perlite and sphagnum peat to lighten mine. Straw may help, but I've never used it. I wonder if the nursery would have potting soil more suitable that you could purchase? Ours has a variety of soils.
The nursery near us is very basic. In the city there is a nursery that has a “seedling” soil, but I’ll have to wait. It’s over 4 hours away. I think I’ll try the coco, perlite, worm casing mix with a small amount of the black soil. So frustrating …..
It's worth a try. Good luck. And check the weight of the pot when it's wet, then when it's dry. Check the weight each day and water when the pot is light. But if you see the leaves droop and it's light, time to water around the outside of the pot.
Yes that’s good, I forgot to mention I’m running a Kingled 400w LED light. It’s 28” above my seedling.