My best guess when plants fail to develop as they should is lack of the proper nutrients. You seem to be providing sun and good drainage with organic matter to hold moisture so the low nitrogen plant food you are using may be the reason. I suggest you try a full spectrum plant food that contains not just the big three nutrients (NPK) but also the micro nutrients such as copper and boron that could be limiting growth. I use Osmocote Plus on my vegetables along with compost and other organic matter. The Osmocote contains 11 essential nutrients. Why not do a trial and use the Osmocote around one group of seedlings and then continue to feed the way you did last year for a similar row of seedlings. By the harvest season you should be able to tell if my guess about a lack nutrients is the answer to your question about poor root development. Let us know what happens and Keep Growing, Marianne Binetti