The slender metal tool on the rim of the flat is a widger, a tool for transplanting seedlings from their starter medium (which is usually a very low nutrient, sterile, finely textured mix) into a more nutritious medium for their next stage of growth.
My father-in-law, an engineer, used to remind us that it was important to use a tool designed expressly for whatever task was at hand (“the right tool for the right job”). On Friday, I used my widger to lift the delicate little potato seedlings from their very small starter flat, and to open up planting-holes in the potting mix that fills the larger flat that will be their home for the next few weeks. It was the right tool for the job.
However, before I had my widger, I used a table knife for the same purpose, and a good friend uses an old paring knife. My father-in-law might not have approved (he has been gone for ten or so years), but it is unlikely that I would have harmed the table knife by using it in this way, and it worked just fine.
He would have been glad that I finally used the appropriate tool, though. Happily, a widger is not an expensive item. Mine is from Bountiful Gardens, and it cost $5 plus S/H.
Joe (husband) made the wooden flat to fit in the baker’s rack that stands by the back door. While the seedlings are small, two fluorescent bulbs lie across the top of the flat. As the seedlings grow, those lights will be raised slowly (suspended by strings) to stay just an inch or two above the tops of the plants.