Looks more like a fungus than a mineral deficiency,
simply because the leaves are not curled.
Look at this photo (closest I could find
in the Texas A&M guide):
http://aggie-horticulture.tamu.edu/t...eaves/19b.html
Here is a page of mineral deficiency pictures:
http://4e.plantphys.net/article.php?ch=5&id=289
One problem is that those pictures are of more advanced
stages of deficiency disease than your seedlings could
have yet, but in everything that even looks close there
is leaf curl, too.
Here is another set:
http://4e.plantphys.net/article.php?ch=5&id=289
You can see that manganese deficiency is the closest
in those photos, but even there it shows some leaf curl
at the edges.