I'm getting anxious to put some early plants in the ground but it's just too soon. Witness my temperature taking device. Poke it in the ground and see how far the frost is. Although in the AM before the sun warms the ground the stick doesn't stick. Just as well, it's not scientific and I don't think it works.
I do notice that the south side is farther ahead than the other beds. Too bad that is my "wild" garden. But the slightly raised bed on the west is also getting warmer. Sometime I will put the details of my garden spots up.
About soil temperature though... I read that peas like 45F and you can measure the midday temperature for a few days to check that. I also found a great link for the NDAWN (North Dakota Agricultural Weather Network). You can pick a location and get soil readings for the area. Of course individual yards vary and special conditions like raised beds, soil covers, and wind protection make a big difference.
I have a link on the sidebar to a Victory Seeds Company page that republishes a table about how days to germination are effected by soil temperature. Wow, 9F makes a huge difference (136 days vs 31 days for onions). I wish I could find a good link to the original source. I have a book that includes the same table but also another about percent germination by soil temperature. Even though you "can" get spinach to germinate at 77F in 5 days, only 28% are reported to be successful (not to mention the success of the adult plants in heat).
So what's the bottom line? I want to plant, but for the most part I have to wait a bit longer. Since I have a small garden I can try some experiments and just put something in the ground and see.