Just had to throw my 2 cents in here. I think high tunnels are a great idea for season extension, but I have to agree that heating them all might be a bit much. You should be able to get an extra month of growing season on either end without worrying too much about additional heat. I'd also not necessarily use them for just tender crops, everything tends to germinate and grow better with a little more heat in the springtime. I tried beets and lettuce in my greenhouse this year, the leaf lettuce was ready by the end of April and the beets by the end of May.
Also regarding the grass growing better near the straw bales: my bet would be this has far more to do with moisture than with any nutrients in the straw. Which is another plus of plastic cover come to think of it. Straw (and sawdust) tend to suck nitrogen out of the soil while they rot, although they release it back when they're done. Nitrogen is the main limiter fro grass growth as I understand it. Also, the area immediately under your bale would have had nothing growing due to no light, so the roots of the grass around the edge had less competition to deal with on one side as well. Also, on the south side of the bale there would have been a warmer microclimate for the grass, although I don't know if that would have much effect.
Another note regarding soil fertility: in my greenhouse I never really meant to grow much directly in the ground so I dug it down for extra cold protection. The soil in there is pretty much all the brown lower layer. And yet the beets and lettuce did fine. I grew tomatoes and peppers as well and had some issues with fruit set and blossom end rot, but still managed a far larger crop than I would have outside in better soil.
Anyway, just saying plants have a lot of nutrient requirements, but sometimes people overlook the biggest ones which are sunlight and water.
