"...stacks of "hay" bales start smoldering, and eventually burning, all on their own in central CA. Spontanious combustion the fire guys say..."

Yes, that does happen with hay (probably straw, too) when they were too moist when baled. It's a form of composting, but with the large amount of hay stacked together, the heat can't dissipate.

When someone is building a straw bale house, they have a little meter with a probe on the end, and they insert it into the bales to check for moisture. It has to be less than 12 or 14%, IIRC.

Sue