From the list of crops you listed, I would say the soybeans would be most sensitive to low pH.  You should probably have soil pH close to 6.5 for the soybeans, although there could be a bit of leeway in that. 

All the other crops, I think you would want to have at least 6.0, with between 6.0-6.5 being preferable.  Oats can handle a lower pH, as well as rye.  Still, I would want to see that pH up to 6.0 for these crops as well. 

What are your soil sulfur levels like?  If you are driving down the soil pH with the elemental sulfur, is it time to back off on the elemental sulfur a bit?  Just a question I am asking.

With your inherit soil pH levels being 7-8.5 (evidenced by the lower depths), I would determine what you are doing to drive that pH down, and try to moderate those practices a bit - if you can.