exceptions for farmland?

The real crux of the credit lies on the emphasis that sites earning points be both infill site AND sites that were previously developed. The goal is to conserve "greenfield" sites, the "prime farmland" component being one indicator of a greenfield. It's difficult to say if an exception could be mustered for a campus. My "gut" reaction is that it would not qualify; however, you can always attempt the credit and utilize an alternative compliance pathway to see if the LEED Reviewer will award a location-specific exception. You mention that a portion of the site soils qualifies, but if this is only a small portion on a campus within the existing campus boundary, you may have a case. It's worth a try!