I'm not familiar with US Ag data for soils conditions, but perhaps that is common in your area. Do you have a local building department available to consult?

If you are going to build in an unusual site condition and/or where there are know problematic soil conditions, ie, expansive soils, history of erosion and slides, etc., then I would not hesitate to have a specific soils investigation and recommendations made.

Foundations are not where one wants problems or surprises. Exercise prudence! Good luck!