Each project depending on locality is unique in its soil condition because of vegetation, lack thereof,  rock, mineral, water, etc... We analyze the situation and make our determination.  If our job consists of a cut and fill, all the site area must be stripped of all organic material and then amended.  For arena area as an example, it may need a base of small aggregate compacted in before the addition of a good footing material.  The surrounding area's for example may need no more than conditioning and graded out.