Force required to vertically pull a post out of ground with various soil types

I am trying to determine the force required to vertically lift a typical traffic sign post (round, square and/or channel) out of the ground based on typical soil types at standard installation below grades depths of 30" and 36". To make the calculation simpler, assume that the post coming out of the ground is just a 4" stub and that actual sign assembly is not part of the equation.