Concrete is used in two ways with regards to the buried posts. There should be a concrete footing under the bottom of the post to distribute the loading of the building on the soil in a way that doesn't exceed the load bearing capacity of the soil. That generally means a concrete "footer" at least 16" in diameter under the bottom of the post. The second issue is the lateral support of a buried post. Pole barns derive some of their diagonal bracing from the lateral loading of the soil on the buried post. Different soils provide different lateral support, obviously organic soil will compact easily and provides little lateral support. The safest thing to do is backfill the posts with some kind of gravel that beds in solid and interlocks. So avoid backfill like pea gravel or washed rock, you want to use something that's mixed with fines and different size stone. Even better from a lateral loading perspective is concrete, but it's a bit expensive. There is also the issue of uplift of the posts, which is very important since they are the only thing preventing high wind from lifting one side of the structure out of the ground. Posts should have some kind of cleats attached near the bottom that resist forces trying to pull the post out of the ground. If you run a piece of rebar through the post and bed that in the concrete, that serves the purpose.
Pole barn construction technique
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