I can not understand your reluctance to cement the posts. The cement does not have to come completely to the surface of the ground. If you are concerned about the stability of the fences as you say you are, then concrete is the only way to go. I'm sure there would be companies that would just bury the posts in soil if that's what you wished, but if it was my operation, I wouldn't take the job because when it leaned and sagged, it would not reflect well on my company. If anyone did take the job, I would expect them to require you to sign an agreement that you would have no free adjustments when the fence started to lean or sag.