Here is an interesting story by Eddie McGriff from the Southeast Farm Press about the impacts of a layer of hardpan below the surface of an Alabama cotton field on plant growth. One of the most interesting impacts to me as a climatologist was the impact of breaking up the hardpan on floods and droughts. The farmers noted that after breaking up the hardpan heavy rain was able to soak into the ground instead of running off into the ditches beside the fields, resulting in more soil moisture. The disruption of the hardpan also allowed roots of the cotton plants to move deeper into the ground, making them more drought resistant. You can read more about the impacts of the hardpan here.