The Well Grounded Broadcaster

Low power broadcasters who use the AM band can really improve range and audio quality by installing an effective Radio Frequency (RF) ground system. A good RF ground system depends, in part, on a natural phenomenon called "ground conductivity." This refers to the electrical properties of minerals in the local soil. Read more at the FCC web site page titled "Map of Effective Ground Conductivity in the USA"

So how does a broadcaster construct an effective ground? Two transmitter manufacturers have provided some tips here:
Hamilton Rangemaster Grounding Methods
and in the PDF file of Ground Hints

Also some nice info from Information Station Specialists:
Why a Groundplane is Important, How It Works & Is Installed

Ham radio enthusiasts, especially those who work the 160 meter band (which is directly above the AM band at 1800 KHz) provide a rich source of helpful hints on setting up an effective ground system:
Grounding methodology
Good Ground is Hard to Find
Grounding in RF Environments

One last technical note and caution - true RF grounds should not radiate. Most conductors carrying RF current will radiate if not buried in the earth. This phenomenon means a long ground lead can cause compliance problems with the FCC. Instead of constructing an antenna with a ground, you might inadvertently build something engineers call an "off-center fed dipole." Building an off-center fed dipole could earn an FCC citation, like this one:
FCC Citation for Excessive Ground Lead