I was going to suggest trying Dawn. It worked great to get sticker residue out of my daughter's favorite shirt that I thought was a lost cause.
I found this online:
Cover your work surface with a clean towel so red clay dirt stains don't transfer.
Wet the red clay dirt stain on your baseball pants with water, then apply enough liquid hand dishwashing soap to cover the stain. Work the soap into the stain and the fabric by rubbing the fabric together. Leave the dishwashing soap on your baseball pants overnight.
Wash your baseball pants in the washing machine as usual. If the red clay dirt stain on your baseball pants persists, mix a paste of powdered dishwasher detergent and water. Rub the paste into the red clay dirt stain and allow it to soak into the stain for three hours.
Launder your baseball pants again to remove the dishwasher detergent and remnants of the red clay dirt stains. Make sure the stain is fully removed before placing your baseball pants in the dryer.
Read more: How To Clean Dirt-Stained Baseball Pants | eHow.com http://www.ehow.com/how_###-###-####_clean-dirt_stained-b...