Removing Mud/Clay Stains from Soft Doors

[85 USMC M998]

This may seem strange but I cannot seem to get the mud/clay stains off the soft doors. Was bad and let it sit for a few days. The 4 letter word (work) kept me from it.
Tried dish soap and Car wash (different times) with a stiff brush to no avail.
Any thoughts?
Thanks

 

[Floridianson]

12% pool chlorine?

 

[Keith_J]

Bleach works on organic stains, not minerals (clay). Pressure washer, start at lowest pressure and work up. Detergent helps, clays have strong ionic surface charges.