I wrote an answer to this last night, but it did not post for some reason.

I use 1/2 cup Biz with 1/2 to 1 cup of Tide with bleach in just enough hot water to dissolve the powders and cover the item. I soak for about 30 minutes stirring every 10 minutes or so with a wooden spoon. Then I run the item through the washing machine mixing the solution in with the washer's water.

Good luck. BTW - I have always been of the mind that if the clothes soaked with red clay get ruined, then I can live with it since it was already ruined by the clay beforehand. I use this same solution on soiled baby clothes with great results. My son's baseball pants and shirts come out great most of the time.

I haven't tried this personally, but my sister-in-law swears that Resolve Carpet Cleaner gets red clay stains out of clothes. Her son plays baseball and she uses the Resolve on his uniform.

TKPP, STPP or TSP (phosphates) thrown in with your detergent of choice... you end up with super soft water and then the detergent can work.

To add a little humor.....

One person's junk is another one's treasure - LOL
http://www.dirtshirt.com/default.aspx?categoryID=1

Bet they don't need detergent here :-)

Here is a link that might be useful: http://www.dirtshirt.com/default.aspx?categoryID=1

The red in the clay is iron deposits, so try something for rust--starting with a little vinegar on an edge of the stain. Test the fabric first!

My brother has that shirt!