I used Adobe Photoshop CS3
I followed the directions from this tutorial with a few tweaks to fit what I wanted.
I made a grass rectangle bigger than I wanted it so i could crop off the parts I didnt want and shrink it to tile on the larger field.
http://www.myinkblog.com/2008/06/11/...-in-photoshop/
for lines and numbers I did solid white and took down the opacity a tiny bit
then I took one of the scrubbier looking brush tools and did a few layers of different browns for some dirt over the top
(down the middle: a lot between the hashes at the middle and at the goal lines, and then some other spots... Less is more i thought.)
I will post more about my field project as it comes together.