I built a farm wagon on brand new Horst running gear - 20,000# capacity. I bought new ribbed field tires for it at Les Schwab. They are 11L-15SL 10 ply - - whatever that means. Sidewalls say 42psi - that's what I run them at.
However - now that I opened your picture - those tires are what are sometimes called "super wides". I'd start at 22 to 24 psi and see where that gets you.