Fels-Naptha for 9 years. I like white pants to be white. For the really tough stains like blood, grass and clay, I use a scrub brush. Wet the stain first, get some Fels soap on your brush and brush in one direction with the water running over it in the sink. Still not completely gone? Work in some soap with your brush and let pants sit for 10-30 minutes and then throw them in the washer and wash with hot water and regular detergent. Comes out white every time. Also... I NEVER put my kids uniforms in the dryer. Sets the stains, dries out the material and makes them prone to ripping/staining. Also, never use bleach. It destroys the fibers. They will loose their stretch.
A few years ago I watched a video where a guy power washed his kids pants clean. I tried it and was amazed how much dirt came out. I should stay pushed out. Baseball pant material is woven really tight. The dirt and stuff gets into the weave and can be hard to get out. That's why I started using the scrub brush under running water. It pushes the dirt and stuff out.
A couple of seasons ago, because I am the way I am ( 
, I challenged myself, Fels-Naptha, and Under Armour, to see how long I could make one pair of pants last. I only let him wear the same pair...and they lasted through an entire year...still white. I would have kept the experiment going...but he outgrew them, and I will only take crazy so far LOL...