"Why do baseball teams choose white pants? Because the people choosing the pants, usually men, are not the ones who do the laundry. "
We toured the facilities at our local farm team and the biggest question was “how do you get the uniforms clean?”
Answer: Tide makes a product specifically to clean them. But it’s not commercially available.
One of the keys to getting them clean is getting them soaked immediately. If not, the clay (as you know) is ridiculously hard to get out. Treat with a shout/oxi/resolve gel/spray, soak overnight, wash with Tide and then see if you need to repeat. The beauty of high school ball is the uniforms stayed at school so I didn’t have to wash those. The travel ball pants were a different story. 
I use OxiClean gel stick which has nubs on the end to scrub on all the grass and dirt stains. Then I put a scoop of OxiClean powder detergent in the tub of the machine in addition to Tide liquid detergent and wash with hot water. I had seen a video online of someone using Fels Naptha soap to get out grass stains, so I bought some at WalMart, but it did nothing. I buy the OxiClean gel sticks a half dozen at a time.
Well, I think @JazzyTXMom has the answer. White baseball (or any sport) uniforms must be washed by men.
Thanks everyone for the suggestions, this is a good thread. Aside from making men do the laundry, I am determined to keep the pants reasonably clean. Interestingly, my son’s team has a nearly two week trip in July where the coach will do the laundry, as most parents aren’t even going. I hate to see what the pants look like after that trip, they will probably have to be burned in a bonfire.
Now for the first test…Yesterday I put the baking soda goop on the stains and let the pants dry in the sun, then washed them and did a vinegar rinse, and it definitely made a difference on the reddish clay stains! I am going to try some of these other suggestions, including soaking the pants for a longer period of time and we will see what happens. Now, back to the NHL playoffs.
The chemistry nerd in me always gets excited about cleaning products! The science of cleaning!
This is apparently what some pros use to clean their white pants:
https://www.cleanuniform.com/how-do-professional-sports-teams-keep-uniforms-clean/
http://www.clubhouseucs.com/product-page/slide-out-1
Tread carefully. Some of this stuff can be corrosive and nasty! 
I had a soccer goalie child with white shorts. I swear by the Carbona Stain Devils- they come for different stains. The one for “makeup,grass and dirt” ( #6)- they are numbered- is the one you want. I went through a lot of it, and did a lot of soaking.
Ah, white baseball pants… that brings back memories of DS’s high school/travel team days. I didn’t stress too much about the sliding or grass stains because it was evidence of getting down and dirty when needed. But an OxiClean soak usually got the stains out.
My DD’s sport also has white pants for specific competitions. Again an OxiClean soak usually works. My issue with her is armpit (deodorant?) stains on the shirt she wore under a jacket. Never did figure that one out. Suggestions?
The professional baseball cleaner in BunsenBurner’s post #25 intrigued me. The second link is for part 1 of the 2-part system. It looks like a large bottle and costs $90. The label on the bottle says, “No Stain To Tough.” For $90 I would think they could use the correct spelling of “Too” on the label!
I use Oxiclean for stains that won’t come out any other way, but not as the packaging directs. I use a basin, and make a very high concentration of Oxiclean powder in hot water (hot so it dissolves). I don’t measure, but the idea is to really dump the Oxiclean in there and stir until it dissolves, so the resulting mixture looks very milky. I soak the whole garment or just the part with the stain for a long time–even overnight–then put through the wash with an extra rinse cycle to make sure all the Oxiclean comes out. I’ve saved many a garment that way. But I have to admit I never bothered to make that kind of effort with my son’s baseball pants, which would be dirty again ten minutes into the next game. So long as they smelled clean, it was good enough for me.