I get a scrub brush, a large bucket (or wheelbarrow works if you have a lot of them), and mild detergent. And I hand wash mine, and then air dry.
I'll fill the bucket with soapy water and let all the boots soak for at least a few minutes, usually much longer. The longer you soak, the more the dirt breaks up. Doesn't matter if that is splint boots, bell boots, or sport boots. Then I use some "elbow grease" to scrub them clean.
Then I'll have a sprayer end on the hose that I can "gently" pressure wash the boots clean of soap. (I'd never use a real pressure washer, that would probably destroy the boots pretty quick.) Then air dry.
It's a hassle. I hate doing it. But it sure gets them pretty clean.
I've tried washing them in the washing machine. But my hand washiing does a WAY better job of cleaning them. And gentler.
For white boots, you can add bleach to whiten them up.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.