Science Buddies: "Ask an Expert"

I want to know if this example below can be theoretically achieved without ruining my reader's suspension of disbelief:

The climax revolves around a king creating an enormous dust explosion to kill thousands of soldiers marching through a narrow pass between two tall mountains.

he uses the dust the soldiers kick up while walking and ignites it from far away since there is no wind so the dust just kind of hangs in the air as the soldiers March.

The size of the army is about 20 000.

Would this dust explosion be possible narratively speaking and how dangerous would it be?
If not possible, can u please provide me with alternatives?

JohanGallows - In order for a dust explosion to occur, the dust particles must be combustible. Flour, sawdust are clear examples. If the dust is mineral material like fine soil particles, while a dusty cloud forms, because the material is incombustible, it won't explode.

Rick Marz