I'd like to see the grass go a bit dirty at the epicenter... perhaps grow a feathered oval solid out from the center of the explosion just on the grass for a ways with a darker center (scorching) and a feathered edge that moves toward brown on the outside (dirt kicked up and dropped). You could apply these as multiply or something to get it to blend right and adjust the opacity, but it should add a bit more reality to the bottom bit of the explosion... maybe some chunky noise in the brown oval as well to look like bits of dirt or something.