it might be insecticide free.. but is the ground they live use chemical fertilizer?
It can be fatal to Chameleon too..
veiled chameleon don't normally dig the ground to find food.
the only time they do so are for laying egg.
So, i disagree with your thought.
You can try feeding them with earthworm.. but i doubt he will like the taste.
My advise is not to feed them with earthworm and stick with insects, or other common worm feeder (mealworm, butterworm, silkies, etc)