Kousa,
I put it on with a hand spreader and then water in by hand to make sure to get it off the plants. I use one big bag per 8x40 bed. No science there, it's what I can afford. I have not noticed any difference in the soil from it and I agree with keeping it away from food crops.
If you want earthworms add alfalfa. Some folks recommend alfalfa meal but I get mine in the horse aisle at Tractor supply and use the nuggets. Water breaks it down pretty quickly into meal.
