Soil from outside is perfectly fine, do not sterilize it, use several inches deep, plant some grass seed or better still, scrape up some sod (Lawn with grass and dirt included).

It's called "Bio active substrate" and gaining popularity. With bio active substrate feces decomposes on it's own, so cleaning is not necessary when using it.

I have been using bio active soil in some of my cages and it really works well. No need to worry about ingestion or impaction either, Soil passes right through the digestive tract.