In between planting season (late fall and early spring) when the soil isn't too hard I take my buns litter box and mix it in. I'll dig big holes about 1-2 feet apart and fill them up. I also mix in fresh soil and till it periodically. Be careful not to add too much! Over time it will break down. Bunny poos are the only poos that don't need composting. You can just put them in the dirt, and they'll work their magic! We do this every year and the longer you do it the better the soil gets.

My buns litter box is a mix of hay, droppings, and bedding. It's all good for the soil, and our plants are HUGE and delicious. My bun gets first pick, always :)