Why is soil added to dead plant material in composts?

Compost is made of vegetative waste, which most Gardner's know that plants can't be grown in just that alone. So we add soil to the mix to have a balanced ratio of humus to enrich the soil, the students also turned this mixture so air can get in a well, and decompose they leaves and house hold green leftovers, this make a very rich mix, which is great for vegetable gardens, flower gardens etc, it is use to enrich and without the right balance it can kill plants, by growing bad bacteria, mold, and insects etc. You can make a compost bin out of a plastic 42 gal trash barrel. but please put silver dollar size holes all around then make sure you turn this mess often, so it won't stink You can also buy small bins or just put a large hole in the ground . I just bury my left over directly, left over radish, celery, lettuce, any thing green or dried leaves.