You absolutely can put bark on top of soil. In fact, if you want to keep unwanted plants at bay (and don't fancy using ground cover plants to prevent weeds) it's actually advisable, especially if ...

Mulch can be almost any loose aggregate material that will stay put and not significantly reduce air exchange out of the soil. But what kind is best?

One of these is natural pine bark, which is a popular mulching ... and provides excellent cover over the ground which can ...