A friend tows a trailer with his soul no problem. He had the Kia towbar fitted (that is detachable) and the electrics just fold up out of sight. Very neat job and you cant tell its there when not in use.
I always found, many years ago that thw weight you could tow would be roughly the size of the engine. i.e 1.6 engine equated to 16cwts or 750kg of weight. Of course there are rules and the caravan club recommend no more than 75 - 85% of the cars gross weight. The torque of the engine is another factor - maximum torque should be produced at around 50mph in 5th gear. Drive at 50mph in 5th gear, note the revs, now check in the manual and see where max torque is produced - if it is close then is that hurdle crossed. On a personal view, I think the shape of the Soul would make it good as there is not much sweep down at the back.