Bronze, it appears that hydrology_joe gave you very good advice. While some mowers have a bolt that adjusts belt tension, your deck only has a spring. If you belt is slipping, you probably need a new belt. Someone could have replaced the spring with an incorrect one also, but that's not likely. As Joe suggested, Messicks.com is a great source for parts. They show your deck belt and you can get the entire parts diagrams with prices sent to your email address. Here is the picture of belt and tensioning spring that I got sent to my email. Messicks really is a great source for info and parts. I've been extremely happy with their online service for both my NH tractor and my G1800 lawn tractor.
EDIT: The current price of your belt is $36.51 and Messicks has 5 in stock as of today. They don't have any of the tension springs in stock, but could order one for you. The price of the spring is $7.49.