How do you get to the item behind the Resort Area on Pokemon Platinum, on Route 228?

If you are going to Route 228 from the Resort Area, and go a little to the left (into the long gr) you'll see an item ball in a clear patch of ground amidst the trees, roughly above the guy who gives out 2 Nuggets. It's blocked off by a rock on the left side and a tree on the other. I'm sure there must be some way to get to the item, but I can't work it out. Can anyone help me?