The ivory soap works, and I use it in my orchard every year. I cut the bars into thirds, drop a piece into the toe of a hosiery leg or kneehigh, and then tie the stocking onto a branch. Before I did this, I never saw fruit on the lower branches of my trees. Now, I have peaches, apples, and pears.
To help the soap last longer, I like to set upwrapped bars of Ivory out in my closet to dry out so they last longer, letting them set for at least a month before putting them outside in the stocking.