Experts and BHG readers answer.
MY MAGNOLIA TREE
The 2-3 day lifespan of your magnolia blossoms is perfectly normal, and there's not anything you can do make the individual blooms last longer. If you fertilize with a high-phosphorous fertilizer (something with a big middle number like 5-10-5 or 10-30-10) you might promote more blooms, but it won't aid in the longevity of the blossoms. You only need to fertilize once per year with a slow-release pelletized fertilizer.