From University of Georgia Extension Service’s Growing Southern Magnolias for our Bracken’s Brown Beauties:
Once established, growth can be accelerated with light, frequent applications of fertilizer during the first three growing seasons.
During the first growing season after establishment, (2021) apply ½ pound (1 cup) of 8-8-8 or 10-10-10 fertilizer along the perimeter of the planting hole in March, May and July.
The second year (2022), increase the rate of the fertilizer to 2 cups and broadcast it in a donut-shaped area from the tips of the canopy to 3 feet beyond the canopy in March, May and July.
By the third year (2023), increase the rate to 4 cups and spread it in a donut-shaped area from the tips of the canopy to 6 feet beyond the canopy in March, May and July.
By the fourth year, the tree should have a well-established root system and should be able to forage for nutrients on its own.
Magnolia roots have been shown to grow more than three times further than the canopy width of the tree, so they can obtain nutrients applied to nearby plants and turfgrass.