According to North Dakota State University, trees are best transplanted in spring when the ground soil has warmed but before the buds begin to swell. Alternatively, transplant your young magnolia in the early fall after the leaves are shed but before the first winter frost occurs. Early fall planting allows the young tree to establish roots in the soil that will sustain the tender tree over the winter.
Prepare a planting hole twice the diameter of the root ball and just as deep in a location with a full sun to very lightly filtered shade exposure. Ensure a friable, nutrient rich soil with a neutral to slightly acidic soil pH. Amend the soil with elemental sulfur to boost acidity if needed. Add coarse sand to improve drainage if needed. Add compost and aged manure to boost soil nutrients if needed. Mix any amendments into the excavate soil.
Place the young tree in the hole being careful to keeps its root ball and surrounding soil entirely intact. Ensure that the top of the root ball is level with the surrounding soil line. Back fill the excavated soil in around the root ball pausing halfway to water in the soil. Continue filling in the soil and tamp down lightly around the roots when done.
Water your magnolia in well when finished with planting to drench the surrounding soil to a depth of at least 4-inches down. Maintain moist soil at all times never allowing the soil to dry out beyond the top 1/4-inch of soil crust. Depending on your climate and season, this may translate into watering once every 10 days or once every 5 days. Touch test the soil if in doubt about watering. Fell the soil at a depth of 3 to 4-inches, if wet do not water, if dry, water immediately.
Mulch around the root zone of your magnolia with a lightweight organic material such as leaf mold, shredded bark or straw. Apply a 3 to 4-inch thick blanket starting 3-inches out from the trunk and extending to several inches past the drip line of the tree.
Refrain from applying chemical fertilizer for the first year or two that your magnolia is in the soil, unless the soil is very poor or the growth appears stunted. If needed, use a water soluble, complete formula designed for acid loving trees and shrubs. Apply according to product label directions and always err on the side of under- rather than over-fertilizing.