They need about 1 inch of moisture every 14 days. In some areas, rainfall will provide sufficient moisture. In dryer areas, and during dry periods, soak the roots at weekly intervals. But be careful that you do not drown trees growing in poorly drained soil. Fertilizing If magnolias are planted in reasonably fertile soil that is well supplied with organic matter, they seldom need fertilizing.
Trees growing in lawns benefit from fertilizers used on the lawn.
But if you want to stimulate the growth of magnolias, or if they show signs that the soil is infertile, apply lawn fertilizer in late fall or early spring. Signs of low soil fertility are small, sparse, pale leaves and short twig growth. Use 2 pounds of fertilizer per inch of trunk diameter. Spread the fertilizer in a band 2 to 3 feet wide under the ends of the branches. Do not let the fertilizer touch the trunk.