Magnolias are sensitive to nutrient deficiencies and so they perform much better when fertilized. If you live in a region with alkaline soil such as in an area with low rainfall then lack of iron will show up as yellowing between the leaf veins. Magnolias are also sensitive to salt so over feeding can burn the leaves. For this reason a slow release plant food like Osmocote is recommended. Just apply the Osmocote around the drip line of the tree roots every spring. Frequent heavy watering will help to leach the salts out of alkaline soils and also get the plant food to the roots of your magnolia tree.