Why are the leaves on my magnolia turning yellow?

One of two possibilities exists. First, magnolia are extremely sensitive to iron and manganese deficiencies in the soil. A soil analysis will confirm if the soil is deficient and the rate and concentration of iron/manganese fertiliser to apply can be calculated. This in turn will alleviate your problem. Second, magnolia trees prefer to grow on acid soils with a low pH. If the pH of your soil has increased then this will also result in the yellowing symptoms you have described. As before, a soil analysis will confirm if this is the problem and remedial measures can be applied.