Over 100 different species of magnolia trees or shrubs exist, and even more numbers of ornamental cultivars. Overall, these plants are not "heavy feeders"--plants that grow well only if given very rich soil with large, constant supplies of nutrients. To diminish the need or worry of having to fertilize magnolia trees, plant them in ideal growing conditions. Magnolias grow best in a moist, well-draining soil that is rich in organic matter and does not have an alkaline pH.
Hubert Conlon of the University of Tennessee Extension Service urges you to conduct a soil test. Plants require nearly 20 nutrients, and knowing which ones are available to the magnolia tree in your garden allows you to objectively decide whether to fertilize or not. A small sample of soil is taken and analyzed by extension service offices or a private laboratory, and a results sheet is provided to you revealing all qualities and nutritional components of your soil.
Symptoms of Nutrient-Deficient Soil
Several things can occur on magnolia trees that may indicate a lack of nutrients in the soil. A lack of flower production in spring is one factor, especially if the tree is receiving ample sun exposure. The color of leaves is important, too. Stunted growth and yellow leaves in spring or summer can indicate a lack of nitrogen, magnesium, iron or other trace mineral. Lots of leaves, no flowers and very dark green leaves may indicate an over-abundance of nitrogen. The soil test provides the necessary insight to determine if any supplemental fertilization is required to correct the situation.
Types of Fertilizer
Synthetic (man-made) fertilizer products are either slow-release granules or in water-soluble form (powder or liquid). Quality slow-release granules are the best choice as they degrade slowly, supplying nutrients to the magnolia tree's roots over the entire growing season. Based on your soil analysis results, choose a slow-release fertilizer that compensates for any deficiencies. Typically, any all-purpose formula suffices, such as 10-10-10 with micro-nutrients. Since magnolia trees also grow best in acidic soils, often an acid-forming fertilizer is used. Such products lower the soil pH while providing nutrients and should be investigated for use if your soil is neutral to alkaline in pH. If your soil already is in the pH range of 5.0 to 6.5, an acid-forming fertilizer isn't warranted.
The best time to apply synthetic slow-release fertilizers for magnolia trees is from late fall (after leaf drop) through to early spring, according to the University of Tennessee Extension. John Marlar of the Mr. Grow website recommends fertilizing in early spring. Depending on the quality of your soil and the dosage recommendations on the fertilizer product label, you may fertilize twice annually--in early spring and again in fall after the fall frosts have occurred and the tree is no longer growing.
The Role of Mulch
Maintaining an organic mulch over the root zone of magnolia trees provides many benefits, including supplying trace nutrients as the mulch decomposes. The mulch mimics the natural forest floor conditions where magnolias are native so that the soil is constantly being built up and replenished with lots of decaying organic matter. Mulch alone can sustain the health of a magnolia tree if there are no major nutritional deficiencies in the garden soil. Create a mulch blanket about 3 inches deep and extend it out 3 feet or more past the reach of the tree's branches. If you choose to use synthetic fertilizer granules, simply scatter it atop the mulch as rainfall will move the nutrients down into the soil.