Before feeding your magnolia tree determine what it really needs. If your magnolia has healthy foliage and is gaining height and circumference, fertiliser is probably not necessary. For lacklustre magnolias, perform a soil test before fertilising.
Magnolia canopy and root growth require at least 18 nutrients, according to Hubert Conlon, Extension specialist at the University of Tennessee. Nitrogen, phosphorus and potassium (N, P, K) are the most commonly applied nutrients.
Magnolias responded to high-nitrogen slow-release fertiliser (20N-2P-8K) with more root growth and less canopy growth, in Tennessee growth trials. A more balanced formula of nitrogen to potassium (8N-3P-7K) applied monthly produced healthier magnolias.
Magnolias receiving 2 to 3 per cent phosphorus or potassium in the Tennessee growth trials were unaffected.
Nutrient levels are lower and release-rates slower using organic material such as magnolia food. Manures have high N-P-K ratios are not recommended for magnolias. Cotton-seed meal is close to the University of Tennessee experts' recommended ratio. Composted sludge applied with commercial fertiliser produced even magnolia growth in the Tennessee Extension trials.
A study by the International Society of Arboriculture found that mulched magnolias surpassed unmulched trees in branch and canopy growth. Finished compost with known components used as mulch benefits by providing micronutrients as it is watered into the soil.