Magnesium (Mg) deficiency is rare in broadleaves and conifers in landscapes. The deficiency is common in palms, especially date palms.
In magnesium-deficient palms, leaf tips turn bright yellow, while leaf bases and along the midrib remain green. Lower (older) fronds may die prematurely. In magnesium-deficient broadleaves, foliage can become chlorotic or chlorotic and necrotic.
Magnesium deficiency can be remedied by fertilizing soil with magnesium sulfate. Be aware that adding magnesium can reduce potassium availability; conversely, excess potassium makes magnesium unavailable. It may be best to add both potassium and magnesium in combination, such as by using fertilizers especially for palms. To avoid aggravating this deficiency, do not remove symptomatic leaves until they have turned entirely brown. Symptomatic leaves do not recover and must be replaced by new growth. See "Soil and Fertilizer Management" in the California Master Gardener Handbook for more information.
Adapted from the publication above, Abiotic Disorders of Landscape Plants, UC Agriculture and Natural Resources, and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Wide marginal necrosis with the center green due to magnesium deficiency.
Chlorosis due to magnesium deficiency, severe to mild (left to right).
Marginal chlorosis and necrosis of leaves due to severe magnesium deficiency.