Oldest leaves of magnesium (Mg)-deficient palms usually have broad chlorotic bands along the margins with the central portion of the leaves remaining distinctly green (Fig. 1). In severe cases, only the rachis and adjacent portions of the leaflets remain green on the oldest leaves, but younger leaves show progressively wider bands of green along the centers of leaves. In Phoenix spp., leaflet tips on the oldest leaves may be necrotic, but this necrosis is due to potassium (K) deficiency superimposed on Mg-deficient leaves (Figs. 2 and 3). In palmate-leaved species with deeply dissected leaves, the chlorosis appears as broad yellow bands along the margins of individual leaflets (Fig. 4).
Magnesium deficiency symptoms differ from those of K deficiency in that symptom severity of K-deficient leaves increases gradually from base to tip of the older leaf, whereas Mg deficiency symptoms are distributed uniformly from base to tip of the leaf. Discoloration of K-deficient leaves is usually orange to bronze, shading gradually to green at the base of the leaf, whereas Mg-deficient leaves have distinctly green leaf centers and bright lemon yellow to orange (depending on the species) margins extending the entire length of the leaf. There is never any necrosis associated with Mg deficiency, whereas leaflet necrosis is a common symptom on K-deficient palms. It is possible for a palm to show symptoms of both deficiencies on a single leaf.
Magnesium deficiency is caused by insufficient Mg in the soil. Magnesium is readily leached from sandy and other soils having little cation exchange capacity. High levels of nitrogen (N), potassium (K) or calcium (Ca) in the soil can also induce or exacerbate Mg deficiencies.
Magnesium deficiency is very common on highly leached soils in Florida, Hawaii, and other tropical areas. It can also occur in container-grown palms if dolomitic limestone has not been added to the substrate. Also, since palms may remain in a container for up to a year or longer, any added dolomite is usually exhausted after six months or so with Mg deficiency symptoms becoming visible as a result. Most species of palms are susceptible to some degree, but Phoenix canariensis is by far the most susceptible species to this disorder.