Magnesium deficiency

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Characteristics and occurrence


Magnesium (Mg) deficiency may result either from low magnesium content in the soil or from an overabundance of potassium or calcium which inhibit magnesium uptake by the crop. Therefore, the disorder is most likely to occur on sandy soils with low cation exchange capacity (CEC), on volcanic ash soils of high potassium status, or on some calcareous soils. Overfertilisation with potassium may also induce magnesium deficiency. In strongly acidic soils, magnesium deficiency may be induced by the presence of toxic concentrations of aluminium in the root environment, which inhibit magnesium uptake by the plant (see Aluminium Toxicity).


A magnesium-deficient crop will tend to have a pale overall colour. The earliest specific symptom of magnesium deficiency is an interveinal chlorosis of older leaves. Typically, the main veins retain a relatively broad margin of dark green tissue, but the minor veins are less well defined, resulting in radial bands of pale tissue between the main veins. However, in some cultivars the chlorosis is more mottled, composed of isolated patches, or the veins retain little green margin, and appear as a green network on a generally pale leaf. Chlorosis first appears on the oldest leaves, but may spread to quite young leaves. It may be accompanied by upward or downward curling of the leaf margins, or a wilty drooping of the leaf blades.

Purple or red-brown pigment may appear on older leaves in conjunction with chlorosis. Most commonly, pigmentation affects the upper surface of interveinal patches near the leaf tip and margins, but in some cultivars the veins under the leaf may become red.

Chlorosis generally progresses to yellowing and necrosis of the oldest leaves. Leaves may become entirely yellow and wilted before browning off, or necrosis may develop on interveinal and marginal tissues without prior chlorosis. Most commonly, localised yellowing precedes necrotic lesions, which spread from interveinal zones.

The vines of magnesium-deficient plants may become thin and twining, with lengthened internodes, a response similar to etiolation.

Possible confusion with other symptoms

Potassium deficiency may also produce interveinal necrotic lesions surrounded by chlorosis in oldest leaves. However, in the case of magnesium deficiency, necrosis is usually in a more regular pattern around the leaf, confined to interveinal and marginal zones. In addition, the senescing leaves are generally wilted, and the necrotic tissue is usually paler and remains soft.

Diagnostic soil and plant tissue tests

The critical concentration for magnesium in sweetpotato leaves is not yet well defined, as various sources of information are not in good agreement. In particular, estimates based on solution- or sand-culture experiments tend to be lower than those observed in the field. In solution culture experiments, a critical concentration (leaf blades 7-9 at 4 weeks) of 0.12% magnesium was estimated for magnesium deficiency, while healthy plants were observed to contain 0.15-0.35% magnesium. This agrees well with reports from sand culture studies. However, symptoms observed in field crops in Uganda and Australia have been associated with concentrations in the index leaves of 0.29-0.37% magnesium and in USA 0.40% magnesium has been reported in an magnesium-deficient crop, suggesting a critical concentration greater than 0.40%.

Soil tests have not been calibrated for sweetpotato response to magnesium. For other field crops, neutral 1 M ammonium acetate-exchangeable magnesium levels of greater than 100-200 mg magnesium/kg are generally considered adequate. European standards recommend consideration of soil test K levels also, with a K/magnesium ratio of less than 5, on a weight-of-element basis, being desirable.


Cultural control

Magnesium deficiency may be corrected by incorporation of dolomitic lime or magnesium oxide into acid soils (20-50 kg magnesium/ha), or by band application of kieserite or fertiliser-grade magnesium sulfate (10-40 kg magnesium/ha). As magnesium sulfate is the most soluble of these sources, it is the preferred source where it is necessary to correct an observed magnesium deficiency in an established crop, although it is often more expensive than other sources. If top-dressing of the established crop is difficult, magnesium sulfate may be applied as a foliar spray or by fertigation.


Bolle-Jones, E.W. and Ismunadji, M. 1963. Mineral deficiency symptoms of the sweet potato. Empire Journal of Experimental Agriculture 31, 60-64.

Doll, E.C. and Lucas, R.E. 1973. Testing soils for potassium, calcium and magnesium. In: Walsh, L.M. and Beaton, J.D. (eds.) Soil Testing and Plant Analysis, revised edition. Madison, USA, Soil Science Society of America Inc., p 133-151.

Edmond, J.B. and Sefick, H.J. 1938. A description of certain nutrient deficiency symptoms of the Porto Rico sweetpotato. Proceedings of the American Society for Horticultural Science 36, 544-549.

Leonard, O.A., Anderson, W.S. and Gieger, M. 1948. Effect of nutrient level on the growth and chemical composition of sweetpotatoes in sand culture. Plant Physiology 23, 223-235.

O’Sullivan, J.N., Asher, C.J. and Blamey, F.P.C. 1997. Nutrient Disorders of Sweet Potato. ACIAR Monograph No. 48, Australian Centre for International Agricultural Research, Canberra, 136 p.

Reuter, D.J. and Robinson, J.B. (eds.) 1986. Plant Analysis. An interpretation manual. Inkata Press, Melbourne.

Spence, J.A. and Ahmad, N. 1967. Plant nutrient deficiencies and related tissue composition of the sweet potato. Agronomy Journal 59, 59-62.


Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



A Mg-deficient crop showing interveinal chlorosis and bronze-red pigmentation on oldest leaves (J. Low).

Typically the whole crop looks pale, and older leaves develop yellow bands between main veins (P. Blamey).

Interveinal chlorosis, with a margin of green either side of main veins, progresses to yellowing (J. O'Sullivan). 

Symptoms sometimes observed are purpling of veins under older leaves, and thin, elongated twining growth of vine tips (J. O'Sullivan).

If necrosis develops before leaves wilt and fall, there are light brown lesions spreading in interveinal and marginal chlorotic areas (J. O'Sullivan).