Zinc toxicity

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Characteristics and occurrence

Incidence of zinc (Zn) toxicity is rare and has not been reported in sweetpotato crops. Cases in other crops have been reported on acid soils high in zinc, or on soils overfertilised with zinc fertiliser. Zinc is also contained in some fungicides, and may accumulate if these are used persistently.


Zinc toxicity may cause severe growth reduction or prevent the establishment of transplanted cuttings. In solution culture experiments, concentrations higher than 10 µM Zn in the root medium decreased sweetpotato growth. At 50 µM Zn, growth was completely stopped and death of roots was evident.

Specific visible symptoms are not always evident. Symptoms include the development of dark pigmented spots or blotches on older leaves, or in more severe cases, red pigmentation throughout the vine, particularly on petioles and around margins and veins of the leaf blades. However, red pigmentation is not observed in all cultivars. Severe damage to the roots at high zinc concentration may cause general yellowing and wilting.

High levels of zinc inhibit the uptake of iron, and it is common to find symptoms of severe iron deficiency induced by zinc toxicity. Iron deficiency is characterised by a pale yellow to white interveinal chlorosis on the younger leaves, and may eventually lead to necrosis of the leaf blades and growing point. Cultivars which normally have purple pigmentation in the youngest leaves become bright pink at the tip.

Possible confusion with other symptoms

Symptoms of zinc-induced iron deficiency may not be easily distinguished from those caused by an inadequate supply of iron, unless other symptoms such as increased red pigmentation are expressed. Zinc toxicity may be revealed by chemical analysis of the leaf tissue.

Diagnostic soil and plant tissue tests

Concentrations of more than 70-85 mg Zn/kg in the 7th to 9th youngest leaf blades were associated with reduced growth due to zinc toxicity in solution culture experiments. Concentrations above 150 mg Zn/kg corresponded to severe stunting, whiles concentrations as high as 900 mg Zn/kg were measured in severely affected plants.

In soil, a DTPA-extractable Zn concentration above 10 mg/kg is considered potentially harmful in acid soils. ‘Total Zn’ concentrations in soil (perchloric acid extractable Zn) usually fall in the range 10 to 300 mg/kg, with concentrations above 150 mg/kg regarded as high (Landon, 1991), and likely to result in reduced plant growth.


Cultural control

Should zinc toxicity be found to occur in field-grown sweetpotato, liming to raise the pH of the soil may alleviate the problem by reducing the concentrations of plant-available zinc. Large applications of phosphorus fertilisers may also have a beneficial effect.


Landon, R.J. 1991) (ed.) Booker tropical soil manual: a handbook for soil survey and agricultural land evaluation in the tropics and subtropics. Booker Tate Ltd; Longman, London.

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.


Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



Reduced growth at higher  levels of Zn. Yellowing of  leaves is due to induced Fe deficiency (J. O'Sullivan).


Dark red-brown pigmentation develops in some varieties, especially near veins on older leaves (J. O'Sullivan).

Symptoms of iron deficiency are commonly induced due to root damage preventing iron uptake (J. O'Sullivan).