Copper toxicity

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Characteristics and occurrence


Toxic levels of copper (Cu) rarely occur naturally in soils. However, copper may accumulate due to application of sewage sludge, pig slurries or mine slag, or more commonly through persistent use of copper-containing fungicides or fertilisers. Mild symptoms of copper toxicity have been observed on sweetpotato in northern Australia on land with a history of heavy fungicide use.



Excess copper is highly toxic to sweetpotato, concentrations as low as 5 µM in the root zone being sufficient to cause significant growth reduction. Concentrations above 20 µM prevent root growth and result in the death of the transplanted cutting. Root damage may cause severe wilting, and death of the shoot tip. Unlike zinc toxicity, there is little or no general chlorosis of the leaves, nor increased red pigmentation.

Moderate levels of copper toxicity may be accompanied by a pale green to white interveinal chlorosis on mature leaves. Often the symptom appears on only a few leaves on the vine, with both younger and older leaves lacking chlorosis. Small interveinal areas become pale and slightly sunken, while the tissue around both major and minor veins retains normal colouration. The effect may vary from a faint mottle to a striking, sharply defined pattern. The symptom is often unevenly distributed across the blade, affecting one region, such as the leaf tip or a lobe, more than others.

Like toxicities of manganese and zinc, copper toxicity may also induce symptoms of iron deficiency, seen as a conspicuous pale yellow interveinal chlorosis on the younger leaves.

Possible confusion with other symptoms

The interveinal chlorosis may be confused with the pitting caused by manganese deficiency. In the case of copper toxicity, it is most intense on mature rather than on younger leaves and is not accompanied by a general chlorosis of the leaf blade. Symptoms of manganese deficiency typically affect the entire blade uniformly, while those of copper toxicity may be localised or unevenly distributed.

It may be difficult to distinguish between severe copper toxicity and zinc toxicity, although the latter more frequently causes increased red pigmentation on the vines. Some knowledge of the site history and a chemical analysis of leaf tissue may be necessary to confirm the diagnosis.

Diagnostic soil and plant tissue tests

No soil tests have been calibrated for copper toxicity in sweetpotato. For many crops, ‘total’ soil copper concentration (perchloric acid extractable) above 100 mg/kg have been regarded as high (Landon, 1991), while concentrations of DTPA-extractable copper above 20 mg/kg are potentially toxic in acid soils (CFL, 1983). For citrus, soils containing more than 112 kg/ha total copper in the top 15 cm, using Spencer’s test (Spencer, 1954), are considered at risk, particularly if the soil pH is below 6.5 (Koo et al., 1984).

Copper toxicity appears to affect primarily the plant roots, and therefore the concentration in the above-ground parts may be a poor indicator of plant status. Alva et al. (1995) found no significant increase in foliar copper concentrations in citrus trees, as soil copper was increased to levels which decreased growth. In sweetpotato, critical tissue concentration for toxicity of 15.5 mg copper/kg in the 7th to 9th youngest leaf blades was determined in solution culture. However, analyses of healthy field-grown plants have been found to commonly exceed this concentration. In crops where no fungicides have been applied, concentrations above 30 mg copper/kg have been recorded, while crops receiving fungicidal sprays may contain much higher concentrations without ill effect. From the author’s experience in Queensland, a concentration of 270 mg copper/kg was measured in plants showing mild symptoms of copper toxicity, while over 800 mg copper/kg was measured in leaves of an apparently healthy crop. These high concentrations are probably achieved by direct adsorption of copper onto the leaf surfaces from fungicidal spray, and reflect the recent spraying history of the crop. In contrast, toxicity is more likely to result from accumulated spray residues in the soil, and depends on the extended history of the site, combined with the ability of the soil to fix copper.


Cultural control

As copper toxicity usually results from excessive application of copper, prevention rather than correction should be stressed. Heavy applications of P fertilisers may reduce the availability of excess copper to the plants. Liming may be beneficial, as copper becomes less available to plants at high pH. This strategy has been used effectively in citrus crops, where maintaining the soil pH above 6.5 has been recommended for amelioration of copper toxicity (Koo et al., 1984).


Alva, A.K., Graham, J.H. and Anderson, C.A. 1995. Soil pH and copper effects on young ‘Hamlin’ orange trees. Soil Science Society of America Journal 59, 481-487.

CFL 1983. Soil Analysis Service Interpretation Charts. Consolidated Fertilizers Limited, Morningside, Queensland, Australia.

Koo, R.J.C., Anderson, C.A., Stewart, I., Tucker, D.P.H., Calvert, D.V. and Wutscher, H.K. 1984. Recommended fertilizers and nutritional sprays for citrus. Bulletin 536D, Institute of Food and Agricultural Science, University of Florida, Gainesville FL, USA.

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.

O’Sullivan, J., Loader, L., Asher, C., Blamey, P. 1997b. Troubleshooting nutritional problems in a new industry: sweet potato in North Queensland. Proceedings of the First Australian New Crops Conference, Gatton, July 1996. Rural Industries Research and Development Corporation, Australia.

Spencer, W.F. 1954. A rapid test for possible excesses of copper in sandy soils. University of Florida Agricultural Experiment Station Bulletin No. 544.

Tiller, K.G. and Merry, R.H. 1981. Copper pollution of agricultural soils. In: Loneragan, J.F. et al. (ed.) Copper in soils and plants. Academic Press, North Ryde, NSW, Australia. pp 119-137.

Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



Growth reduction and chlorosis on mature leaves in nutrient solution with trace, 8 µM, and 12 µM copper (L to R) (J. O'Sullivan).

Wilting due to severe root damage (J. O'Sullivan).

A mild mottling between veins of an old leaf, in a field where copper-based fungicides had been frequently used (J. O'Sullivan).

Whitening of areas between veins of a mature leaf (J. O'Sullivan).