Aluminium Toxicity

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Characteristics and occurrence

Aluminium (Al) toxicity is the most common cause of growth reduction on strongly acid soils. Aluminium is the most abundant metal in the earth’s crust, but its solubility at neutral to alkaline pH is very low. However, below about pH 5.0 (measured in water), Al solubility increases rapidly and may reach toxic concentrations. Solution culture experiments have shown that sweetpotato will grow normally at pH values as low as 4.0 in the absence of Al. Therefore, in the field, sweetpotato crops probably encounter problems of Al toxicity before suffering any direct effects of low pH.

Sweetpotato is considered to be moderately tolerant of Al. It was also found to be more tolerant of soil acidity than taro, yam, tobacco, maize, green bean or soybean, but less tolerant than cassava.


The primary effect of Al toxicity is to reduce root development (Figures a-c). Roots appear short and thickened, with short laterals, and may be discoloured yellow to brown. Root hair development is suppressed. Where the Al concentration increases with soil depth, the downward extension of the roots may be restricted, resulting in a very shallow root system.

In addition to poor growth and stunted appearance, a number of symptoms may appear in the tops as a result of poor root development, depending on which secondary factor is most limiting. Symptoms of water stress are common. As Al in the soil solution inhibits the uptake of Ca and Mg by roots, symptoms of Ca or Mg deficiency may develop. In addition, the solubility of P is reduced in the presence of high concentrations of Al, so that P deficiency is often associated with Al toxicity .

(a)  0  µM Al  

(b)  25 µM Al 


(c)  50 µM Al

Fig. a-c. Root growth comparison in sweetpotato cv. Meriken grown  in nutrient  solution with  Aluminium (V. Ila'ava).

 Possible confusion with other symptoms

As mentioned above, Al toxicity is often associated with the expression of other nutritional disorders, such as a deficiency of Ca, Mg or P. Carefully uprooting plants may reveal a root system of limited extent, and brown in appearance.

Diagnostic soil and plant tissue tests

Aluminium toxicity is best diagnosed by testing the soil. Soils with a pH below 5.0 (measured in 1:5 soil:water) are at risk of Al toxicity, and in soils below pH 4.5 (other than organic soils), Al is likely to severely reduce sweetpotato production.

Tests for exchangeable Al and Al saturation may be useful. Abruna et al. (1979) found that Al saturation correlated well with the response of sweetpotato yields to liming on several tropical oxisols and ultisols, with the greatest beneficial effect of lime on those soils with greater than 60% Al saturation. Liming a soil with 35% Al saturation resulted in only a 10% yield increase. Ila’ava et al. (1996) demonstrated that the tolerance of sweetpotato to solution Al is greatly increased in the presence of high concentrations of Ca, and it may be necessary to take Ca availability into account when using soil tests to predict crop response to Al.


Cultural control

Correction requires elevation of the soil pH, through the incorporation of lime or dolomite into the soil. In addition, maintenance of high soil organic matter levels through the return of crop residues, cover cropping, or mulching will help as components in the organic matter are capable of binding free Al into non-toxic complexes. Organic matter also helps to slow the rate of acidification of the soil, which may result from sequential cropping. Choosing nitrate fertilisers instead of urea or ammonium fertilisers may also reduce acidification.

Phosphate forms insoluble complexes with Al. Some of the beneficial effect of adding P fertilisers to acidic volcanic ash soils appears to result from decreasing solution Al concentrations.

Host plant resistance

Sweetpotato cultivars have been shown to vary in their tolerance and sensitivity to high concentrations of Al. Cultivars which yield well under acid soil conditions may be selected.


Abruna, F., Vincente-Chandler, J., Rodriguez, J., Badillo, J. and Silva, S. 1979. Crop response to soil acidity factors in ultisols and oxisols in Puerto Rico. V. Sweet potato. Journal of the Agricultural University of Puerto Rico 63, 250-267.

Abruna-Rodriguez, F., Vincente-Chandler, J., Rivera, E. and Rodriguez, J. 1982. Effect of soil acidity factors on yields and foliar composition of tropical root crops. Soil Science Society of America J. 46, 1004-1007.

Blamey, F.P.C., Vermeulen, W.J. and Chapman, J. 1984. Inheritance of boron status in sunflower. Crop Science 24:43-46.

Ila’ava, V.P., Blamey, F.P.C. and Asher, C.J. 1996. Sensitivity of sweet potato lines to Ca and Al stress in solution culture. In: Craswell, E.T. Asher, C.J. and O’Sullivan, J.N. (eds.) ACIAR Proceedings No.65: Mineral nutrient disorders of root crops in the Pacific. pp 53-57.

Munn, D.A. and McCollum, R.E. 1976. Solution culture evaluation of sweet potato cultivar tolerance to aluminium. Agronomy Journal 68, 989-991.

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.

Ritchey, K.D., Wright, R.J., Baligar, V.C. and Murrmann, R.P. 1991. Evaluating sweet potato tolerance to aluminium toxicity: comparison of rapid test method and field results. In: Wright, R.J., Baligar, V.C. and Murrmann, R.P. (eds.) Developments in Plant and Soil Sciences Vol. 45: Plant-soil interactions at low pH. Kluwer Acad. Publ., Dordrecht, pp 939-945.

Sangalang, J.B. and Bouwkamp, J.C. 1988. Selection of sweet potato for tolerance to aluminium toxicity: screening procedures and field tests. Journal of the American Society for Horticultural Science 113(2), 227-281.


Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



Stunted plants, with chlorosis and bronzing present on one variety but not the other (J. Low).  

Symptoms of phosphorus deficiency (reddening, yellowing and shedding of oldest leaves) on a severely stunted plant (J. Low)